Living under a rock

Living under a rock and feeling completely out of touch with everyone else.

I rarely buy new items unless I have to due to me being a tight Scotsman however recently I have bought a few new items and their performance surprised me somewhat and made me feel a little like I’ve been living under a rock. I thought this would make a good topic for an HPR show.

SSD 256GB USB stick (Refer to picture 1 and 2)

• Wikipedia article about SSD’s
• This is my first ever SSD, its made by Netac
• It is a double ended USB thumb drive with a USB A one side and USB C the other.
• I couldn’t believe how fast it was on my Mac; copying a 700MB file seemed almost instantaneous.
• I couldn’t actually believe it had copied the file and thought it had been an error!
• I tried it on my old PC running Ubuntu and was getting 40MB/sec write speed!
• Even more surprising was copying files between a USB hard drive and the SSD usb stick, with both connected to my Raspberry Pi. This was achieving a sustained write speed of 60MB/Sec.
• I’ve never seen anything faster than 40MB/Sec.
• I think I’ve been living under a rock for a number of years while the rest of the world moved on!
• My only concern with this technology is that I am not totally convinced about the long term reliability of SSDs. The SSD is great for transferring files back and fourth to different devices however for now I’ll stick to spinning hard drives on my PC for long term storage as speed is not so important to me.

Picture above shows my new Netac USB SSD stick

Picture above shows how the USB SSD stick opens

LED Lantern (Refer to pictures 3)

• This LED Lantern was recommended by my mother
• We purchased it from Tesco’s.
• Tescos in the UK is a British multinational groceries and general merchandise retailer
• I think it only cost me £8.40. (UK pounds)
• It even came with AA batteries !
• The LED Lantern is 9 inches x 3.5 inches
• The light output is phenomenal. It is diffused and fills a good-sized room with light.
• It has a low output mode, which is still very usable and will increase the running time.
• It also has a flashing option, which is likely only useful in an emergency.
• I think the reason they were selling them so cheap was that the date code of the included AA alkaline batteries was almost out of date.
• Alkaline batteries can have a very long shelf life.

Picture above shows the new LED latern that I bought from Tescos Super Store in the UK

Laser printer HP M209dw (Refer to picture 4)

• Wikipedia article about laser printers
• Wikipedia article about inkjet printers
• I needed to replace my old Epson inkjet printer with a new printer as it recently failed after 20 years of use! I covered the failure of my old printer in HPR episode HPR4552
• My new printer is an HP M209dw
• I'm finding it really nice to have crisp, clear, trouble-free printing again.
• It’s also very nice that the printer does double-sided (duplex) printing.
• I’ve been putting my new printer to good use printing out various SSH documentation to help me try and solve an SSH problem I’m having.
• As of May 2026 i’ve now had the printer about 6 months and according to the printer I’ve used about 50% of the toner cartridge.
• I believe it may be necessary at some point to remove the cartridge and gently rock it in order to use up all the remaining toner. I’m slightly nervous of doing this as I'm concerned it could prove messy.
• Time will tell how long the cartridge ultimately lasts and how much a new toner cartridge will cost. I also think it unlikely that this printer will last 20 years like my last Epson printer. Come to think of it I might not last that long either. :)

New Raspberry Pi4 server install

• Wikipedia article about the raspberry pi 4.
• Current stumbling block is SSH .
• My new raspberry pi 4 is running an up to date distribution. This distribution has a corresponding up to date version SSH . This newer version of SSH is refusing to connect to another raspberry pi running an old distribution.
• I can’t update the distribution on the older raspberry pi because it has a PiFace command and display add-on board which I covered in HPR episodes hpr2933 , hpr2951 , hpr2963 , hpr2976 . The drivers for this board are not available on an up to date raspberry pi Linux distribution.
• Getting the driver software installed for the add on PiFace command and display board on and up to date raspberry pi Linux distribution is non trivial.
• I also need to be able to make this work in the other direction. ie the old pi needs to be able to SSH into the new pi. This is proving tricky.
• I did a search within the SSH release notes and found over 20 references to possible incompatibilities between the latest version of SSH and the old version of SSH running on my PiFace Raspberry pi!

Car tyre problems

• Mrs. X’s car had a long-standing tyre problem.
• The air pressure in one of her front tyres kept going down.
• This caused a warning light on the dashboard to come on.
• At first, it was taking a long time for the light to come on after it was pumped up.
• Over time the leak got worse and the light came on more quickly after the tyre was inflated.
• We eventually took her car to our local garage; they couldn’t find the fault.
• The tyre leak continued to get worse.
• We eventually took the car to a car tyre specialist called Farmer Autocare near Medowbank in Edinburgh Scotland. They discovered that air was leaking from the tyre valve.
• They informed us that they were unable to repair the tyre because it was fitted with a TPMS “Tyre Pressure Monitoring System” .
• They said that the tyre valve had a sensor and this couldn’t be separated from the tyre valve and that we would need to go to one of their specialist branches that can replace this. This would require a new sensor which would need to be reprogrammed. They warned us the repair could be expensive.
• The following week, we visited the specialist Farmers Autocare in Corstorphine Edinburgh. They took a closer look and found that it was actually leaking around the edge of the valve. They ground down any corrosion around the alloy wheel, refitted the valve and added sealant.
• In the end, the repair bill ended up being very reasonable.
• We were warned that each of the four valves has a battery installed and that they have a finite lifespan and that due to the age of the car, any one of the four batteries could fail at any time.
• Each of the valves would be expensive to replace!
• I guess this is the price you pay for the complexity in modern cars.

Kindle pre 2013 No longer supported by Amazon

• I have one of the affected devices. It’s a pre 2013 Kindle 4 .
• It was the last model before they brought in a touch screen. My wife complains about her newer model with a touch screen. She preferred the earlier model like mine.
• In an email Amazon said they were glad we had enjoyed using our kindle for a long time. Going forward we can no longer purchase books from Amazon as they are cutting Kindle Store access.
• What surprised me most was that the email stated that a factory reset of the Kindle would make the device unusable for good!
• I was under the impression that I can still download books manually from Amazon. However this may not be true as I noted down the following comment from somewhere. Unfortunately I can’t remember where:

The statement 'You can still log into Amazon’s “Manage Your Content and Devices” page on the web, download your owned Kindle books as files, and transfer them manually via USB.' is not really true. There is no longer (in April 2026) any "download" facility on that page, even for titles I "own". That's the whole point.

• During the episode I refer to the fictitious TV character Victor Meldrue from the UK TV show “ One Foot In The Grave ” whose catch phrase was “I don’t believe it!”.

Roku streaming stick Feb 2026

• Our old Google Chromecast started to become very buggy and slow. This is an old model that you use by casting programs from your phone or tablet device.
• My mother has a newer version of the Google Chromecast . When I visited her I also found this a bit slow and buggy.
• I previously tried replacing our Google Chromecast with an Apple TV streaming box costing over £100!
• I found this frustrating as some of my existing apps would not cast to my Apple TV streaming box.
• I promptly returned the Apple TV streaming box to Amazon for a full refund.
• Mrs X came up with the idea of buying a Roku streaming stick.
• I knew very little about Roku and didn’t expect to be much good due to the low price (£27 UK).
• We bought the Roku Streaming stick plus 4K HDR
• This is a streaming device similar to a Google Chromecast and Amazon’s Firestick
• We have found it extremely good value at only £27 (UK)
• The remote control is excellent and also controls all the TV major functions like volume & turning the TV off and on.
• The interface seems very responsive and I can cast all my apps from my phone and tablet to it with no problems.
• The only negative thing I have found is that it seems to have stopped my TV remote working with Kodi . I have Kodi running on a raspberry pi under my TV. This works by sending the remote control signals received by the TV and passing them to the raspberry pi via the HDMI cable connected to the TV. These control signals are sent using CEC . My hunch is that during the Roku installation it has upgraded or reconfigured the CEC on my TV stopping it from now controlling Kodi . Luckily this is not too much of a problem as I don’t use Kodi very often these days and I can also control Kodi using an old Xbox remote control and Xbox infrared Sensor that I have plugged into my raspberry pi.

Provide feedback on this episode.

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01 Introduction

This is the third episode in an 8 part series.

02

In the previous episode we covered

* The early history of computers in industrial control

* The early history of PLCs, including how they got their name

* Who the major brands are

* What they look like physically

* A basic description of the abstract machine architecture

* A very brief look at what a PLC program is like

* The scan concept

* The main PLC programming languages

* The minor PLC programming languages

* The relative popularity of each of the programming languages

03

In this episode we will begin by taking a look at one of the early PLCs from the era when they first began seeing widespread use.

I will not even attempt to try to be comprehensive, I will just give a broad overview in the limited time I have available here.

I have started with an older model because its simplicity and limited features allow for an easier introduction to the topic.

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04 Allen Bradley PLC/2

The Allen Bradley PLC/2 was introduced to the market in 1977.

It was not their first foray into this field, but it was their first really successful one.

I am not aware of a comprehensive history of their early product line, but one of their early major selling models was the PLC2/30.

This is also known as their 1771 series.

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05 Physical Layout

The CPU module was a large metal box which sat beside the I/O rack and connected to it with a cable.

The I/O rack was a box with an open front and an series of slots into which tall narrow boxes containing the I/O could be slotted.

06

A backplane ran across the back, connecting the I/O to the CPU.

Multiple racks could be connected together by cables.

For the PLC2/30, you could have a maximum of 896 digital I/O points.

07

Racks were 315mm high, and between 247mm to 610mm wide, with racks coming in 4, 8, 12, and 16 slot varieties.

The CPU was the same height as the rack and roughly square in outline.

08

As micro electronics advanced, the CPU module was able to be shrunk down in size such that it would fit into a slot in the rack, which became the norm for PLCs.

