The PLC Guy

Systems School

2012-02-23T20:58:00.000-08:00

I had just (it seemed) been through this school and now I was re-joining it as an instructor. I had been taught about forms of voice radio (VHF and UHF frequencies). navigation (TACAN and Inertial Navigation) approach control (ILS), and radar identification (IFF). Now I was to teach these subjects.

My first assignments were to reacquaint myself with the materials to get ready to teach these subjects. I thought back to the remark by my fellow students that I would never succeed because I had no 'war stories' to tell.

I was among instructors who had years invested in teaching this material but it seemed, without anything against them, that they were mostly just presenting the same old information. I had been through this and I didn't get any enthusiasm for the subjects. Now this is fairly hard to imagine in these dry electronic subjects but it seemed that enthusiasm is just what was missing.

I had already introduced, in Fundamental Electronics, a glimpse of my instructor style. I had presented the 'Mary Circuit'. A particular circuit in digital electronics (named the Exclusive-OR or XOR circuit) which receives two inputs, each of which can be ON or OFF. The output of this circuit is ON only if ONE, and only ONE of the two inputs is ON.

So I had introduced the 'Mary Circuit' - Mary has accepted an invitation to the prom. In fact she had accepted invitations from two different boys, Joe and Tom. Now it is 'Prom Night' and Mary is up in her room. Does she go down to the front door if no one is there? No. Does she go down to the door if just Joe is there? Sure. Does she go down to the front door if just Tom is there? Sure. Does she go down if both Joe AND Tom are there? Noooo!!! Thus the Mary Circuit.

So I received inquiries in Systems School about this 'Mary Circuit'. They were very literal and not very receptive of innovations. This would change.

The navigational signal presented by a TACAN (Tactical Air Navigation) antenna was very complex. It provided, in one signal, both direction and distance information to any aircraft which utilized its signal. The signal presented one indication when its rotating signal pointed East, with additional signals which indicated how far from East it had moved. Simultaneously it presented information to an aircraft how far away from the antenna the aircraft was. Obviously this was a very complex signal.

But this signal was to be taught with a few diagrams in a workbook. This was unacceptable. I, instead, had students in front of the class, spinning with an flashlight pointing out toward the class, another flashlight pointing upward and emitting 'beep' sounds when pointing in a particular direction. The students, after many hilarious minutes, came to understand how they could determine their relative bearing from the antenna (student wielding the flashlights) by listening to the beeps and watching the rotating flashlight. Similarly they could determine their distance from the antenna/student by listening to the pattern of other beeps.

The understanding of this signal was allowed, in the official teaching guide, a full day for presentation. In one hour my students could not only identify the different components of the signal but their importance in providing the information intended. In fact other classes, composed of cross-training older students, asked specifically for me to present the class. 'Students' older than me ended the presentation in laughter but also with full understanding of the subject matter.

Some of the components of teaching - engagement of the students, the importance of humor in attracting attention - which I had learned in my college classes - came to full use in these classes.

At some future time I may relate my use of the 'Official Invisible Air Force Bowling Ball' in teaching the principles of inertial navigation.

"Funsies"

2011-03-20T12:25:00.000-07:00

Fundamental Electronics ('Funsies' as we sometime referred to it), as I noted before, is a "Modular, Self-Paced" teaching environment. But it looks a lot different from an instructor's side than from the student side.

A new class is typically made up of students just out of high school. There were usually ten or twelve students. Most were only about four years younger than me. I was a E-2 rank. I had one whole stripe on my sleeve. The rank's name is 'Airman'. But the Air Force, to encourage longer enlistments, was granting to those who signed up for 6 years a rank of E-3 (two stripes - 'Airman First Class') right out of Basic Training. So some some of the young students already out-ranked me.

The highest ranked student in each class became the 'class leader' and performed various leadership duties. Occasionally higher ranking Air Force members would 'cross-train' from their current job into another, beginning their instruction at the Fundamental level. I had one class where a 5 stripe sergeant was a student and was therefore the 'class leader'. In this particular class one of the young 'Airman First Class' was attempting to exert what they perceived as a rank advantage over me, the instructor. To say the least I wasn't very proficient at exerting my own authority at this early point.

