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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.

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Id Day Date Title Host 4696 Mon 2026-08-03 HPR Community News for July 2026 HPR Volunteers 4697 Tue 2026-08-04 Correcting the Dates of Files Vance 4698 Wed 2026-08-05 ID3 Tags and Vorbis Comments Whiskeyjack 4699 Thu 2026-08-06 Sunshine, Moonlight, Playnite !? operat0r 4700 Fri 2026-08-07 Robert A. Heinlein: The Juveniles Ahuka 4701 Mon 2026-08-10 Elsbeth muses with Lee on her experiences and opinions on AI Elsbeth 4702 Tue 2026-08-11 MakeACopy is an open-source document scanner app for Android Ken Fallon 4703 Wed 2026-08-12 Ebooks and Audiobooks Ahuka 4704 Thu 2026-08-13 HPR Beer Garden 17 - Double DDH IPAs Kevie 4705 Fri 2026-08-14 Free Software Is Wasted On You Teens Trollercoaster 4706 Mon 2026-08-17 Quirks and Customisations Lee 4707 Tue 2026-08-18 UNIX Curio #12 - expr Vance 4708 Wed 2026-08-19 Programmable Logic Controls - Episode 1 Whiskeyjack 4709 Thu 2026-08-20 Peertube: my two penneth Kevie 4710 Fri 2026-08-21 Civilization VI Ahuka 4711 Mon 2026-08-24 Second-Hand Phone TrumpetJohn 4712 Tue 2026-08-25 friedcamp interviews part 1: Wim van Gool Kristoff 4713 Wed 2026-08-26 July2026 Community News response, and Future Plans TheDUDE 4714 Thu 2026-08-27 HPR Beer Garden 18 - Amber Ale Kevie 4715 Fri 2026-08-28 friedcamp interviews part 2: Thomas Farstrike Kristoff 4716 Mon 2026-08-31 PocketMod TrumpetJohn Comments this month
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hpr3766 (2023-01-09) "ACER Nitro 5 laptop review" by Bookewyrmm.

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candycanearter07 said: "re: a refreshing contrast..." (2026-08-12 02:54:05)

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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)

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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)

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Archer72 said: "Amateur Radio Field Days" (2026-08-02 11:50:11)

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hpr4697 (2026-08-04) "Correcting the Dates of Files" by Vance.

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candycanearter07 said: "quirks in hpr" (2026-08-05 15:52:18)

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Trollercoaster (our of character) said: "Woopsie" (2026-08-16 19:25:41)

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candycanearter07 said: "cool ep!" (2026-08-30 20:15:40)

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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)

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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.

This show has been flagged as Clean by the host.

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



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01 Introduction







This is the second episode in an 8 part series.







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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.







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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.











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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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







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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.







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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.











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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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.







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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







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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.







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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.







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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.







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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.











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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.







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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.







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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.







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The PLC has a watchdog timer.



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







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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.







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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".







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







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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.







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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.







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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.







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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.







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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.







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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.







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a = (b or c) and not d







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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.







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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.







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a = (b or a) and not d







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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.







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







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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.







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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.







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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:







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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







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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.







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Nearly all PLCs support programming in Ladder.



Some support programming in instruction list as well as ladder.



Anything else is much less common.







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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.







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



Number two is C, at 11.55%.







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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







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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.







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