Hey. So, we are upgrading an old Symax PLC to CLX and I've come across a "SHORT" instruction in one of the ladder rungs. I've downloaded the manual and can't seem to find a reference to that instruction. Do you guys know what it does? Thanks
So, I'm not sure I get the point of it. It says that only bits 1-16 are alterable and it is always de-energized, so it cannot be altered and it is normally closed. how does it affect the logic??
That's what I mean, it does nothing. Why bother putting it into the ladder logic if the PLC will process the same way, with or without it? It seems to serve no purpose.
I can't really speak to why the original programmer left it in the logic. Sometimes I have left an AFI (Always False Instruction (or Input?)) in my PLC program just because ...
Thanks again guys. I get putting an AFI or OPEN in series with a rung to void it from the program. And I also get putting an empty branch or SHORT in parallel with a rung to bypass a certain section or instruction. I don't get why a SHORT would be in series with a rung.
EX.
|--[ ]----[ ]----[/]----SHORT----------( )--|
I guess I should've mentioned that earlier, but do you get why I am confused now?
Perhaps the original programmer had OPENs in that location during debugging, then, instead of removing them, they simply changed them to SHORTs?
I find it amusing that, according to the PDF, both OPEN and SHORT are 'always de-energized', so you use a normally-closed contact to make the SHORT always true. Um, you could also do that with the OPEN bit, so why bother having two bits for this purpose? I think if I saw a --]/[--- contact with the label 'SHORT', I would read that as NOT SHORT and would expect it to solve false. Apparently, that's not the case.