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09 The Electronics Inside

Early model PLC/2s used some sort of 8 bit processor, some sources say an Intel 8080.

10

They also used four AMD 2900 bit slice processors as a logic co-processor.

If you are not familiar with bit slice processors, these are chips which are like a 4 bit vertical slice through a processor, and can be joined together with logic chips to form a complete CPU.

These are what were used to construct minicomputers.

11

They were used as coprocessors in early PLCs because microprocessors on their own were simply too slow to handle running the user program rapidly enough to allow a useful size program.

One or more microprocessors were used as well in order to coordinate the overall operation and system management.

12

As microprocessors grew faster and more powerful, the need for logic coprocessors declined and they were eventually dropped.

Early versions used magnetic core memory.

Later ones switched to some sort of solid state RAM, probably static RAM of some sort.

13

Details of what sort of processors are inside any PLC are actually very hard to come by as manufacturers don't generally talk about that sort of thing.

They wish the user to see it as just a black box.

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14 The Data Table

From a programmer's perspective, the most important thing to understand first is the data table.

The data table is the PLC's data memory.

15

For a PLC2, this is an array of 16 bit words.

Each word consists of two 8 bit bytes.

Addressing for both bits and words is in octal.

16

For those not familiar with them, octal numbers follow a counting system that goes from 0 to 7.

The next number after 7 is 10.

Counting then proceeds from 11, 12, 13, etc. going to 17, 20, 21, etc.

Each octal digit takes exactly 3 bits.

17

Individual bits in PLC2 notation may be addressed by specifying the word followed by a slash, and then the bit.

For example, 030/12 is the 12 bit in word 030. Remember that this is octal, so 12 is not the 12th bit if you are counting in decimal.

18 Memory Organization

On the PLC2/30, the data table has the following organization.

19

Word addresses 000 to 007 are Processor Work Area number 1.

This is not accessible by the user.

20

Word addresses 010 up to, but not including, 100 are the Output Image Table.

This is a memory mapped image of the I/O outputs.

010 to 077 is for Rack 1.

020 to 027 is for Rack 2.

This pattern continues up to Rack 7, which is 070 to 077.

21

Word addresses 100 to 107 are Processor Work Area number 2.

This is also not accessible by the user.

22

Word addresses 110 up to, but not including, 200 are the Input Image Table.

This is a memory mapped image of the I/O inputs.

This is laid out in the same way as the output image table, and goes from 110 to 177.

110 to 117 is for Rack 1.

120 to 127 is for Rack 2.

This pattern continues up to Rack 7, which is 170 to 177.

23

You will notice that the output image table and input image table appear to address the same rack slots.

They do in fact do this.

As to which address a specific slot in a specific rack maps to depends on whether there is an input card or an output card in that slot.

24

Addresses 200 to 277 are for Timer/Counter accumulated values.

An accumulated value is the current time or count.

25

Addresses 300 to 377 are for Timer/Counter preset values.

A preset value is the target time or count which when reached causes the timer or counter to indicate that it has reached the desired value.

26

The memory above 400 can be configured to split it into a data storage area and user program area.

The data storage area is where you would store data that your program needs to use which is not part of the I/O or times and counters.

You need to strike the correct balance between user data and program size.

27

There are various things that can be changed and configured with respect to the above, but I am not going to cover that in any depth as this is not a tutorial on the PLC2.

=

You should however have a pretty good idea of the memory of an early model PLC.

The things to understand are that I/O are mapped to memory addresses, and

different memory ranges are used for different purposes.

28

All memory management was manual.

It was up to the user to keep track of which memory addresses were to be used for what purposes.

Allen Bradley helpfully provided paper forms which you could photocopy which you could use to plan out and document what each address would be used for.

Part of the programmer's job was to make efficient and logical use of memory, while also leaving space for future changes.

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29 The User Program

The user program is made up of instructions.

Each instruction typically takes one word of memory.

However, complex instructions can take up to 8 words of memory.

30

There is a main program.

You can think of this as like the "main" function in C.

If you don't find that analogy helpful, then just think of it as this is where your program starts.

The main program continues with one rung after another until it reaches the END statement.

31

There is also a subroutine area.

The main program calls a subroutine by using a Jump to Subroutine, or JSR instruction.

32 The T3 Programming Terminal

When the PLC2 came out, things like laptops were still far in the future.

Even the first Compaq Portable suitcase style PC was still some years away.

Indeed, the first PLC/2 came out not long after the first Altair PC kit.

33

Programming initially therefore was done using a special programming terminal known as a T3.

The T3 was a suitcase size box with a small CRT in the end of it, and a keyboard attached to it below the screen.

If this sounds like the early portable PCs, keep in mind that this in fact predated them by a number of years.

34

The keyboard was not a typewriter or QWERTY style.

It had a membrane keypad with graphical symbols.

Recall the previous episode on control diagrams using relays and how these were documented using schematic symbols on drawings.

The T3 terminal keypad had symbols corresponding to a simplified and modified version of the symbols used in those schematic drawings.

35

When a programmer pressed one of those symbols on the keypad, the corresponding symbol appeared on the screen in the current cursor location.

The programmer could then enter the data address that the symbol was to correspond to using the numeric part of the keypad.

36

An engineer, technician, or electrician could therefore create and enter programs using a fully graphical environment that used his existing knowledge of control using hardware components.

They did not see this as programming a computer, they saw it as something else entirely which was unrelated to computers.

37

Programming took place with the T3 connected to the PLC while the PLC was in program mode.

The instructions were entered directly from the terminal into the PLC memory.

Programs could be saved or restored to cassette tape.

The program would run when the PLC was set to run mode.

38

What is more, the T3 terminal allowed on line debugging of programs.

When the T3 was connected in run mode, the ladder diagram would update live.

When a logic condition was true, it would be highlighted.

When the logic condition was false, it would be shown in an non highlighted state.

39

By observing a specific rung, you could see if the expected series of graphical instructions were all highlighted in a pattern which would result in the final output of the rung to be true or false as expected.

This was an invaluable troubleshooting aid in finding both software bugs and hardware faults.

Programming terminals became an indispensable part of an industrial maintenance electrician's essential equipment.

40

Eventually as portable PCs and later laptops became available, the T3 became obsolete and these functions were transferred to software that ran on standard PCs.

However, they still work essentially the same way.

Additional capabilities were added, such as the ability to write programs while not attached to the PLC.

Descriptive labels and comments could be added to memory addresses to document them and aid in reading the program,

Cross referencing functions would make it easier to find which addresses were used for what,

etc.

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41 Instructions

I will now turn to listing the sorts of instructions which were available to use.

I won't try to explain everything, but it should at least give you a vague idea of how these were programmed.

I will use Allen Bradley's names for these, which I should emphasize do not correspond to what other manufacturers call them.

42 Relay Type Instructions

These are boolean instructions and consist of the following.

43 Input instructions

These examine bits in memory and yield a logic state depending on the value of the bit they address.

Input instructions start at the left of the rung and work their way right as more are added.

Examine on - This is true when the addressed bit is set to 1.

Examine off - This is true when the addressed bit is set to 0.

44 Output Instructions

These turn bits on or off depending on the sum of the boolean logic conditions leading up to them.

Output instructions are on the right of the rung.

There must always be at least one output instruction.

Output Energize -When the logic conditions leading up to it are true, it sets a bit to 1.

When the logic conditions leading up to it are false, it sets a bit to 0.

45

Output Latch - When true, the bit is turned on.

When false, it does nothing.

Output Unlatch - When true, the bit is turned off.

When false, it does nothing.

46 Branch Instructions

These do nothing themselves, but they make wiring connections on the diagram.

These are important to allow the instructions listed above to be placed in parallel in the same rung so that they can do things like specify OR conditions.

You can think of them as essentially turning an examine on or examine off into an OR or OR NOT instruction.

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47 Timer and Counter Instructions

Timers and counters are as the name implies, instructions which do timing and counting operations.

They have separate preset and accumulated values.

They are a type of output instruction in that they reside on the right side of the ladder rung and their operation depends on the state of the rung logic to their left.

48

Timers and counters work in Binary Coded Decimal, or BCD values.

BCD values each take 4 bits of memory.

However, unlike hexadecimal, only values from 0 to 9 are valid.

What would be values from A to F in hexadecimal are not valid values in BCD.

49

The preset and accumulated values are stored in 16 bit words. The first 12 bits are used to store up to 3 BCD digits.

The remaining uppermost 4 bits are used to store instruction status bits such as whether the timer or counter is done and various other features.

50 Timers

The timer time base corresponds to the timer resolution.

The time bases are 1.0 second, 0.1 seconds, and 0.01 seconds.

So a timer with a 1 second time base can time from 0 to 999 seconds to one second of resolution.

A timer with a 0.1 second time base can time from 0 to 99.9 seconds to 0.1 seconds of resolution.

51

There are three types of timer,

Timer On Delay (or TON),

Timer Off Delay (or TOF),

Retentive Timer (or RTO).

52

On delay timers time while the rung condition is true and reset when it goes false.

Off delay timers time after the rung condition goes false and reset when it goes true.

Retentive timers are like on delay timers but retain their accumulated value until reset by the Retentive Timer Reset (or RTR) instruction.

Generally, you start a timer and then monitor its "done" bit somewhere else in your logic.

53 Counters

Counters count events.

These come in the following varieties.

54

Up-counter (or CTU).

Down-counter (or CTD)

Counter Reset (or CTR)

Scan Counter (or SCT)

55

Up counters count upwards from zero, and down counters count downwards from the preset.

The scan counter counts the number of program scans.

This is not something you would typically find a use for, although you can use it to for example calculate the scan time.

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56 Data Manipulation Instructions

Data manipulation instructions are used to read, write, and compare byte and word data.

