Recently had an issue that I tracked down to a frozen rung of logic. The PLC is an AB PLC5/40E. the rung of logic is simple. (I hope the text formats correctly)
--][-----[ons]---------(U)
-B3/0 -- B3/1 ------- B3/0
There is another branch of the output that moves a number into an N7 register, but that's harder to draw in text, and I don't think it could cause this. So what I was getting was that B3/0 and B3/1 were both on and staying on. Neither of the bits is used anywhere else in the program. B3/0 is set by my MMI, and the file is scanning. All I had to do was toggle B3/0 and the rung started acting normally again. But since then I've been trying to make the rung lock up again and cannot do it.
There are about 100 of these rungs in the PLC and its been running for probably 5 years now, apparently without having this issue. I say that because I can't see any way for it to reset without intervention from a programmer, and when it does happen it disables an important portion of the code. I actually saw it on my test system. So has anyone seen anything like this, or can you think of a way this can happen? I would feel much better if I knew a way to reproduce it.
--][-----[ons]---------(U)
-B3/0 -- B3/1 ------- B3/0
There is another branch of the output that moves a number into an N7 register, but that's harder to draw in text, and I don't think it could cause this. So what I was getting was that B3/0 and B3/1 were both on and staying on. Neither of the bits is used anywhere else in the program. B3/0 is set by my MMI, and the file is scanning. All I had to do was toggle B3/0 and the rung started acting normally again. But since then I've been trying to make the rung lock up again and cannot do it.
There are about 100 of these rungs in the PLC and its been running for probably 5 years now, apparently without having this issue. I say that because I can't see any way for it to reset without intervention from a programmer, and when it does happen it disables an important portion of the code. I actually saw it on my test system. So has anyone seen anything like this, or can you think of a way this can happen? I would feel much better if I knew a way to reproduce it.