Imagine a machine with some number of pumps. We have the logic for each pump in its own routine: Pump1, Pump2, etc.

Somewhere in our program we want to know if any of the pumps are running. You write a rung of logic like this:

This machine gets changed a lot and the number of pumps changes frequently. Any time we add or remove a pump we need to remember to revisit this rung and modify it. By definition, this rung is separate from the Pump logic (it comes after it).

If we remove Pump2 and remove the tags or variables associated with Pump2, then hopefully the editor will be smart enough to tell us that we have a contact referencing a tag or variable that no longer exists.

But what if we’re adding a pump? When we add Pump4, what prompts us to revisit this rung? What if I’m not even familiar with this code because it’s been a long time, or I wasn’t the original programmer who wrote it? Maybe it’s not pumps, but steps, or missions, or so on. There could be hundreds. The fact is this is a very common problem that pops up often in ladder logic programming, and we just live with it. We shouldn’t. We should ask our PLC vendors for better tools.

Now, there are hacky ways to solve this problem to make sure that when I copy a pump routine that I don’t have to remember to go update another routine.

For one, I could put all the Pump Running bits in an array from 1 to a maximum number of pumps, and replace my pleasant little Any Pump Running rung with a FOR loop. That only works if my pumps are numbered sequentially of course, and it’s really ugly and makes it difficult to look at the AnyPumpRunning coil and follow it back to see which pump is running. But I could do it. If I had no self respect.

Another hacky solution is to use a Reset (unlatch) instruction on a coil at the beginning of the scan, before any of the pump logic… let’s call it “tempAnyPumpRunning”. Then in parallel with each Pump Running coil we could use a Set (latch) instruction on the tempAnyPumpRunning coil. Finally, after all the pump logic you could use the tempAnyPumpRunning coil to drive the real AnyPumpRunning coil. I mean that would work. It’s less ugly than a FOR loop, but still a bit ugly, and suffers from the same cross referencing problem as the FOR loop. I’m embarrassed to say I’ve done this. It turns out I have no self respect.

Each time I did it, I cursed ladder logic for not being expressive enough.

I’d like to pause for a moment and talk about how this problem is solved in a programming language with any kind of functional programming features. I’ll use C# as an example, because I’m familiar with it. In C#, if you can have a collection of objects, let’s say of type Pump, and each pump has a property called Running, and if you want to know if any of them are running, you can do this:

bool anyPumpRunning = pumps.Any(p => p.Running);

The expression in the brackets, p => p.Running is a function with one input (Pump p) that returns a boolean, and the .Any(...) extension method evaluates this function on every object in the pumps collection and if any one of them returns true, then the .Any(...) function returns true. Even if we change the number of pumps, this line of code never has to change.

Similarly, C# has many such useful features:

bool allPumpsRunning = pumps.All(p => p.Running);

int howManyPumpsRunning = pumps.Count(p => p.Running);

double totalLitersPumped = pumps.Sum(p => p.LitersPumped);

double maxPumpRuntime = pumps.Max(p => p.Runtime);

In the PLC world, we presumably created a UDT or a structure called Pump, and we created variables or tags of type Pump called Pump1, Pump2, etc. The PLC knows where all these variables are located in memory (or it could). It should be possible to create new PLC instructions that act on all variables of a given type, like for Any Pump Running:

You can get close to this now. You could create an AOI (or custom function block) but it would have to take an array of pumps as a parameter, and use a FOR loop inside to evaluate it.

The AOI solution still has a cross referencing problem. It’s not obvious which pumps are on. But the new instruction I described above could have a feature where you click a button and it pops up a cross reference of all the Pump Running bits, sorted by which ones are on, and double-clicking could take you directly to where that coil was being set. It would be EPIC!

It’s just a thought. Take it or leave it.

There are many ways to visualize your program at a higher level. Take a look at this model for a second:

I tend to focus on Part Tracking as a core part of my programming work. You can think of the part tracking as the PLC’s internal model of the outside world. That is, it’s the world that can’t be sensed directly by the inputs. In some cases the part tracking information can be used in spite of the current state of the inputs. Let me explain.

Imagine a work cell where a robot places a part in a fixture, a nut feeder feeds a nut onto the part, and then a weld gun extends from the top, and welds the part.