Operations take place when the rung goes true.

57

GET reads a 16 bit word from a memory location.

PUT writes a 16 bit word to a memory location.

LES resolves to true if the value read by the GET instruction is less than a specified value.

EQU resolves to true if the value read by the GET instruction is equal to a specified value.

58

The LES and EQU instructions can be combined in various ways with each other and with GET to give all the possible comparison permutations such as greater than, less than or equal to, etc.

59

GET BYTE reads a 8 bit byte from a memory location.

LIMIT TEST resolves to true if a byte value is between the limits stored in the upper and lower bytes of a word.

LIMIT TEST is used in combination with GET BYTE.

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60 Arithmetic Instructions

Arithmetic instructions also operate on BCD word values.

Operations take place when the rung goes true.

The uppermost byte is used to indicate things like arithmetic overflow or underflow.

These include

61

Add

Subtract

Multiply

Divide

Convert BCD to Binary

Convert Binary to BCD

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62 Block Transfer Instructions

Block Transfer Instructions are output instructions that are used to transfer up to 64 16 bit words of data between the data table and I/O modules.

These are used with intelligent I/O that worked with words rather than individual bits.

63

Examples of these are things like

Analogue I/O that read and write varying voltages rather than just on or off.

PID that handle closed loop control of things like temperature.

Thermocouples that read temperatures.

Servo motor control that command positions of servo motors.

Stepper motor control.

Position encoders.

64

I won't go into details on block transfers, but this should be enough to give you a general idea of how the PLC could handle that sort of equipment as opposed to the simple on/off states of digital modules.

Obviously these can be used in combination with GET, PUT, and other data manipulation instructions which can read and write memory locations.

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65 Jump Instructions and Subroutine Programming

These include the following instructions.

66

Jump (or JMP)

Label (or LBL)

Jump to Subroutine ( or JSR)

Return (or RET)

67

Jump will jump execution to a label position.

You can think of it like a GOTO instruction.

68

Labels are numeric and can be from 00 to 77 in octal.

Jump instructions are rung outputs, and when the rung goes true, execution jumps to the label number specified in the instruction.

Labels are input instructions and appear at the start of the rung where execution resumes.

69

Normal jumps cannot jump into a subroutine, and they can only jump forward, not backwards.

This means that loops are not possible using jump.

This would be a deliberate design decision to prevent programs from getting stuck in a loop and preventing the scan from proceeding.

70

Subroutines appear after the end of the main program.

They are defined by numerical octal labels using the label instruction.

Subroutines are called using the Jump to Subroutine (or JSR) instruction.

At the end of the subroutine, the Return (or RET) instruction returns execution to the next rung following the JSR which called it.

You can nest calls to subroutines up to 8 deep.

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71 Data Transfer File Instructions

Despite what you may think from the name, there are no disk files in the PLC2.

A "file" instruction rather moves blocks of words from one data memory location to another.

=

These include source and destination addresses, and various other parameters which control the operation.

There is actually quite a bit involved in these, but I'm not going to bother going into the details.

You just need to know that it is possible to move blocks of data memory around.

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72 Shift Register Instructions

These operate on entire series of words and include

=

Shift File Up

Shift File Down

FIFO Load

FIFO Unload

73

These can operate on 1 to 999 words.

A typical use for this sort of instruction would be to track a part as it moves through the various stations of a machine.

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74 Bit Shifts

These operate on individual bits and include the following.

75

Bit Shift Left

Bit Shift Right

Examine Off Shift Bit

Examine On Shift Bit

Set Shift Bit

Reset Shift Bit

76

These do pretty much as you would expect in terms of shifting bits left or right.

However, the shift register can be defined to be anywhere from 1 to 999 bits long, so it is not confined to the size of a word.

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77 Sequencer Instructions

Sequencer instructions are a software emulation of electromechanical drum controllers.

Drum controllers allowed a long sequence of operations to be performed.

These used a rotating drum which had a regular series of parallel holes in the outer surface.

Above one row of these holes was a row of limit switches.

By inserting pegs into some of these holes which made contact with the limit switches, the drum controller could turn on patterns of relays or valves.

78

Some of the limit switches could be connected to sensors on the moving elements of the machine.

When the parts of the machine were in the correct position for that step in the sequence, the pattern of pegs would cause an electrical path to complete through the limit switches and energize a solenoid which would ratchet the drum to the next step.

79

This is often compared to a music box or to an old style of washing machine control.

However in the case of the drum controller, the pegs would program the input match conditions as well as the desired output.

The PLC/2 drum controller would emulate this sequencer.

This feature came to be very widely used in equipment used in the automotive industry and other assembly oriented manufacturing processes and often formed the heart of the PLC/2 program.

80

I won't go into details of the sequencer instructions, but there are separate input and output sequence instructions.

The input sequence instruction would try to match the states of the sensors, and the output sequence instruction would turn the various outputs on or off, controlling things like solenoid valves.

The sequence itself would be defined by a series of words in memory, with each bit corresponding to an individual input or output.

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81 File Logic Instructions

File Logic instructions amounted to array operations on words, although the term "file" is used for what in computer terminology would be an "array".

These instructions included

AND

OR

EXCLUSIVE OR

Complement

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82 Special Programming Techniques

These are miscellaneous instructions.

These consist of the following.

One Shot Leading Edge

One Shot Trailing Edge

83

A one shot is an output instruction that causes the addressed bit to turn on for one scan upon a change of state in the rung logic condition.

One shots could be triggered on a false to true change or a true to false change, depending on which of the two instructions were used.

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84 Why Did the PLC2 Use BCD?

You are probably wondering why the PLC2 used BCD numbers instead of normal integers.

This reflects the operating environment that an early PLC would have to work in.

Early operator panels using mechanical numerical input devices and simple electronic output displays which typically worked in BCD.

85

Thumbwheel switches were small drum like rotating devices with numbers 0 to 9 printed on the periphery.

Several wheels would be stacked beside one another.

They looked somewhat like the mechanical odometer that you would find in an older car.

The operator could however move each digit wheel separately.

Each digit would be read by wiring 4 wires back to 4 inputs on the PLC.

Thus each digit was inherently BCD due to how the thumbwheel hardware worked.

86

By turning the individual wheels separately, the operator could adjust things like temperatures, speeds, time delays, etc., and so modify process parameters.

Output operator displays were typically 7 segment LED, neon, or vacuum fluorescent displays.

These displays or devices similar to them preceded PLCs and so were common industrial devices that had to be handled.

Each numeric digit had 4 inputs that would be wired to 4 outputs on the PLC.

Seven segment displays can use hexadecimal, but that wasn't very useful if you are trying to display a temperature in Celsius to an operator.

Thus again, BCD was the most practical sort of number to work with.

87

As PLCs and their user programs grew more capable and more sophisticated, they began working with normal integers and even floating point numbers.

However, many if not most PLCs were to retain instructions to translate to and from BCD.

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88 The Influence of the PLC2

The PLC2 was a hugely successful product for Allen Bradley and firmly established them as one of the leading PLC vendors, with a number one position in the North American market, and in the top 5 world wide.

However, it had a number of weaknesses, particularly the very primitive memory and subroutine architecture and the focus on octal and BCD numbers.

It was focused on replacing control relays and it did this fairly well.

89

The next really successful successor to the PLC2 for Allen Bradley was the PLC5.

This took the basic PLC2 instruction set and married it to a new memory system with new data types, separate name spaces, and parameterized functions.

90

It was like switching from programming in Basic with line numbers and jumps to Pascal.

However, it could still use PLC2 I/O racks and modules, an important consideration given that the bulk of the cost of a PLC was in the I/O system.

91

The PLC 5 was in turn succeeded by the Contrologix series which modernized things still further, while being in a smaller form factor.

Allen Bradley remain one of the major PLC vendors today, operating under the Rockwell name.

--------------------

92 Conclusion

In this episode we focused on one specific but very widely used early PLC, the Allen Bradley PLC2.

We covered the following subjects.

The internal electronics.

The data table.

The user program.

The T3 programming terminal.

And a very brief overview of the instruction set.

93

This was a fairly simple and in some ways primitive PLC, but you should have a very rough idea of the concepts behind a PLC, including the data table and scan concept and the relay-like instruction set.

94

In the next episode we will take a look at a PLC from the largest vendor word wide, Siemens.

In this example we will look at a model from slightly later than the PLC2, the S5 series.

This could be seen as a second generation PLC.

Since we won't need to repeat the basic concepts covered already, we can focus more on what is new and different about the S5 compared to the PLC2.

95

This has been the third episode in an 8 part series.

--------------------

Provide feedback on this episode.

jasper from the Free Latern

https://www.thefreelantern.com/

LyricConductor

https://github.com/jnuyens/LyricConductor

ModuleJail:

https://www.modulejail.com https://github.com/jnuyens/modulejail

Smart Dinosaur - Space for Fantasy

Youtube: https://www.youtube.com/watch?v=qnUyW2yiiwo&list=OLAK5uy_kCrIROefktTWed3iPyDGhCUKl4gTvqXDU

Spotify: https://open.spotify.com/album/3J2qdIgJFtk78O5IhYWbOH?si=eba8037fe0b34339

Provide feedback on this episode.

