Symax unknown "short" instruction

KevinO · Mar 20, 2015

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

rdrast · Mar 20, 2015

What processor?
I'm not familiar with that from the 300's or the 50's.

danatomega · Mar 20, 2015

In this document:
http://v1.graybar.com/automation/ga_manuals/Hardware/SyMax/Troubleshooting%20Guide/Chapt03-Pt2_Proc.pdf
in PDF page 16 of 18 it mentions register 8176-19 '"SHORT" This bit is always de-energized. A normally closed contact of this bit is used to display "SHORT" in the ladder program on the programmer.'

Also makes mention of another bit called "OPEN"

KevinO · Mar 20, 2015

Thanks a lot guys. It is a 400 processor.

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??

danatomega · Mar 20, 2015

It affects the logic this way : if it's always off and normally closed then it's always passing logic.
It's static and unchanging.

KevinO · Mar 20, 2015

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.

danatomega · Mar 20, 2015

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 ...

mellis · Mar 20, 2015

Think of it like this:

In the AB world you can put an empty branch around a contact if you want to bypass it (make it irrelevant to the logic). This is a short.

In the AB world you can put an AFI instruction and it will always be false. This is an open.

KevinO · Mar 20, 2015

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?

Eric Nelson · Mar 21, 2015

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.

-Eric

KevinO · Mar 21, 2015

Ya, in any case, I'm pretty sure just ignoring it when programming the CLX logic, won't affect how it works. Thanks for your help again guys!

Symax unknown "short" instruction

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