There are various places where we may place sensors in this situation, and you sometimes don’t have much control of it. Perhaps there’s a sensor in the fixture detecting that the part is there, but sometimes the mechanical engineers can’t find a place to fit one in. Sometimes the robot gripper has a sensor (or sometimes we use vacuum sensors to detect a part we’re picking up with vacuum). Perhaps neither is the case and we just know the robot has a part because we knew there was one in the infeed fixture and we know the robot went there and gripped, so we just assume it’s there.

Part Tracking gives you a way to remove some of this uncertainty. Create a memory bit (M1) to indicate there’s a part in the robot gripper. If the robot gripper moves to the fixture position and opens, then clear memory bit M1 and set memory bit (M2) indicating there’s a part in the fixture.

Keep this part tracking logic separate from your fault logic (so your part tracking logic is predictable and similar from project to project). If you happen to have a sensor in the fixture, create a separate rung that seals in a fault if the M2 bit is on and the fixture part present sensor is off, and use a delay timer of 100 ms or so just to allow for a sensor blip. Make sure the fault stops the cell. In this case I typically require the operator to go into the HMI and manually clear the “ghost” part in the fixture (i.e. clearing the M2 bit) before they can reset the fault. That way I know that they know the part really is missing. Likewise, a second fault rung seals in if M2 is off, but the sensor indicates a part.

Now as I said, sensors can sometimes be unreliable, and nowhere is this more common than in a weld cell. The high magnetic field generated by the weld current, and the weld expulsion splattering around is a surefire way to mess with both inductive proxes and optical sensors. Separating part tracking from sensors gives you a way to deal with this. For instance, when the welder is firing, mute out the faults so they don’t trigger if the sensor suddenly reads incorrectly.

To put it another way, use your part tracking to make decisions, and use the sensors to validate the part tracking, but only at times when you’re sure of their validity.

Furthermore, you can use inputs to initiate actions without requiring them to be on for the entire duration of the action. For instance, you can require that the fixture sensor is on to indicate a part is present before you fire the welder, but you can seal in the Fire_Welder bit around the sensor contact so the sensor can flicker after the output turns on without causing the output itself to flicker on and off.

I want to talk about the concept of “idioms” or the idea of “idomatic” when it applies to programming languages.

Python is said to have “strong idioms“:

One reason for the high readability of Python code is its relatively complete set of Code Style guidelines and “Pythonic” idioms.

When a veteran Python developer (a Pythonista) calls portions of code not “Pythonic”, they usually mean that these lines of code do not follow the common guidelines and fail to express its intent in what is considered the best (hear: most readable) way.

Many years ago I did some programming in Perl. A main philosophy of Perl is the acronym TMTOWTDI (There’s more than one way to do it). That’s an example of a language with “weak idioms”. This goes hand-in-hand with Perl’s apparent lack of focus on readability. In fact, some detractors of the language claim it’s a “write-once, read-never” language.

When I started writing the Patterns of Ladder Logic Programming page, I wasn’t thinking of it at the time, but in retrospect I was trying to document “Idiomatic Ladder Logic.” Yes, there are many ways to accomplish the same task in a PLC, but you should stick to idioms when they help communicate the meaning of your code.

When you deviate from these idioms, you’re communicating to the reader that something about this case is different. If I expect a fault coil to be sealed in, and you make it a set-reset, you’re communicating to me that this fault condition needs to survive a power outage. That’s useful information. If your fault coil isn’t sealed in, that must mean you intend it to be self-clearing.

Even your deviations should be idiomatic. Don’t make fault coils self-clearing by putting a Reset instruction for the same coil somewhere else in your code. That won’t be obvious to the reader, and is the reason for the idiom “don’t use a coil more than one place in your logic.”

Most or all new PLCs can be programmed in multiple languages, from the IEC-61131-3 specification. These languages are very different, and the idiomatic way to do something in one language isn’t necessarily the way to do it in another language. For instance, doing any kind of loop (for, while) is non-idiomatic in ladder logic, but is certainly an idiomatic construct in structured text. That means part of our job as programmers is to pick the correct language to express our intent.

Data collection, string parsing, and math is naturally expressed in structured text (ST), but control logic is naturally expressed in ladder diagram (LD). Part tracking logic can go either way. Sequential function chart (SFC) is perfect for expressing a sequence, ladder diagram is a good runner up using the Step Pattern, and structured text requires that you define a state machine, which is the least expressive option.