• Introduction & Setting : Recorded on a sunny August afternoon in a Merton garden with Trurl exploring a wide-ranging philosophical and analytical journey through mathematics, games, and literature.
• Literary & Sci-Fi References :
  • Stanislaw Lem : Discussion of the robot character Trurl from The Cyberiad , who alphabetically creates everything before reaching the letter "N" for "nothing" and destroying everything. Learn more about Lem on Wikipedia .
  • Vladimir Nabokov : Analysis of The Luzhin Defense , a novel exploring chess obsession, psychological breakdown, and the "horror of chess." Explore the book on Wikipedia .
  • Edgar Allan Poe & Thomas Hobbes : Discussion on 19th-century precision in writing, stylistic inflections, and the value of rigorous language.
  • Martin Amis : Praise for his modern precision, with highlights including Money and Information .
• Core Philosophical & Mathematical Themes :
  • Socratic Epistemology : Exploring the ancient Greek view from Plato's dialogues that equates knowledge directly with practical skill.
  • Active vs. Empirical Learning : Trurl's theoretical aversion to traditional science labs in favor of pure conceptual frameworks, contrasted with hands-on computational tinkering.
  • Stephen Wolfram : Discussion of A New Kind of Science and the computational approach to universal systems.
• Applied Puzzles & Strategic Games :
  • Chess Ratings : Investigating the rating system, Elo mechanics, and statistical anomalies suggesting top players are statistically underrated.
  • The Drake Equation & Astrobiology : Quantitative modeling of extraterrestrial life inspired by Stephen Webb's If the Universe Is Teeming with Aliens... Where Is Everybody? .
  • Economics & Libertarianism : Transition from early socialist upbringing to decentralized economic structures, cryptocurrency architectures, and the Lightning Network.

Provide feedback on this episode.

In a bonus episode for the reserve queue, Kevie and Dave talk about how they review beers:

1. Aroma - what do you smell from the glass and bottle or can
2. Taste - take a sip, swirl it around your mouth and allow it rest a second before drinking; don't gulp. What flavours are you getting.
3. Wait - What flavours are appearing in the aftertaste.
4. Opinions - what are your personal thoughts on this beer.
5. Score - pick a score and rarely give any beer full marks.

Connect with the guys on Untappd:

• Dave
• Kevie

The intro sounds for the show are used from:

• https://freesound.org/people/mixtus/sounds/329806/
• https://freesound.org/people/j1987/sounds/123003/
• https://freesound.org/people/greatsoundstube/sounds/628437/

Provide feedback on this episode.

Obviously, I'll need to introduce why I'm the best person to explain this concept and how I did my research.

After that, I dive into the core of he episode: the research.

The Ultimate Guide To Spotting AI Generated Text That Every Writer Needs To Read.

That title already has two tells:

• Title case : every word capitalised, which conveniently cannot be read out loud
• Hyperbole : a promise of ultimacy, for a ten minute podcast

For most of this episode I am not myself, I am the machine, and every sentence I say is an example of the thing it describes. This page is the key to that part of the show: the tells in the order you hear them, with what they sound like in the recording.

Most names come from tropes.fyi , so you can look them up there and get the longer explanation.

As also stated in the podcast: these aren't errors. They are ordinary figures of speech that human writers have used for centuries. The tell is the density.

Part one: Vocabulary

• Prestige vocabulary : words nobody uses out loud. e.g. delve, harness, utilise, robust, foster
• Grand nouns for ordinary things : e.g. tapestry, landscape, realm, ecosystem, paradigm
• Magic adverbs : imply significance, carry no information. e.g. quietly, fundamentally, arguably, remarkably
• Avoiding the verb "to be" : plain "is" swapped for something that sounds busier. e.g. serves as, stands as, represents, marks
• Invented concept labels : an abstract problem noun (trap, creep, paradox, inversion) welded onto a domain word, then used as if it were established. e.g. "the fluency trap", "prestige creep"
• Vague attributions : authority without a name. e.g. "experts agree", "industry reports suggest", "observers have noted"
• Appeal to familiarity : consensus claimed rather than proven. e.g. "a classic", "famously", "as we all know"
• Promotional register : describing turns into selling. e.g. unlocks, seamless, unprecedented

Part two: Sentence shapes

• Negative parallelism : the "it's not X, it's Y" pattern, and the most reported tell of all. e.g. "This isn't a stylistic quirk, it's a structural signature."
• Not X. Not Y. Just Z. : the same move tripled, building tension for a reveal that needed none. e.g. "Not a habit. Not a tic. A tell."
• Sets of three : one tricolon is elegant, three in a row is a stuck needle. e.g. "it arrives in threes, it lands in threes, it exhausts in threes"
• Short punchy fragments : emphasis manufactured by full stops instead of earned by content. e.g. "Short. Parallel. Relentless."
• Self-answered questions : asks what nobody asked, answers it immediately. e.g. "The problem? Nobody asked."
• Anaphora : the same sentence opening until it becomes furniture. e.g. "They open sentences the same way. They open the next one the same way."
• Trailing "-ing" analysis : shallow significance bolted onto a finished sentence. e.g. "highlighting its enduring importance", "reflecting broader trends"
• False ranges : "from X to Y" where there is no scale and no middle. e.g. "from cesspit to citation"
• Filler transitions : connect nothing to nothing. e.g. "it's worth noting", "importantly", "notably"
• The interjection reflex : every sentence broken in half, whatever the punctuation. e.g. em-dash, double hyphen, comma pair, colon
• Comma-clipped tail : a short phrase hung off a comma instead of landing the point. e.g. "above the content, and save."

Part three: Tone

• Manufactured suspense : the buildup arrives, the revelation does not. e.g. "here's the thing", "here's the kicker"
• Patronising analogy : teacher mode, uninvited. e.g. "think of it as a Swiss Army knife for condescension"
• Forced figurative language : a metaphor picked for cleverness rather than clarity. e.g. "Picture a bicycle. Now picture that the bicycle is prose."
• Stakes inflation : everything is historic. e.g. "a pivotal moment in the evolution of reading itself"
• False vulnerability : honesty performed, never risked. e.g. "let me be honest with you here. Genuinely honest."
• Assertion in place of evidence : e.g. "history is unambiguous on this point", "the evidence is clear"
• "Imagine a world where..." : the futurism sales pitch, pleasant outcomes if you accept the premise. e.g. "imagine a world where every paragraph carries a quiet intelligence"
• Quotable one-liners : slide bait that survives being pulled out of context because it never had any. e.g. "Every bite is a chance to be here, now, fully."
• The collaborative "we" : one author, plural pronoun. e.g. "we will unpack each one in turn"
• Sycophancy : not a figure of speech but a training artefact, and the fastest way to spot raw chatbot output. e.g. "What a genuinely profound question."

Part four: Formatting

• Title case headings : see the top of this page
• Bullet lists where prose would do : the document turns into a slide deck. e.g. this page, from here to the bottom
• Bold-first bullets : every item opens with a bolded keyword and a colon. e.g. yes, also this page, I know
• Emoji as punctuation : rocket for aspiration, key for insight, sparkles because the paragraph ended
• Em-dashes : mid-sentence, twice, sometimes three times, where a comma would have done the job
• The find and replace tell : double hyphens everywhere, because someone removed the character and kept the habit
• Unicode decoration : arrows and curly quotes nobody typed on a keyboard. e.g. Input → Processing → Output
• One thought per line : paragraphs abolished. e.g. "It happened. Alone. On a Tuesday."

Part five: Composition

• Announce then answer : says what the text is about to do instead of doing it. e.g. "This guide will explore what those signs look like."
• Rapid-fire historical analogies : authority borrowed from a list, with no argument underneath. e.g. "Marconi. Sony. Apple. Print, radio, web, mobile."
• The metaphor that will not end : e.g. the garden that stays a garden for six sentences, then becomes a north star that moves the needle
• Self-echo : a word from earlier returning as if it were being paid off, when it is the same narrow vocabulary surfacing again
• One point diluted : the same idea in five coats, sounding like progress. e.g. "put differently", "to rephrase", "in other words"
• Fractal summaries : every section summarises itself, including the one you are inside. e.g. "In this section we explored composition."
• The tie-back : closes by looping the answer back to the original question, long after the point was made. e.g. "So, to answer your question..."
• Concede and dismiss : e.g. "despite its fluency, it faces challenges typical of automated systems. Despite those challenges, it endures."
• Signposted conclusion : competent writing does not need to announce that it is ending. e.g. "in conclusion", "to sum up", "in summary"
• The conclusion that refuses to end : clause stacked on clause, quieter and more meaningful each time. e.g. "the real AI generated text was the paragraphs we scrolled past along the way"
• Engagement bait : e.g. "Agree?" plus a pointing emoji

Tells on the list that did not fit in the episode

The recording would have run to forty minutes. These are worth knowing anyway.

• Reasoning leak : the text narrating its own decisions instead of just making them. e.g. "I want to be exact about my own role here."
• Premise stacking : a paragraph of evidence in front of the point, so the point has been made twice before it arrives
• Compulsive counting : states the number of items before listing them. e.g. "Five things we wish to discuss"
• Excessive enumeration : a listicle disguised as prose. e.g. "The first wall is... The second wall is..."
• Belaboring the unnecessary : defending a minor point against an objection nobody was going to raise
• Synonym cycling : refusing to repeat a noun. e.g. the dashboard, then the interface, then the portal, then the analytics hub
• Wh-headings : the default shape a model reaches for when naming a section. e.g. "What this means", "Why it matters"
• Content duplication : the same paragraph appearing twice, because the model lost track of what it had written

Why this is not a detector guide

Liang, Yuksekgonul, Mao, Wu and Zou, Stanford, published in Patterns (Cell Press) on 10 July 2023. Seven commercial GPT detectors were run over 91 TOEFL essays written by non-native English speakers and 88 US eighth grade essays.

• Average false positive rate on the TOEFL essays: 61.3 percent.
• 89 of the 91 essays were flagged as AI generated by at least one detector. All seven agreed on 18 of them.
• The US eighth grade essays came back near perfect. Plainer word choice, lower perplexity, reads as machine to the software.
• Feeding the TOEFL essays through ChatGPT with the prompt "enhance the word choices to sound more like that of a native speaker" dropped the false positive rate to 11.6 percent.

So the way to stop being accused of writing like a machine was to have a machine rewrite you. Do not take my word for it, the paper is open access:

• https://www.cell.com/patterns/fulltext/S2666-3899(23)00130-7
• Preprint: https://arxiv.org/abs/2304.02819

The pointers, for real

• Write with things in it: names, dates, prices, the model number of the drive that died.
• Say something a machine has no way to know, because it happened to you.
• Be willing to be wrong out loud.
• Keep one weird sentence that your editor would cut.

Links and references

• The list this episode leaned on: tropes.fyi , by ossama.is
• The detector study: Liang et al., "GPT detectors are biased against non-native English writers", Patterns, 2023
• "Not an Addict", K's Choice, 1995. It's not a habit, it's cool, I feel alive.
• "Friends, Romans, countrymen" and "I come to bury Caesar, not to praise him": Shakespeare, Julius Caesar, act 3 scene 2
• "Veni, vidi, vici", attributed to Julius Caesar by Suetonius and Plutarch

Over to you

There is a comment section under this episode. Tell me which tell I missed, and which one you are guilty of. Better yet, record it. Read your worst, most machine sounding paragraph out loud and post it to Hacker Public Radio. The queue is always hungry.

Provide feedback on this episode.

Lieven Blancke:

https://mastodon-belgium.be/@lieven

asterisk:

https://www.asterisk.org/

Asterisk audio files:

https://downloads.asterisk.org/pub/telephony/sounds/

Audio of PSTN call:

https://en.wikipedia.org/wiki/File:1TR110-1_Kap8.3_Freiton1.ogg

Provide feedback on this episode.

As we are currently going through a warm patch (well, at the time of recording anyway), the HPR Beer Garden returns to provide some refreshment in the heat with a glass of cold and refreshing lager. Dave chooses Super Bock from Portugal, whilst Kevie picks City Limits Amber Lager from the tiny Dog Falls Brewing Co in Inverness.

Connect with the guys on Untappd :

• Dave
• Kevie

The intro sounds for the show are used from:

• https://freesound.org/people/mixtus/sounds/329806/
• https://freesound.org/people/j1987/sounds/123003/
• https://freesound.org/people/greatsoundstube/sounds/628437/

Upcoming beers:

• Mild
• Bitter
• Pale Ale

Provide feedback on this episode.

New hosts

Welcome to our new hosts:

• Kristoff
• number9

Last Month's Shows

Comments this month

Past shows

• hpr3708 (2022-10-19) "Insomnia as a Hobby" by TrumpetJohn.
  
• Robert St.Preux said: "Insomnia as a hobby" (2026-08-30 00:56:59)
  
• hpr3766 (2023-01-09) "ACER Nitro 5 laptop review" by Bookewyrmm.
  
• wevaba6950 said: "A refreshing contrast between enterprise gear and low-power hardware" (2026-08-10 09:19:47)
  
• candycanearter07 said: "re: a refreshing contrast..." (2026-08-12 02:54:05)
  
• hpr4581 (2026-02-23) "Sharp Intake of Breath City (A.K.A.) How I learnt to stop worrying about the fork bomb" by not_toby.
  
• Windigo said: "Fantastic episode" (2026-08-08 16:51:00)
  
• hpr4655 (2026-06-05) "Tips for Glasses" by norrist.
  
• Archer72 said: "Glasses and age" (2026-08-06 11:41:39)
  
• hpr4676 (2026-07-06) "HPR Community News for June 2026" by HPR Volunteers.
  
• Archer72 said: "hpr4657 :: UNIX Curio #8" (2026-08-02 19:29:26)
  
• hpr4677 (2026-07-07) "UNIX Curio #10 - Checksums and Hashes" by Vance.
  
• Whiskeyjack said: "Checksums and HPR4677" (2026-07-31 00:37:40)
  
• hpr4678 (2026-07-08) "High Resolution Elapsed Time in Shell Scripts" by Whiskeyjack.
  
• Archer72 said: "Reply to Whiskeyjack and candycanearter07 " (2026-08-03 10:12:47)
  
• hpr4692 (2026-07-28) "Noise Music Tutorial 2: Using Audacity to Make Noise" by TheDUDE.
  
• Windigo said: "Audacity for noise music" (2026-08-06 16:09:59)
  
• candycanearter07 said: "re: Audacity for noise music" (2026-08-12 18:55:18)
  

This month's shows

• hpr4696 (2026-08-03) "HPR Community News for July 2026" by HPR Volunteers.
  
• Archer72 said: "Amateur Radio Field Days" (2026-08-02 11:50:11)
  
• Archer72 said: "hpr4673 First contact" (2026-08-03 10:16:09)
  
• candycanearter07 said: "re: mic issues" (2026-08-03 22:21:42)
  
• Th3Dud3 said: "response to Kevin not being able to get cream ale" (2026-08-06 00:12:38)
  
• hpr4697 (2026-08-04) "Correcting the Dates of Files" by Vance.
  
• Jim DeVore said: "Useful episode!" (2026-08-04 10:53:12)
  
• Vance said: "Modifying metadata" (2026-08-07 01:44:51)
  
• Henrik Hemrin said: "Exif in digiKam" (2026-08-08 19:20:13)
  
• candycanearter07 said: "useful!" (2026-08-10 16:08:18)
  
• hpr4698 (2026-08-05) "ID3 Tags and Vorbis Comments" by Whiskeyjack.
  
• xmanmonk said: "Great episode!" (2026-08-05 15:19:01)
  
• candycanearter07 said: "quirks in hpr" (2026-08-05 15:52:18)
  
• Whiskeyjack said: "Reply to xmanmonk in HPR4698 - ID3 Tags" (2026-08-06 00:12:26)
  
• Whiskeyjack said: "Reply to candycanearter07 in HPR4698 ID3 tags" (2026-08-06 00:28:48)
  
• hpr4703 (2026-08-12) "Ebooks and Audiobooks" by Ahuka.
  
• candycanearter07 said: "useful!!" (2026-08-12 02:30:58)
  
• Archer72 said: "Audiobooks and Audible" (2026-08-12 09:06:40)
  
• Jack said: "Misleading" (2026-08-12 17:05:21)
  
• hpr4705 (2026-08-14) "Free Software Is Wasted On You Teens" by Trollercoaster.
  
• Archer72 said: "Brilliant! " (2026-08-03 14:08:08)
  
• Trollercoaster said: "One left in my six pack" (2026-08-14 07:53:38)
  
• old guy said: ":-)" (2026-08-14 09:24:15)
  
• Trollercoaster said: "old guys together" (2026-08-14 13:14:48)
  
• candycanearter07 said: "good satire again" (2026-08-14 21:42:55)
  
• Trollercoaster said: "Who's talking about satire?" (2026-08-15 07:54:33)
  
• Trollercoaster (our of character) said: "Woopsie" (2026-08-16 19:25:41)
  
• hpr4706 (2026-08-17) "Quirks and Customisations" by Lee.
  
• candycanearter07 said: "cool ep!" (2026-08-30 20:15:40)
  
• hpr4707 (2026-08-18) "UNIX Curio #12 - expr" by Vance.
  
• xmanmonk said: "Another great show" (2026-08-18 22:22:56)
  
• Whiskeyjack said: "HPR4707 Unix curio - dc" (2026-08-19 14:31:02)
  
• Vance said: "dc" (2026-08-20 04:47:32)
  
• Whiskeyjack said: "HPR4707 Unix curio - Stack Machines" (2026-08-21 16:00:48)
  
• Trollercoaster said: "Suggestion for Linux Curio" (2026-08-27 10:42:43)
  
• Whiskeyjack said: "Obscure X Window programs" (2026-08-27 16:16:15)
  
• candycanearter07 said: "re: Suggestion for Linux Curio" (2026-08-27 18:13:51)
  
• Archer72 said: "re: Suggestion for Linux Curio" (2026-08-28 10:07:31)
  
• Vance said: "Thanks for the ideas" (2026-08-28 16:39:28)
  
• Whiskeyjack said: "Reply to Vance on episode research" (2026-08-30 04:14:01)
  
• candycanearter07 said: "re: re: Suggestion for Linux Curio" (2026-08-30 12:06:31)
  
• hpr4708 (2026-08-19) "Programmable Logic Controls - Episode 1" by Whiskeyjack.
  
• Archer72 said: "Thanks you" (2026-08-08 10:49:34)
  
• Whiskeyjack said: "Reply to Archer72 on HPR4708 - PLCs Episode 1" (2026-08-08 17:11:10)
  
• Trey said: "Memories of Ladder Logic" (2026-08-19 13:01:41)
  
• Whiskeyjack said: "Reply to Trey on HPR4708 - PLCs Episode 1" (2026-08-20 02:02:44)
  
• Trey said: "Cyber in ICS" (2026-09-02 22:31:51)
  
• Whiskeyjack said: "Reply to Trey on Cyber in ICS" (2026-09-03 15:24:51)
  
• hpr4709 (2026-08-20) "Peertube: my two penneth" by Kevie.
  
• xmanmonk said: "Thanks for the episode" (2026-08-20 19:15:57)
  
• hpr4711 (2026-08-24) "Second-Hand Phone" by TrumpetJohn.
  
• candycanearter07 said: "interesting!" (2026-08-24 19:17:44)
  
• Whiskeyjack said: "HPR4711 - TCL Flip phone" (2026-08-24 22:22:11)
  
• hpr4712 (2026-08-25) "friedcamp interviews part 1: Wim van Gool" by Kristoff.
  
• Archer72 said: "Interview" (2026-08-26 23:11:49)
  
• Laindir said: "Fascinating interview!" (2026-08-26 23:18:33)
  
• hpr4716 (2026-08-31) "PocketMod" by TrumpetJohn.
  
