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