The 'class leader' came to me and suggested that I might want to go get a coffee for a few minutes. I thanked him and stepped out but only far enough that I could still hear what went on. The sergeant then proceeded to make it very clear - in a surprisingly normal tone of voice - that 'that badge' (he was referring to the 'Instructor' badge I wore) made me the highest rank in the class and if the 'Airman First Class' had a problem with that they could discuss it outside. When I came back from my 'coffee break' I stepped into a very calm class. I gave the 'class leader' a discreet thankful nod. I had a lot to learn myself.

The students would receive a study manual which had text, pictures, study questions, practical equipment exercises and quizzes based on the hardware exercises. As they mastered the work in the manual they would take a proficiency test, in a separate testing area, then progress to the next manual.

The 'instructor' in Fundamentals managed the paperwork for each student, kept class statistics and helped each student as they progressed, answering questions and helping with misunderstandings. In the early few days of a class most of the students were on the same manuals so it wasn't too difficult to keep things straight. But as better student finished more rapidly and surged ahead in their manuals the class would become fragmented in what they were studying. There was less opportunity to address class problems as a whole.

This became even worse as the students advanced past very basic electronics. The students were preparing for different specialties. These would require different sets of knowledge at the basic level. If a specialty did not require knowledge of a subject - for example “lasers” - they would not receive that manual but would receive others that they needed. So a class became even more fragmented and the instructor was answering an even wider variety of questions. It was like a rapid-fire version of Jeopardy with ten subjects at the same time.

It had been found that, relying on the aptitude tests given before enlistment, the 'Reading Ability' pre-test came out ahead as a predictor of success in learning Electronics over scores on the the 'Electronics' test. But even with preliminary testing the predicted ability didn't always show itself. We had one student who, by pure memorization, finished each of the tests with barely a passing score. He was 'progressing' but it was increasingly apparent that he lacked actual understanding of the subject and was becoming more and more despondent over this situation. He did not give up but he had to work twice as hard as the other students just to achieve this level.

He had been with a few different instructors so we got together and discussed him and his progress. We then took the matter up with our supervisors. Even though he was 'technically' passing we urged that something be done to help him. There was no way he would be happy or productive in the Electronics field. Our supervisor called him in and talked about his progress listening to his level of enjoyment for the course. He was then reassigned to another skill field, with no negative mark whatsoever. We later received a letter from him expressing his delight in his new assignment (something very different than Electronics.) We couldn't be happier for a student who couldn't make it through our classes. We felt that we had properly applied another aspect of being an 'instructor'.

Being an Air Force Electronics instructor was more-or-less an eight hour a day, five day a week job (more on this later). At times we would be relieved of one class, which had fragmented in their studies enough to be grouped together in other classes with students in similar areas of instruction. A number of students had also failed to make the grade (even though their 'Electronics' pre-tests had predicted great aptitude). So at times we were between classes. We were given duties such as reviewing training materials and tests for updating and special instruction for students having specific problems. (I mentioned this near the end of my Instructor Training post.)

While I was 'between classes' my supervisor, a Master Sergeant, said to me at the end of a Friday “See you Tuesday”. I was confused. Was there a holiday on Monday I didn't know about? But luckily another instructor grabbed me. He said that that was my supervisor's method of giving an 'extra day off'. He said that my supervisor knew exactly what he had said, there was no holiday and if I go back in to ask he will correct it to “See you Monday” thus losing that day off. Interesting times.

After one and one half years of teaching in the Fundamental Electronics school I was re-assigned to the Systems School.

Instructor School

2011-03-05T13:33:00.000-08:00

I began my career as an Electronics Instructor by sitting in on the same 'Fundamental Electronics' classes that I had graduated from some months before. But this time I would have to be proficient in all the materials taught in the Fundamental Electronics School, not just the areas I had taken as a student. So about one third of the classes were totally new. I did this until the Basic Instructor Course opened up. So I sat in with students, doing the practical exercises, reading the same training materials and taking the same tests (except as an Instructor I had to take all the various forms of the tests which had different questions). I had to pass the tests with a 'missing only one at most' score. Obviously an Instructor should have a grasp of the material that they can easily pass all the tests.

I was excited when the Basic Instructor Course opened up. I reported to the training building and found that I was the lowest ranking individual in the class. There were even officers there each taking the same course Basic Instruction. There were also civilians who would be teaching Air Force courses (primarily in the Accounting and Procurement areas). My trepidation at being with OFFICERS was quickly lessened as we each introduced ourselves. Since the class was being lead by an upper-ranking sergeant it was like a buffer between our ranks. We always addressed each other with the proper rank (or 'Mr' or 'Mrs' for the civilian students). There were only about ten students in the class so at times it became somewhat informal but we were always careful not to cross lines.