• Kightlinger said: "I love PocketMods!" (2026-08-31 17:35:43)
  
• candycanearter07 said: "analog solutions" (2026-08-31 20:52:55)
  

Mailing List discussions

Policy decisions surrounding HPR are taken by the community as a whole. This discussion takes place on the Mailing List which is open to all HPR listeners and contributors. The discussions are open and available on the HPR server under Mailman.

The threaded discussions this month can be found here:

Events Calendar

With the kind permission of LWN.net we are linking to The LWN.net Community Calendar.

Quoting the site:

This is the LWN.net community event calendar, where we track events of interest to people using and developing Linux and free software. Clicking on individual events will take you to the appropriate web page.

Provide feedback on this episode.

Heinlein published his first novel in 1948, and within a decade had published some major works. What distinguishes these early novels from his later ones is that he has not pushed the boundary of what he can write about to the degree he does in the 1960s and later. But there are fine novels in this period.

Heinlein’s Earlier Novels

During the 1950s Heinlein was busy with his Juvenile series, but also found time to write some adult novels, and some of them are very good. A few of them were expansions of earlier, shorter works that he turned into full novels.

• Beyond This Horizon (1948) – This is Heinlein’s first novel after the lackluster For Us The Living (which never was published during his lifetime; Heinlein knew it was sub-par). It is set in a future society that is practicing a form of eugenics, with a breeding program designed to produce the perfect human by selecting for health, longevity, and intelligence. The protagonist, Hamilton Felix, is not the one, but he is near to the end. And this society has also made it a point to keep a certain number of people who are not part of the breeding program, as a form control for the experiment. Hamilton Felix is in fact the penultimate step in this program, but has so far shown no interest in marriage or procreation. But in the course of the novel he is persuaded. His children are shown to be the end point of the program at the end of the novel. A sub-plot involves his friend Mordan Claude (in this book everyone puts their family name first) who falls for a woman he meets. When she sneezes, he begins to suspect she is one of the “control naturals”. They are protected, but clearly this is a class society, which is to be expected in any society that practices eugenics, and should be off-limits for Claude. But love conquers all, and then he discovers that she is not a control natural after all, which to my mind is a somewhat weaker outcome.
  
  Another aspect of this society is that most people are armed, and dueling is common. This is explained as “An armed society is a polite society,” since if you offended someone you could find yourself in a duel very quickly. A lot of gun enthusiasts have picked up on this and repeated it as if it was revealed truth, but even a cursory examination of societies with a lot of guns shows it to be false.
• Sixth Column (1949) – This novel was originally a serial written in 1941, when the Sino-Japanese war was in full swing, and this motivates some of the ideas in this book. The setting is a United States that has been conquered by the Pan-Asian Federation. This Federation had previously absorbed the Soviet Union and India, and is portrayed in very much Yellow Peril manner. John W. Campbell had a lot to do with this, and it makes the novel something that you can skip without losing much. The Pan-Asians regard the Americans as slaves. And their policy towards their slaves is “Three things only do slaves require: work, food, and their religion.” In an American military outpost hidden in the mountains of Colorado a miracle weapon is found that can kill, and it can somehow be tuned to only affect the Asians and not the Americans. Since the Pan-Asians have allowed religion (better to pacify the slaves), the Americans create a religion and use that to infiltrate the Pan-Asian facilities. Heinlein himself had a low opinion of this novel, and deservedly so.
• The Puppet Masters (1951) – This novel is a product of the paranoia of the times regarding Communists. But this novel is more of an allegory than a direct anti-Communist screed. It is fundamentally about “the enemy within”, in the form of alien parasites called slugs that can take over a human by controlling the nervous system, making the human a powerless slave to the alien. And they are attempting to conquer the world, though all of the action we see takes place in the United States. This is essentially The Body Snatchers plot, and has been done many times. But both this novel and the previous Sixth Column reflect the feeling of paranoia prevalent in society when people thought there were communists under every bed looking to corrupt the youth and subvert The American Way of Life. And it is notable that the organization that leads the fight against the slugs is a top secret outfit established without the knowledge of Congress and responsible only to the President.
• Double Star (1956) – This is when Heinlein as a novelist starts to really click, and it was shown by his winning his first Hugo for best novel. The plot is somewhat reminiscent of Prisoner of Zenda. In a future solar system, Earth, Venus, Mars, and the Jovian satellites are united under a parliamentary democracy, which is in turn under a constitutional monarchy. Think of it as similar to the UK, only in this case it is the House of Orange that are the monarchs. A politician named John Joseph Bonforte is the leader of the Expansionist coalition, and one of the most prominent politicians in the solar system. And he has been kidnapped sat a crucial time. His staff find an actor, stage name The Great Lorenzo, who is similar enough physically to impersonate Bonforte, while they try to get the real Bonforte back. They do succeed, but he has been very badly treated by the kidnappers, so the actor has to continue for the election campaign. And he wins the election, but the real Bonforte dies. The actor then continues to impersonate him to the point of really becoming him.
  
  The two things that make this novel so interesting are first, the focus on acting as seen through the eyes of Lorenzo; and second, the focus on the machinery of politics. And this draws upon Heinlein’s own experience in electoral politics. Though he was not successful in getting elected, he clearly had absorbed a lot about the how-to of the process. And he did write a non-fiction book called Take Back Your Government (1992), which was published posthumously. It was subtitled A Practical Handbook for the Private Citizen Who Wants Democracy to Work, and was originally written in 1946. It was based on his experience with EPIC and in trying to get Upton Sinclair elected Governor of California in 1934.
• The Door Into Summer (1957) – This is another good novel, which features several themes that would reappear through Heinlein’s work. The first is that Heinlein was a very confirmed cat lover. The title comes from a cat who went from door to door in the middle of winter, trying to find the door into summer. Second, the protagonist is an engineer, which is of course Heinlein’s background. The third is the time travel element. People can travel into the future through a type of suspended animation called Cold Sleep. But for the purposes of the plot a scientist has invented a time travel device that sort of works. It sent a grad student named Leonard Vincent a few centuries back in to Italy where he became Leonardo da Vinci. But it sends our hero, Daniel Boone Davis, back exactly where he wanted to go. We see a few things in this novel that are of interest. First, the cat, Pete, has the full name of “Petronius the Arbiter”, who was the author of the Satyricon, so Heinlein’s interest in sex is starting to make an oblique appearance. Second, when Dan goes back in time, he lands in the middle of a nudist camp, and that is also something Heinlein was very interested in.
  
  The plot can be summarized as Dan gets robbed of his inventions by his business partner and his secretary/fianceé, gets put into Cold Sleep, then goes back in time to just before he got robbed and fixes things to his satisfaction, then takes the Cold Sleep again to finish his plans. It is a pretty good yarn.
• Methusaleh’s Children (1958) – This was originally a novella written in 1941, then expanded into a novel. It is part of the Future history. It introduces Lazarus Long, who would go on to be Heinlein’s alter ego through many of his future books, and thus implicitly makes those books part of the Future History, though most scholars will dispute this. What cannot be disputed is that this novel is a cornerstone for where Heinlein would go in many of his future stories.
  
  The story begins with a successful businessman named Ira Howard who dies of old age at around the age of 49 in the 19th century. In his will he leaves his estate to trustees with the mandate to find a way to extend human life span. The best they can come up with is to encourage (financially) people who had long-lived grandparents to marry each other and have children. They carry this on and by the 22nd century the Howard Families, as they call themselves, have an average life span of 150 years. But they have to hide this form the rest of society, and arrange from time to time to “die” and then become someone else. This masquerade helped them survive the years of Nehemiah Scudder and his successors, but a few of them reveal themselves during the Covenant government. This has a bad result because people want the secret that they assume the Howard families are hiding, and they might be willing to kill to get the secret. So the Howard families steal a starship and make their escape. One of the people onboard is Andrew Jackson “Slipstick” Libby, who we first met in the short story Misfit, and he manages to come up with a Faster-Than-Light drive. They finally decide to go back to Earth, where they discover that scientists have found ways to prolong human life and they are no longer enemies, in fact they are welcomed for bringing the FTL drive.
  
  This is definitely a must-read novel for anyone who wants to get into the Heinlein universe. So many other works are based on it.

Links

• https://en.wikipedia.org/wiki/Beyond_This_Horizon
• https://en.wikipedia.org/wiki/Sixth_Column
• https://en.wikipedia.org/wiki/The_Puppet_Masters
• https://en.wikipedia.org/wiki/Double_Star
• https://en.wikipedia.org/wiki/Take_Back_Your_Government
• https://en.wikipedia.org/wiki/The_Door_into_Summer
• https://en.wikipedia.org/wiki/Methuselah%27s_Children
• https://www.palain.com/science-fiction/the-golden-age/robert-a-heinlein/heinleins-earlier-novels/

Provide feedback on this episode.

Outline of this radio program:

• Short history of multimeters
• Differences between VTVM, VOM, DMM
• Why you may need a VTVM
• Analog vs Digital
• Current manufacturers
• Why you may want to use an analog meter
• Safety and CAT
• Probes
• Functions of the modern DMM and notes
• Wrap up

Provide feedback on this episode.

--------------------

01 Introduction

This is the second episode in an 8 part series.

02

In the previous episode we discussed the predecessors of PLCs, in particular relay logic.

In this episode we will discuss how relay logic came to be expressed in software rather than in actual hardware components.

03

The topics to be covered include

* Early computers in industry.

* The first PLC.

* PC versus PLC - what's in a name.

* Who the major brands are.

* What does a PLC actually look like.

* Machine architecture.

* PLC programs.

* The scan.

* PLC programming languages.

* Relative popularity of PLC programming languages versus more conventional computer programming languages.

