The AFI (also known as Always False Instruction) is a ladder logic rung instruction that sets the rung output to false; generally for the purpose of debugging and trouble shooting the logic.

In the world of Rockwell-AllenBradley, AFI instructions set is only available in RSLogix5 (PLC5) and RSLogix500 (ControlLogix). There is no built-in AFI instructions set in RSLogix500 (SLC500).

Therefore, if an AFI function is required in SLC500, you will have to “whip-up” your.

The ladder logic below shows how you can do it in two ways.

METHOD 1: AFI using Inverse Shunt
The logic is very simple and needs no further explanation.

METHOD 2: AFI using OTU coil (Output Unlatch)
The OTU instruction is a retentive output instruction that can only turn off a bit (it cannot turn on a bit). We can use this characteristics to our benefit for the purpose of AFI instruction set.

Both AFI implementation has worked for me in past projects and maintenance support contracts.

I hope that helps anyone looking to for RSLogix500 AFI instructions set.

Cheers!