In college, one of my Psychology courses was 'Psychology of Instruction' which covered methods of preparation and presentation of materials in different situations. It also covered various forms of measurement of progress and ranking of students. I had liked that course.

So here I was in the Air Force learning its methods of preparation and presentation of instruction and it turned out to follow the techniques I had learned in college. We learned about the basic concept of instruction - 'You are not a mind reader!'. The desired outcome of instruction was always in terms of directly observable behavior. There was none of the 'the student should understand ...' stuff. The final result had to be in terms of behavior we would observe. 'What does the student actually do that makes you satisfied that they have mastered the subject?'

As part of 'teaching a physical skill' I gave a presentation on changing a broken string on a guitar. I had sat in my apartment working out EXACTLY the steps and checks on changing a string. Before my presentation I had placed an old string on my guitar and started the presentation with the opening ... "You are at Carnegie Hall preparing to give the performance of your life when ..." (and I pulled on the string breaking it - I had worried that it would whip back and cut me but that didn't happen.) The presentation went well.

In another presentation which was to be a pure lecture format it had been emphasized that we had to take into account the background of our audience/students. I began with a fairly complex presentation when, as expected, it appeared that the students (my fellow instructor students) were not following. I gave a feigned frustrated sigh and ripped up my notes. I got an audible 'gasp' from the others then launched into the simpler presentation I had prepared which followed the format that we were expected to give.

After my presentation, which went over well, the instructor asked why I had done what I did. I noted that, following the background of the audience, I knew I would capture the best attention if it appeared, at first, that I was utterly failing in my presentation. He remarked that no one had done that before. I grinned.

The final test of the Basic Instructor School was to give actual face-to-face instruction to real students. But, as I had mentioned in an earlier posting, the presentation in the Fundamental Electronics Course was of individual student learning from training materials, often with many in the class on totally different areas. The only face-to-face instruction was with students who had worked through a subject area but had failed the test for that area. They then had to take 'Remedial Instruction' which often took place in any old empty classroom.

The students, who could be from a very wide assortment of the training areas, brought their training manuals. It was up to the 'Remedial Instructor' to determine individually what area each student had been studying and which concepts in that area they had failed to grasp. The 'Remedial Instructor' had to instantly give a presentation for each student clarifying the areas they missed. I did this over the course of one hour for three students.

After the 'Remedial Instruction' class I met with teaching instructor. He was amazed. The usual class preparation which should take hours had been compressed into mere minutes. And even though he had not studied the area of Electronics I had presented he said that he understood it very well from my quick instruction. I grinned again. I was an Air Force Electronics Instructor.

Systems Training

2011-01-02T16:31:00.000-08:00

Once done with the Fundamentals of Electronics, and my leave to visit home, I entered into the 'Systems' part of my training. My training was in the area (as designated at the time) '326X1B'. The 'X' in the code was replaced by a numeral relating to the level of advancement in the area.

The training specifically was titled "Integrated Avionics Component Specialist". The training was on Communication, Navigation and Identification systems on the FB-111 aircraft referred to as the 'Aardvark'. It was a sweep-wing fighter-bomber. There is a Wikipedia entry if you are curious. Even as I was undergoing training, there were those who could cite all the various differences between the many variants of this aircraft. In short, I didn't know and unless it was relevant, I didn't care.

The 'Systems' training used the fundamentals we had learned to build an understanding of the various units on the aircraft we were to maintain. There were voice communications which had various ways of establishing frequencies and operations. There were navigation units, one which sampled various air pressures to result in readouts for true airspeed and other calculations. Another which received transmissions from radio stations to establish the position and speed of the aircraft. Finally identifications units (ominously named 'Identify Friend or Foe') which responded to radar sweeps with information coded in numbers which identified the aircraft.

These were very interesting classes as they ranged from pure analog (the 'air data computer' using chains, gears, and 3D cams to calculate its results) to purely digital (the derivation of the codes sent back by the identification equipment). The digital electronics were of mid-1960's technology (I was receiving this training in 1973). The digital logic was in 'flat packs' incorporating multiple AND and OR gates.

