using Math instruction alot in program, will it harm ?

leonlai · Dec 20, 2012

Hi guys,
I have a stupid question, I used a lot of Math & Compare instruction (MUL, CPT, .. )in my PLC, will it harm or one day PLC will cause Faults after a long time of run ???

for example, PLC run good now, then after million scans PLC and thousands time of ON & OFF power supply, any residuals value somewhere somehow left in PLC and cause STUCK ???

If yes, how can I prevent it happened in furture ?
If not, may be I think too much >_<

thanks

TimWilborne · Dec 20, 2012

You didn't mention the type of PLC, but I think in general the answer is the same for all.

The main thing thing that lots of math instructions, or any instructions for that matter will affect is the scan time.

Most, if not all, PLCs go through some type of power up housekeeping step to clean up all those things you are worried about.

Tharon · Dec 20, 2012

No, the instruction types used would not cause your PLC to fail faster or slower.

Unless you do things like pulsing relay outputs very quickly... or other programming methods that could cause physical wear.

leonlai · Dec 20, 2012

Thank you all

shooter · