Hacker Public Radio

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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the reason they were selling them so cheap was that the date code of the included AA alkaline batteries was almost out of date.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.







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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.







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







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.







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







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.







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







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.







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







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.







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







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.











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







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.







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







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.







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







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.











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







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







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







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.







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







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.











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







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.







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







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.







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







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.







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







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.







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







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







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







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.











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







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.











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







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.







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





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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












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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.











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This show has been flagged as Clean by the host.

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





Aroma - what do you smell from the glass and bottle or can



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.



Wait - What flavours are appearing in the aftertaste.



Opinions - what are your personal thoughts on this beer.



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/








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