Rslogix 5000 V20 Latch followed by unlatch

So came across a rung today with always on bit driving a latch output followed by an unlatch output with the same tag name. What is reason behind this? does this not defeat the rung?


---I I---------------------(L)---------(U)---
Always on .. valve 1 .. valve 1


Any insight?

Looks like someone wanted to disable the output from coming on without deleting the run. That's my guess.

JaxGTO said:

Looks like someone wanted to disable the output from coming on without deleting the run. That's my guess.

Click to expand...

This is a possibility. However its not advisable with an IO address considering that the L5K platform has asynchronous IO updates. It is unlikely, but still possible, for an update to happen at just the wrong time, turning on the output until the next IO update.

My guess is someone who misunderstood something about PLCs and what they were doing programmed that.

JaxGTO said:

Looks like someone wanted to disable the output from coming on without deleting the run. That's my guess.

Click to expand...

A rung can be disabled with a AFI command. this is the first time I have seen two outputs on the same rung, one would think it should generate some sort of error.

this sort of "two output instructions in series" is valid in RSLogix5000 projects ... (ControlLogix and CompactLogix) ...

it is not valid for RSLogix5 or for RSLogix500 ... in those older platforms, yes, you WOULD get an error and the rung would not verify ...

going further:

A rung can be disabled with a AFI command.

Click to expand...

well, not exactly ... at least not in the same sense that most people interpret the word "disabled" ...

the AFI rung will still be executed by the processor ... specifically, it will NOT be "skipped over" ... in other words, suppose that you use an AFI at the beginning of a rung - which has an OTE instruction at the end ... in that case the OTE will indeed be executed with FALSE logic - which will cause the processor to "go write a ZERO" into the OTE's bit/box ...