Again I was surprised that the training was so logically built on previous instruction that the concepts came easily. I had no problem with these classes. The best part was the mid instruction break each day. The Airman's Club was right next door and we could get a good cup of coffee and doughnuts to pick up our mornings.

Near the end of instruction a curious request was made of us (in a military service most 'requests' are 'orders' but this wasn't.) As far as I understand there were many Electronics instructors who were rapidly approaching retirement or end of enlistment and insufficient preparation had been made for their replacements. The request was of those who were placed highest in their classes to become 'Instructors'. We were informed by classmates that this could never work because, as opposed to instructors who had come from years of line operations, we would have no 'war stories' to tell to fill odd moments of our instruction. I said 'yes'.

Beginning Technical Training

2010-10-09T16:22:00.000-07:00

My Air Force technical training was at Lowry Air Force Base on the east side of Denver, Colorado. (The area now belongs to the city of Denver, I believe.) I remember the flight from San Antonio approaching Denver's Stapleton International Airport. I saw the Rockies in the distance and I rejoiced. The Rocky mountains have the same relationship to Denver as the Cascade Mountains have to Yakima, Washington, my home town. I seem to need mountains for orientation. The flat landscape of San Antonio had left me - well - disoriented (in the literal sense of not knowing which way East lies.)

On arrival there was the usual amount of paperwork mix up. We were finally assigned to our respective training flights and barracks. These were two man rooms , a big difference from the open barracks of Basic Training. As the particular class in school we were assigned to would not enter for a couple of weeks we reported daily for miscellaneous work details. These could be anything from lawn mowing (this was June of 1973) to moving stock at the commissary warehouse. But we had a lot of free time and the Airman's Club served inexpensive drinks. Fortunately I found the library and spent more time reading than drinking.

The format of the Fundamental Electronics training ("Fundies" or even "Funsies" as we referred to it) was a 12 week (nominal) course. Six hours a day was spent in a "self-paced' instruction mode. This meant you had manuals and test equipment at your own work station. As you completed reading the training manuals and performing the practical electrical setups and measurements you were then tested on that section's material. If you passed you moved on to the next training unit. The students in a class did not progress together so there was some moving between classes with different instructors and classmates.

Between physics and math in high school and the minimal amount of electronics I picked up while working toward my novice amateur radio license in college I handled the early stages of the training easily. The later sections built rather logically on the previous as electronic circuits (oscillators, amplifiers, RF transmission, digital electronics) were introduced. I remember missing one question on all the tests in this Fundamental Electronics.

I completed the "12 week" course in 6 weeks. I was allowed to take some leave and fly home to visit my family. I was impressed that wearing an Air Force uniform allowed an upgrade to a fortunately empty seat in first class.

Basic Training

2010-09-19T17:07:00.000-07:00

Basic Training in the Air Force consists of only 45 training days (these are 'week days' so 9 weeks usually.) We arrived just before Memorial Day so our first three days (the Memorial Day weekend) didn't count as 'training days'. The first days were spent learning how to evacuate our barracks in time in case of a fire. As I remember, after endless drills, we were never fast enough. Each drill involved jumping out of a freshly made bed then, after the drill was over, we had to re-make the bed to Air Force standards before we got back in them to do yet another drill.

Were we glad on Tuesday to be issued something other than the civilian clothes we arrived in (they said bring 3 days clothes, but not all did.). The haircut, while very close to the scalp, felt good in the San Antonio heat and humidity.

As I had guessed during my first few hours I was the oldest in my flight. (I also quickly learned that the 'flight' was the smallest organized unit in the Air Force. There was a mnemonic to remember this - "How many new airmen will get sore feet?" (I've cleaned up the last two words) The first initials of the words stand for - Headquarters, Major air command, Numbered air force, Air force, Wing, Group, Squadron, Flight.) Most of the members of my flight were just out of high school. When they learned my age (an ancient 22 years compared to their probably 18-19) and level of schooling I was dubbed "the professor". The TIs (Training Instructors) gave me the task of keeping the flight's paper records (a job referred to as the 'house mouse'). This didn't get me out of anything else. It was just an added duty I inherited, lucky me.

One of the first tasks was to record the level of stock on the flight's supplies (toilet paper, cleaning supplies etc.). There was a minimum level of each item as indicated by black marked outlines on shelves and walls where the items were stocked. A system few could misinterpret. (I was beginning to appreciate the efficiency - little did I know.) A few items were low so I asked the TI where to get more. He indicated a building about two blocks away and said I could just make it if I hurry.