--------------------

04 Early Computers in Industry

Computers came to be used in industry fairly early on.

Minicomputers were used in some large industries to control things like electric power plants.

For example, the DEC PDP8 was used to control the refuelling machines in CANDU nuclear power plants.

These would load and unload fuel from the reactor, which happens continually on a daily basis while the reactor is running.

05

The first microprocessor based computer sold on the commercial market was the MICRAL from France, based on the Intel 8008.

It was sold as a cost effective replacement for minicomputers used in industry.

It preceded what are generally considered to be the first Personal Computers.

So you can see that industry were not reluctant to adopt new technology.

06

However, mini computers were large complex systems that did not fit well into a factory floor.

They had particular niches in very large complex integrated systems, but were not suited to controlling many individual machines in a factory that produced things like automobiles or appliances.

07

What was needed was something that would fit into a standard electrical enclosure,

withstand the temperatures found in a factory,

could stand up to vibration and noise,

was tolerant of voltage fluctuations,

interfaced directly to sensors and actuators,

and could be readily programmed by engineers, technicians, and tradesmen who were familiar with the processes to be controlled, but had little or no experience with computers.

08

This required a complete integrated package covering hardware, software, and product distribution through industrial supply retailers.

Something that could meet these criteria is what was needed to become the PLC.

--------------------

09 History

Many histories point to a US developer that became Modicon.

However, their first hardware was more a proof of concept than a viable commercial product.

In fact multiple companies in the US, Europe, and Japan were working on the problem and released comparable products all at around the same time in the early 1970s.

10

The idea preceded the implementation by a number of years.

For example, General Motors had asked industrial control equipment suppliers in the mid 1960s for some sort of programmable control device to replace relay logic.

This showed potential suppliers that there was a market for this sort of thing, and gave them a clearer idea of what customers were looking for.

11

What made this possible was the development of the first 8 bit microprocessor in that time period, combined with readily available bit slice processors from the minicomputer industry and other components.

12

People knew what to do, the problem was waiting for the development of suitable component hardware.

Minicomputers were already being used in industry, but were normally located in control rooms.

PLCs were an effort to take that technology out of control rooms and put it on the shop floor.

13 PC Versus PLC - What's in a Name

In the early days, these systems were known as either PLC, which means "Programmable Logic Controller", or PC, which means "Programmable Controller".

Different vendors favoured different terminology, but both were common.

I have a book which was at one time one of the standard reference handbooks for this industry which was published in 1989, and refers to them as Programmable Controllers, or PCs.

14

The term PC was also used to mean "Personal Computer", but that wasn't really a problem in the early days.

However, a certain large company decided to call their entry into the personal computer market the "IBM PC", and suddenly "PC" became a generic term for desktop computers.

15

After a long struggle to keep referring to their products as "PCs", even the biggest vendors caved in and gave up the fight and switched to using the "PLC" term.

I will therefore use the term "PLC" in this podcast series even when talking about products which were originally called "PCs" at the time they were introduced.

16 Who the Major Brands Are

The companies that came to dominate the industry fairly early on were generally companies that already made industrial electrical hardware.

These were

Siemens

Allen Bradley, later known as Rockwell

Schneider

Mitsubishi

Omron

17

These are still the dominant companies in the business, although there are many small brands, particularly at the cheaper end of the market.

All of them were suppliers of a wide range of industrial control hardware, such that you could build all or nearly all of the parts of your control system using only their products.

18

Each of these sells an integrated hardware and software package, including development software, that is completely proprietary.

If you thought that the mainframe business had a lot of vendor lock-in, you haven't seen the industrial market.

--------------------

19 What Does One of These Things Actually Look Like?

At this point you are probably still very confused as to what a PLC actually is.

I will attempt to describe one in basic terms by giving an example of one.

The smallest and simplest ones are what are termed "shoebox" PLCs.

These are basically a rectangular box with a plastic case.

20

Here are the dimensions for a typical low end model, a Mitsubishi FX3U-32M.

According to specs in the manual, it is 150mm wide, 90mm high, and 86mm deep.

21

It can be mounted to the panel in an electrical enclosure by snapping it onto what is termed a DIN rail.

DIN rails are standard mounting systems which use a strip of metal which is U shaped with a lip at the top of each end of the U.

You screw the DIN rail to the electrical panel in the enclosure, and most industrial components such as PLCs, terminal blocks, circuit breakers, and all sorts of other things will simply snap onto it and be ready to wire together.

22

Along the top and bottom of the PLC are terminals to which you can attach wires from the things you want to sense or control in the rest of the machine, such as push buttons, pilot lights, proximity sensors, and valves.

23

Inputs are along the top, and outputs are along the bottom.

The FX3U-32M has 16 inputs and 16 outputs.

These I/O can be 24 volts DC, or 100 to 120 volts AC, depending on the model.

Alternatively it may have small relays for outputs.

While AC I/O once predominated, 24VDC became the standard in most industries several decades ago as it interfaced with electronic devices more readily and also allowed for cheaper and more compact wiring.

24

More inputs and outputs can be added by connecting additional I/O modules to the main PLC next to it on the same DIN rail, up to a total of 256 I/O

The connection is typically via short ribbon cables which plug into the adjacent module.

These I/O modules look like the main PLC, but just add I/O.

25

Inside the PLC are a CPU, memory, and firmware.

This model has 64K "steps" of memory, which can be thought of as how many instructions you can have.

The program runs in RAM, but you can add a small flash memory device to save the program to.

There is a run/stop switch that allows you to start or stop the user program.

There is a port that you can use to connect the PLC to a laptop computer with a cable so you can download your program to it.

26

The biggest part of the market for PLCs is for these "shoebox" style.

However, there are bigger ones as well which have more I/O, more memory, faster CPUs, etc.

These are meant for tasks such as coordinating large assembly lines and things like that.

These are what are termed rack systems, where the rack is an empty box which is open at the front and has a backplane bus running along the back.

You plug the main CPU into the bus, normally in the leftmost position, and then plug I/O modules into the rack.

The I/O modules are tall, thin, fully enclosed boxes with the I/O terminals along the front.

27

You should have a rough idea of their appearance at this stage, so I'll leave the physical description aside for now and go on to the concepts behind how they actually work and what makes them different from something like say a Raspberry Pi.

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28 Machine Architecture

It is not really the hardware which defines the PLC.

Rather, it is the software system architecture.

Every PLC that I am aware of has a common set of features.

29 Data Table

A data table is simply a block of memory which may or may not be subdivided into different parts.

All logic, and all, or nearly all, I/O works by writing to and reading from addresses in the data table.

30 I/O

Every input and output point maps to a fixed memory address in the PLC.

To read the state of an input, you read its memory address.

To change the state of an output, you write to its memory address.

31

These digital I/O appear as single bit boolean values.

The PLC programming language has instructions which allow single bits to be read or written to directly without have to mask off other bits as you would have to do if you were using a conventional programming language.

I have so far only mentioned digital I/O, which are single bit on/off values.

32 Other types of I/O

Many PLCs also have other types of I/O such as

Analogue I/O, which represent variable voltages rather than just on/off values.

For example you may wish to read a temperature from a thermocouple where the voltage level varies with the temperature.

Often these are mapped into the data table as byte or word values.

33 Internal Bit or Boolean Memory

There will be a range of single bit or boolean memory which is used for internal logic state.

Your program may need to come up with a series of intermediate logical values, and you store them in these internal bits or flags.

34 Timers and Counters

Control of machinery often involves timing and counting.

Timer and counters are mapped into the data table.

Timer and counter presets and values can be read as words, and there will be bit addresses which indicate the status of the timer or counter, such as whether it has reached its preset and is "done".

35 Integer and Floating Point Values

There will be word addresses where integer and floating point words can be stored.

If you need to do some mathematical calculations and store the results, you would save them in an integer or floating point word.

36 Typed Memory

Unlike when programming something like a PC where you simply have a range of undifferentiated bytes and it is up to your software to impose meaning on it, PLC data table memory has defined types and meanings and the system firmware enforces the correct memory access methods.

37 Size of Data Table

Data tables vary greatly in size.

A bigger data table allows for bigger and more complex program.

Generally, new PLCs will have bigger data tables than older ones,

and more expensive PLCs will have bigger data tables than cheaper ones of the same generation.

38 Example

Having picked a current PLC as an example for physical dimensions, I will pick an old and obsolete one for an example of a data table.

The Siemens S5-100 series was a small PLC that came in several sizes.

The basic S5-100 had the following data table size.

39

Digital I/O had a maximum of 128 inputs and outputs taken together.

Analogue inputs and outputs had a maximum of 8 taken together.

Flag, or bit, memory was 1024

Timers - 16

Counters - 16

40

The S5-103 offered more of everything in the same basic package, but of course at a higher cost.

Digital I/O had a maximum of 256 inputs and outputs taken together.

Analogue inputs and outputs had a maximum of 32 taken together.

Flag, or bit, memory was 2048

Timers - 128

Counters - 128

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41 The PLC Program

A PLC will come with what amounts to an operating system and virtual machine built into it.

You as the programmer use the vendor's proprietary development software, what is generally called the "PLC programming software" to write an application for it.

42

You then connect your PC to the PLC using a cable of some sort, and download your program into it.

The program gets stored in the PLC's RAM.

There may be flash memory which serves to back up the RAM when the power is off so you don't lose the program.

Before flash was available, a battery was typically required to hold the static RAM memory.

43

There is just one program in the PLC.

There is no file system, just the program memory and the data table.

When the PLC starts up, it runs the program that you wrote.

It continues running the program until you turn off the power or you set the run/stop switch to the stop position.

44

I will come back to programming later, but we needed to cover these basic points before I could describe some further concepts we need to cover.

--------------------

45 The Scan

A key thing to understand about PLCs is the "scan" concept.

