I was using indirect addressing to fill an array with diagnostic data. I whacked an AFI in front of the rung to 'disable' it until I was back in Australia to look at it properly, but 20 minutes later, the PLC faulted again, on the same rung.

I learned it after I put some diagnostic code in a PLC from my hotel room in Papua New Guinea, only to find that it caused the PLC to fault the next day (that'll teach me for trying to write overly complex diagnostic code after a 12 hour day on a terrible internet connection). The timer was just an example - there are much more, uhm, 'creative' ways of finding out this difference. Of course, that is why I visit these forums every day. I always used online edits for testing logic like that. I guess I have just never used it on a timer, counter, etc so I have never ran into the issue. Thinking back on it though, I can see this changing other logic if the rung is parsed after another rung using a counter or timer. Of course, that doesn't mean it isn't out there, but I wouldn't consider it a good programming practice. You are absolutely right on this one, but I can't think of any example where you would have something energize with a single XIO Timer.TT alone controlling a bit. Unfortunately, there's no way that I know of to 'comment out' a rung of ladder code - only in ST (which doesn't apply to LogixPro). There's a difference between a rung being false and a rung being not there. If it's rung-in condition is false, then it sets it's.

Just because a rung has an always false at the start, doesn't mean it doesn't do anything.

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