So I rush over, my list in hand, to the supply office. I was thinking that this part of the assignment should be easy. I reach the supply office and state my flight number and what supplies were needed. (Simple right?) They said, where is your 'Requisition Slip' (I could hear the quotes!). I said, "I don't have one, where do I get one?" They said, "From your TI." So off I run back to the barracks. (The brighter among you know where this is leading, but I hadn't caught on yet.) Back at the barracks I ask the TI for a 'requisition slip'. He says, "Oh, we're out of those. We'll need another pad." I ask (a feeling of dread beginning to grow) "Where do I get another pad?" He replies - wait for it - "At the supply office."

On the way back to the 'supply office' I realize that they could keep this up all day and I couldn't do anything about it. I prepared myself to spend the afternoon running back and forth between the barracks and the supply office. But - miracle of miracles - either they took pity on me, which I doubt, or they had had their fun. I received the supplies as well as a new requisition pad.

Almost immediately the fine system which chose the recruits showed its weakness. We began to loose members of our flight (one member of or flight ended up in the corner mumbling for his mother, at times somewhat incoherently. He was taken away gently.) as well as gain other members. I had to line out the names of those who had been removed as well as add the names of the new recruits on seemingly endless number of roster lists. A properly updated list had to be presented at each and every activity (physical exercise, classes on Air Force history and rules etc.) So I would be up well past midnight (because my own duties of 'guarding' the barracks at night and maintaining latrines were not lessened in the least) updating these lists. Fortunately each week we received a pack of newly updated roster lists (though it was already a day or two out of date) so the job was lessened somewhat. This job continued through my Basic Training days.

US Air Force - First Day

2010-09-18T17:32:00.000-07:00

To start, this was a trade, only the Department of Defense can decide if it was even. I traded 4 years of my life for electronics training, food, clothing, housing etc. Even though I was not immediately in peril (late 1972) of being drafted into some strange field I recognized that I needed to get a handle on this process.

Visiting the various recruiter's offices I quickly narrowed it down to the Navy and the Air Force. The Navy offered an officer's commission for ONLY 6 year's commitment. Not wanting 6 years and not being particularly keen with being on (or possibly under) the ocean I opted for the Air Force even though they couldn't offer me an immediate officer route.

The Air Force offered a variety of fields of training. I wanted one which provided the best electronics training. Even then, the way things were progressing in electronics was becoming very apparent. I took various tests which showed I was suitably prepared to enter into electronics training.

In May of 1973 I made the fateful trip to Spokane,Washington where I would take my oath of enlistment and join the ranks of the military. My younger brother was concerned about air flights. He wondered what would happen if my flight was delayed. I observed that, once I enlisted in the Air Force my fate was totally in their hands and if a flight they assigned me to was late it was their problem.

I swore my allegiance; made the flight and my way to basic training at Lackland Air Force Base outside of San Antonio, Texas. I first became aware of problems when we arrived and the humidity was intense. I thought I was in trouble. But trouble of a different kind soon raised its head.

At the induction center (at 1:00 in the morning, all the arrivals were seated on the floor in a room) the training instructors (TI) asked for a simple action. If you had your Social Security card then, when your name was called, stand, hold the card in front of your face and recite the numbers. If you didn't have the card then, when your name was called, simply stand and say "I do not have my Social Security card."

Okay, I figured, fairly simple task (I had my card, but even if I didn't they had exactly defined only one other way of responding). When my name was called (with 'C' initial I was fairly early in the order) I stood, raised the card in front of my face, and read off the numbers. But there were many, and I mean MANY, who stood saying "I don't have my card man but I know my numbers, they are ..." at which point the TIs would scream, shout and raise holy hell. The rest of us would groan as they obviously had a procedure to deal with those who didn't have their card with them and each idiot who stood with a "but I know my numbers" was just slowing things down. (Now, I didn't think that being a recent college graduate provided any more ability to respond correctly compared to what I perceived as mostly recent high school graduates. But they had said, 'do this' or 'do that' without any other option. It wasn't rocket science! Hence my beginning of a bit of despair.) The heat and humidity dragged on.

When things s l o w l y came to a finish we were loaded into buses and transported from the receiving station to our barracks. There we were given our initial briefings, filled out various forms then were finally allowed to go to sleep at 3:00 AM. The TIs said that, since we got into bed late we would be allowed to sleep in. But at 5:00 AM (2 hours later for the mathematically challenged) the bells went off and we were ordered to get up and 'were we going to sleep our lives away in bed?' I realized, this is it. They control my life from here on out for four years.

