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Object-oriented programming vs. ladder logic in PLCs

Oct. 7, 2024

Could you be using OOP without even knowing it?

A manager recently said all his people used ladder logic and didn’t know object-oriented programming (OOP). He went on to explain that the company wants to be able to reuse code.

In essence, even ladder programmers are using OOP because they are utilizing premade functions that are in the default libraries and putting that in their ladders. For example, timers are premade functions already in many libraries, and those are used throughout ladder logic.

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OOP is simply software that is divided into objects. An object can be thought of as a block, a unit or a cannister—the list goes on. But most describe this piece of software as an object so that, when you have a discussion, others may follow.

Within that object, the descriptions, associated data, procedures, interfaces or methods are included and combined to make an element. An element is typically a function. For instance, a motor starter is something that can be found in almost any system. To create an object that is repeatable in a PLC program, or library, the first step is to understand what is being interfaced with physically. Then describe the inputs and outputs in software, determine data types and then create the functionality in the function block that is wanted to be repeatable. Physically, a motor starter is described electrically as shown in Figure 1.

Figure 1: Physical description to apply to function block.

Taking the physical functionality and describing it in code may equate to the function block shown in Figure 2.

The data table is in Figure 3.

Now, the structure is set up to allow the object to be reused. Some ladder logic in the function block could look like Figure 4.

Figure 4: Ladder function block example.

The function block code could also be written in structured text (Figure 5). Both blocks would act the same way.

Once the block is made, then it is usable for being called in a routine. The example in Figure 6 shows the function being called twice. The code is not made around it because the idea is just to show the objects usability. The code in the block is also incomplete for this example because alarming and auxiliary contact were not dealt with. The idea is to inspire the visualization of object-oriented code and how it can be reused.

Let’s review the steps to creating a reusable object. First, understand the physical functionality. Next, list out the inputs and outputs that interface with the PLC. At this time, decide if these inputs and outputs are integers or Boolean and assign a type. Then write pseudo code for the logic required to run the function. Put that pseudo code into executable code in the function-block logic. Remember that it needs to cover the generic function so that it can be repeatable. After that, program, compile and test. Once the bugs are worked out, then it is sharable with the rest of the controls group. The advantage of these reusable code blocks is that code starts looking the same, and it saves time.