27.1 INTRODUCTION

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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.