Early Interests

2010-09-02T20:57:00.000-07:00

One of the first influences I remember is Disney's "Man In Space" series which had Werner Von Braun as a consultant. As a 5 year old, watching, probably in my dad's arms, it is a memory I cherish. This was the first of several series on space and science by Disney (possibly the pinnacle of Disney's influence, pure cartoons next.) It's incredible how close they were to the actual steps of space development. They even include a possible space disaster (like Apollo 13). Even re-watching it now I feel the possibilities I felt as a young child. What a rush!.

Later on the series of "All About ..." books from Random House provided much of my young science education. It's ironic but a close friend had all of these books in his own home library but with dyslexia had difficulty reading and enjoying them, I didn't know this (the dyslexia) until I was an adult. As a child/adolescent I just envied him. I rode my bike to the library downtown each week to check them out and read them. But I hated math. My mother had to bully me through long division.

Electronics fascinated me. We had a neighbor, an amateur radio operator, who had a 4 character call sign. (Think of the age of commercial radio stations with 3 letter call signs.) I understood he had learned radio and operated during World War I. One of the first science oriented Christmas gifts was a crystal radio set. How interesting that a coil of wire and a small "cat-whisker" crystal (which I now understand as a point-contact diode) could pick up radio broadcasts. I also had the typical Erector sets and an Electrical building set.

In high school I learned basic math (Algebra and Geometry). I did not have the chance to take Trigonometry and Calculus. I had Chemistry and Physics. One influence is my instructor's insistence (this was 1965) on learning the use of the slide rule. What a quick education on the relationships of numbers! (thank you Father Lane!)

In college, despite a 'Liberal Arts' education (History major, philosophy and psychology minors) I also pursued Amateur Radio as a pastime. After college, with the last remnants of the 'War In Vietnam' and 'The Draft' looming I opted to pursue a more predictable path in the US Air Force. OK, so I'm not GI Joe. My 'draft number' (if you don't know about that check Google) was 11. My friend had the enviable number of 366 (remember leap years). Imagine two fingers symbolizing '11' spread apart in the symbol for 'Peace'.

Just Beginning

2010-08-28T15:48:00.000-07:00

I have a web site, www.theplcguy.com, where I try to present some of my observations on the PLC systems I have used, along with various things that interest me from time to time. But I'm going to use this blog as a 'how did I get here'. But first an introduction. (Those familiar with the basics can skip.)

I can't imagine someone coming here without a familiarity with PLCs but for those who don't, let me give a brief explanation. Current machines of any moderate complexity and up have a computerized controller at their heart.

Large machine used to, up to the middle 1960's, have banks of relays, timer units and counter units occupying huge cabinets to control part of their operations. As solid-state electronics were developed the logical sequence of processes were transferred to banks of 'chips' to perform the decision processes. The MODICON controller (mid 1960's) was one of the first of these solid-state controllers.

Later, as microprocessors, essentially 'computers-on-a-chip' were developed ( mid 1970's), the control was transferred to computer programs running using these new devices. The development of small computer systems has encouraged even smaller machine developers to use the ever cheaper computerized control systems to use these units. Current technology produces controllers for little more than the cost of a couple of relays. Entry systems may feature free software to program them.

The whole family of computers which are designed for machine control are labeled PLC or Programmable Logical Controller. (The label was originally 'PC' for Programmable Controller until some upstart company called IBM appropriated that acronym for their product, the Personal Computer.) As an ironic development the IBM PC format has become the development platform usually used to generate the 'programs' mentioned below.

A PLC controls a machine or process by first using 'inputs'. This is information from the machine or operator. These 'inputs' are usually in the form of individual ON/OFF states (indicating the presence or absence of a state) or a number (indicating the level of a state). These can come from sensors on a machine or as data from an operator via pushbuttons or an operator display (sometimes called a Human-Machine-Interface or HMI).

The PLC then, using rules established in a 'program' (which can be written in a number of competing control languages - each claiming to be better than the others) processes the 'inputs' and remembered information.

Using the results of the program 'rules' the PLC controls 'outputs'. These, again, may be discrete ON/OFF outputs or levels to devices which can actually affect operations on the machine.