Lets say I want to show on the HMI when an 24V air valve opens and closes

Hmm so it would be like using the signal from the sensors on the cylinder that turns on when the cylinder is fully out or fully in since they pic up the magnet that's in the cylinder because I'm not sure fully on what u mean with the limit switches I guess I might have to mess wit it for a bit.
But I do know what u mean about the problems we have had al l.j them problems

so it would be like using the signal from the sensors on the cylinder that turns on when the cylinder is fully out or fully in

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Yes

On the HMI you can display both the PLC outputs showing which solenoid output is active, and the two sensors showing if the cylinder reached the desired position. This will help to show how the cylinder should be operating, and the combination of output/input can also be used for alarms.

Hey man, we have all had to deal with undocumented equipment. Generally, in the past, I would just make an io list and also use that list to label io in the program. It is time consuming, but it is the gift that keeps on giving.

You could use pressure switches on the air valve outputs to monitor state...