How do I use variables in ladder logic

Say I have tag.0 and I want to increment it on a certain trigger. I'm sure there would be some syntax such as tag.var where var would increment from 0 to 1 to 2 and so forth but I don't know how to do this.

A second request is what if I want var to change randomly instead of sequentially between 1 and 20?

What brand and model PLC are you using?

As I type this I don't know what you are programming in. If you are using ladder logic, this should get you close.

I'm using Studio 5000 v28 and I'm using ladder logic. The PLC is emulated.

"Randomness" is tricky in a PLC, as they are intended to be deterministic. The simplest I've seen is to read the system clock. I've also seen things mocked up where you smash a bunch of trig functions together and see what comes out.

Regardless, you usually use the smallest digits out of the result, not the whole thing.