This works as follows.

There is a repeated cycle called the scan.

46

First, the PLC reads all the physical inputs and updates the corresponding input addresses in the data table.

Next it runs the user program once.

Finally it reads the output addresses in the data table and writes them to the corresponding physical outputs.

Then it repeats the scan from step 1.

47

A scan can take anywhere from a few milliseconds to hundreds of milliseconds, depending on the model of PLC.

Older PLCs and cheaper PLCs tended to be slower.

However, as integrated circuit technology advanced, programs got faster.

48

The PLC has a watchdog timer.

This is a timer which runs in the background and resets at the start of a scan.

49

If a scan takes too long, the watchdog will trip and stop the program and set the outputs to some defined state, either turning them all off or holding them at the last value.

This is known as "faulting" the processor.

50

A watchdog fault may be caused by a program that is too long, or uses a lot of very slow instructions, or in some cases it may be due to a bug in your program.

However, this last cause is not as common or as easy to cause as you may think when it comes to faults.

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51 PLC Programming Languages

The key to all of this is the unique way in which the programming languages used in PLCs work.

I will first briefly describe the two main programming languages and then explain how they work in a PLC as part of a complete system.

52 Ladder Logic

There are several programming languages, but the main one is called ladder logic.

If you listened to the previous episode, the word "ladder" will ring a bell.

When machines were controlled by electromechanical relays, the electrical drawings which specified and documented the types of relays and the connections between them were called "ladder diagrams".

53

PLCs simply take these ladder diagrams and reproduce them on your computer screen in the programming software using the same standard schematic symbols.

54

You do not need to draw the symbols like it was CAD.

Instead you simply use your keyboard or mouse to say that you want this symbol to be entered in the current cursor location or you want this wire to be connected from here to there.

When your rung is complete the software will check at some point if it is syntactically correct.

You then go on to entering the next rungs one after another until you have your complete program.

55 Instruction List

An alternative representation is something called "instruction list", although various vendors may use other terms.

This is a text representation that looks like a sort of assembly language.

However, it's actually really ladder, just shown in a different way as text and some types of programming software will allow you to toggle between ladder and instruction list mode, translating between them automatically.

56

This should give you a clue as to how a PLC can execute a schematic diagram.

Behind the scenes the programming software will translate the diagram into instruction list, and then the PLC will either execute those instructions in an interpreter, or compile them to machine code and execute those.

57 Other Programming Languages

There are other programming languages, but they are rarely seen.

I won't go into detail in terms of describing them, just briefly mention them.

58

One is called Sequential Function Chart.

This is a type of flow chart which is designed to show complex sequences, including ones with alternate or parallel paths.

The original name for this is Grafcet, spelled G R A F C E T.

This stands for "Graphe Fonctionnel de Commande Étape Transition".

As well as being a programming language, it is also a very good design and analysis tool even if the PLC being used doesn't support it.

59

Another is Function Block Diagram.

This is another graphical language which resembles flow diagrams used in process industries.

This is supposedly used mainly in industries such as chemical manufacturing.

However it appears to be very niche and most people who use PLCs will never have seen it, and many will not have even heard of it.

60

Yet another language is Structured Text.

This is very obviously derived from Modula-2 and strongly resembles it.

If you are not familiar with Modula-2, it was created by Nikolas Wirth and intended as the successor to Pascal, which it was derived from.

Structured Text seems to be much loved by a small number of academics, but seems to have little actual use in the field.

You lose pretty much all of the monitoring and debugging facilities built into a PLC if you use it, so it's kind of pointless except perhaps for a few niche applications.

There are a few other languages as well, mainly proprietary ones, often using flow charts or the like.

61 PLC Languages are Non-Blocking

The important thing about a PLC program is that it executes from top to bottom, left to right, without stopping or blocking.

There is no waiting on input or output or for a system call to return.

Each rung immediately yields a result which is written to a memory address or which enables a timer or counter.

Every rung is executed every scan.

62

You can reproduce this in a traditional computer programming language.

In ladder logic it is inherent to the syntax and it is either very difficult or impossible to do it any other way.

I'll give an example in Python of what I mean.

63

a = (b or c) and not d

64

If b or c are true and d is false, then a is set to true.

If b and c are false or d is true, then a is false.

65

Now imagine that we are not executing this statement once, but rather are executing it repeatedly.

This statement will never block execution.

It will always immediately yield a result.

Now let's modify that a bit.

66

a = (b or a) and not d

67

Note that we have replaced c with a.

Now the value of a depends the previous value of a as well as b and d.

However, once a becomes true, the value of b no longer matters because b is in an or condition with a.

Now if we execute it over and over again, only d becoming true will make a go false.

68

This is a standard push button circuit, also known as a "seal in circuit", because it "seals" around the start condition.

69

b is the start push button with a normally open contact.

d is the stop push button with a normally closed contact.

a is a relay with one of its contacts being used to hold it on.

70

If you wired this up with actual push buttons and relays or if you programmed it into a PLC with ladder logic it would work the same way.

A PLC program will consists of rung after rung of logic like this and executes it repeatedly scan after scan, only pausing to update its I/O.

71 Async Programming

Some of you may be thinking that this cyclical scan sounds like the async programming that is all the rage with web server applications these days.

Essentially it is exactly the same principle.

72

With async programming, your program must never use blocking instructions and execution proceeds on a repeated cycle.

PLCs solve this by not having any instructions which block, and many PLCs do not allow backwards jumps.

Tradesmen in coveralls in factories were doing async programming decades before the cool kids heard about it.

73 Most PLC Programming Languages are Visual or Graphical Languages

If ladder, Sequential Function Chart, or Grafcet and Function Block Diagram sound like visual programming languages, they are.

In fact ladder is probably one of the earliest visual languages in commercial use.

Wikipedia defines a visual programming language as:

74

In computing, a visual programming language (visual programming system, VPL, or, VPS), also known as diagrammatic programming, graphical programming or block coding, is a programming language that lets users create programs by manipulating program elements graphically rather than by specifying them textually. A VPL allows programming with visual expressions, spatial arrangements of text and graphic symbols, used either as elements of syntax or secondary notation. For example, many VPLs are based on the idea of "boxes and arrows", where boxes or other screen objects are treated as entities, connected by arrows, lines or arcs which represent relations. VPLs are generally the basis of low-code development platforms. Scratch is an example of a VPL

75

End of quote.

As you can see, what is cool today was on the factory floor more than 40 years ago.

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76 Popularity of PLC Programming Languages

PLCs are about as proprietary as you can get.

Pretty much every vendor has his own proprietary take on each language.

How you can program any particular PLC is determined by that vendor's programming software.

If the vendor doesn't support it, you can't use it.

An individual vendor may even have multiple incompatible product lines which have to be programmed in different ways with different software, although that is not as common these days as it once was.

77

Nearly all PLCs support programming in Ladder.

Some support programming in instruction list as well as ladder.

Anything else is much less common.

78

Ladder logic happens to be on the Tiobe Index by the way.

For those who have not heard of it, the Tiobe Index, that is T I O B E, is a web site that lists the popularity of numerous programming languages based on various criteria.

79

Number one on their list happens to be Python, currently at 19.98%.

Number two is C, at 11.55%.

80

At the time of writing this script, Ladder Logic was at number 46 with a 0.28% rating, which put it just below Erlang and just above Haskell.

I'm not sure whether that means that Ladder Logic is not as obscure as you thought it was, or whether it means that Erlang and Haskell are in fact more obscure than you thought they were.

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81 Episode Summary

In this episode we covered we covered

The early history of computers in industrial control

The early history of PLCs, including how they got their name

Who the major brands are

What they look like physically

82

A basic description of the abstract machine architecture

A very brief look at what a PLC program is like

The scan concept

The main PLC programming languages

The minor PLC programming languages

The relative popularity of each of the programming languages

83

In the next episode we will take a look at one of the early PLCs from the era when they began seeing widespread use.

This PLC was hugely successful and was for many companies the first PLC they used.

This is the Allen Bradley PLC2.

84

This has been the second episode in an 8 part series.

--------------------

Provide feedback on this episode.

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Sources

Full list of sources on the show page: https://darknetdiaries.com/episode/179/

Last week, I discussed the "Second-Hand phone" approach to digital minimalism. In that episode I mentioned a calendar-making site "Pocket-Mod" Here is a review. the site: https://www.pocketmod.com.

Provide feedback on this episode.

MicroPythonOS

https://MicroPythonOS.org/

https://github.com/MicroPythonOS/MicroPythonOS

Thomas Farstrike

https://github.com/ThomasFarstrike

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The HPR Beer Garden returns for it's eighteenth episode with a look at Amber ales. Dave tries out Hobgoblin Amber , whilst Kevie samples Banks's Amber .

Connect with the guys on Untappd :

• Dave
• Kevie

The intro sounds for the show are used from:

• https://freesound.org/people/mixtus/sounds/329806/
• https://freesound.org/people/j1987/sounds/123003/
• https://freesound.org/people/greatsoundstube/sounds/628437/

The next three beer styles to be reviewed are:

• lager
• mild
• bitter

Provide feedback on this episode.

The show I'm responding to

https://hackerpublicradio.org/eps/hpr4696/index.html

The Album I plan on reviewing in the future (not the best album to start with getting into noise, but it's one of the more famous ones)

https://open.spotify.com/album/2T2a5nzJ0uHJVhLkgiMeYA

Reading on why I will never review anything by Boyd Rice (also if you don't mind paying for a patreon, the podcast The Empire Never Ended did a patreon exclusive episode on Boyd Rice and his ties to Neo-Nazi and Neo-Fascist groups)

https://spencersunshine.com/2024/05/21/boyd-rice-neo-nazi-collaborator-video/

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