24. Instruction List Programming

• This is the most basic form of PLC programming that most resembles assembly language programming.

• Until recently this was the most common programming form, but the advances in computer graphics in the 1980s made graphical programming languages (ladder logic) more popular.

• This graphical language is part of the IEC 1131-3 standard.

• A simple example is given below.


• A list of common mnemonics is given below. These include both the IEC 1131-3 values, and those specific to Allen Bradley controllers.


• A list of the official IEC 1131-3 mnemonics is given below,


• Here there are three types of modifiers,

N: negates an input or output

(: nests an operation and puts it on a stack to be pulled off by )

C: forces a check for the currently evaluated results

• This language uses a stack structure, like an HP calculator. When the open brackets are used the operations are pushed on the stack, closed brackets pull them back off.

• An example of an instruction language subroutine is given below. This program will examine a BCD input on card I:000, and if it becomes higher than 100 then 2 seconds later output O:001/00 will turn on.



24.1 Problems

Problem 24.1 Write the ladder diagram programs that correspond to the following Boolean programs.