PLC Programming Methods

A PLC is a Programmable Logic Controller and was first used to replace relay circuits. IEC 61131-3 is the electrical standard for PLC programming methods, though many programmers aren't formally trained and don't even know this standard exists. Each programmer develops his own style and methods for programming, just as computer programmers do.

This is the most common of the PLC Methods. The diagram looks like a wiring schematic for a relay circuit with the power line on the left and the outputs on the right. This is the main programming method for PLCs in industrial controls. It is referred to as a ladder diagram because when you look at it, it looks like a ladder with the inputs and outputs of the program contained on each rung. As an example, you have a proximity sensor that when triggered, sends 24VDC to the PLC. In the program, you want that trigger to power on a motor. The rung for that sequence will look like this: –||—–( )–, where –||– represents the input from the proximity sensor and –( )–represents the motor output.

The function block diagram method is also a pictorial method of programming. It consists of blocks for each function that show the inputs and outputs for more complex sequences and lines drawn between each block illustrating what each output will do and what will affect each input. For example, you may have a scale in your process and if you want an alarm to sound if the weight measured on the scale is too high or too low, then the scale will have a box with the line drawn from the weight output to the variable input of the alarm box. The output of the alarm box for either the too high or too low alarm will go to an alarm horn and/or light.

The sequential function chart method is another pictorial method. It most closely resembles a flow chart, only it's more complex. There are three primary elements in a sequential function chart: steps, actions and transitions. Each step contains the logic for a particular portion of the process. As an example: weighing an item, checking for alarms and sounding the alarm if the weight is out of limits. The actions are the individual activities of performing the steps. Transitions move the process from one step to the next.

This is a text language and is not used often with PLCs, though many manufacturers do allow for this within their PLCs' programming software. It is very similar to Pascal or BASIC, and for people trained with computer programming, it can be the easiest. Complex math or decision making processes are often easier to accomplish with structured text as it can be done on one page versus many rungs of a ladder diagram.

The instruction list method is probably the most complicated method, as it most closely resembles Assembly language. This can be useful for processes that repeat a small function often. Though it is a powerful method, it is often easier to just program the process in a ladder diagram than it is to learn how to program with an instruction list.