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September 26, 2011 / Kasey Mirecki

How to Invert the Logic of an Output – Alternate Circuit

Examples of “How to invert the logic of an output” by using the ON state of an input.

This can simplify your programming logic and allows you to do more with only one sensor connected to the PLC.

Whenever the leading edge of input signal (X0) is detected, the state of output (Y10) is reversed.
The operation can be easily achieved by using Alternative Out (ALT) instruction as follows.

Engineers who have used FPWin GR software and now switching to FPWin Pro (IEC 61131-3 Programming System) may have difficulties performing the same Alternate instruction.

The examples below covers the use of the Alternate instruction:
Ladder Logic – FPWin GR

Ladder Logic – FPWin Pro (direct addressing)

Using global variables – Identifier)

Time Chart

Please note:

Alternate instruction is not supported for FP0 and older PLC models. (FP1, FP3 and FP-M, etc.).You need to implement the logic below to alternate PLC output.


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