I once wrote a bubble-sort routine in ladder logic for a SLC500 PLC. I wish I’d had structured text back then. First pick the right language, and then pick the right idiom.

Decades ago, computer science emerged from the dark ages of assembly language programming and created two new languages: Lisp and Fortran. These are two very important computer languages because they exist at opposite ends of an imagined spectrum in the eyes of computer scientists: functional languages vs. imperative languages.

Fortran “won” the first battle, not least because imperative languages are closer to how the CPU actually does things, so back in the day when every little CPU cycle mattered it was easier to understand the performance implications of a Fortran program than a Lisp program. Plus, if you were already programming in assembly, then you were already thinking about how the computer was executing your code. In fact, the next big imperative language, C, is often referred to as “portable assembly language.” Fast forward to now, and modern languages like C#, Java, Python and Ruby have all grafted a lot of functional programming features onto their imperative programming basic syntax. In C#, for instance, Linq is a direct rip-off of Lisp’s S-Expressions and it now has Closures and lambda functions. Functional languages provide ways to think at a higher level than imperative languages. In a functional program you describe what you want and in an imperative program you describe how to do it.

Here’s an example in C#, using imperative programming:

var data = new int[] { 1, 2, 3, 4, 5 };
var sumOfSquares = 0;
for(var i = 0; i < data.Length; i++)
{
    sumOfSquares += data[i] * data[i];
}

…and the same thing done functionally:

var data = new int[] { 1, 2, 3, 4, 5 };
var sumOfSquares = data.Select(x => x * x).Sum();

In the second case, I’m taking the list of numbers, using Select to translate that into a list of their squares (also known as a Map operation) and then using Sum on the resulting list to compute an aggregate sum (also known as a Reduce operation). It has some interesting advantages. For instance, the original code can’t be split across multiple cores, but the latter can. Also, if you know both syntaxes, the latter is easier to read and understand.

Now take ladder logic. I’ve made the claim before that basic ladder logic (with contacts and coils) is actually a functional language. A simple example might be ANDing two inputs to get an output, which in C# would look like this:

var output = inputA && inputB;

That’s actually functional. If I wanted to write it imperatively I’d have to do something like:

var output = false;
if(inputA && inputB)
{
  output = true;
}

In ladder logic, that would be the equivalent of using an unlatch (or reset) instruction to turn off an output and then using a latch (or set) instruction to turn on the output if the A and B contacts were true. Clearly that’s not considered “good” ladder logic.

Similarly, a start/stop circuit goes like this:

var run = (start || run) && !stop;

Now historically, mathematicians and physicists preferred functional languages because they just wanted to describe what they wanted, not how to do it. It’s worth noting that electricians, looking at ladder logic, prefer to see functional logic (with contacts and coils) rather than imperative logic (with sets, resets, and move instructions).

In recent years we’ve seen all major PLC brands start to include the full set of IEC-61131-3 languages, and the most popular alternative to ladder logic is structured text. Now that it’s available, there are a lot of newer automation programmers who only ever knew imperative programming and never took the time to learn ladder logic properly, and they just start writing all of their logic in structured text. That’s why we’re seeing automation programming slowly shift away from the functional language (ladder) towards the imperative language (structured text).

Now I’m not suggesting that structured text is bad. I prefer to have more tools at my disposal, and there are definitely times when structured text is the correct choice for automation programming. However, I’d like to point out that the history of computer science has been a progressive shift away from Fortran-like imperative languages towards Lisp-like functional languages. At the same time, we’re seeing automation programming move in the opposite direction, and I think alarm bells should be going off.

It’s up to each of us to make an intelligent decision about what language to choose. In that respect, I want everyone to think about how your brain is working when you program in an imperative style vs. a functional style.

When you’re doing imperative programming, you’re holding a model of the computer in your mind, with its memory locations and CPU and you’re “playing computer” in your head, simulating the effect of each instruction on the overall state of the CPU and memory. It’s only your intimate knowledge of how computers work that actually allows you to do this, and it’s the average electrician’s inability to do this which makes them dislike structured text, sets, resets, and move instruction. They know how relays work, and they don’t know how CPUs work.

If you know how CPUs work, then I understand why you want to use structured text for everything. However, if you want electricians to read your logic, then you can’t wish-away the fact that they aren’t going to “get” it.

As always, be honest with yourself about who will read your logic, and choose your implementation appropriately.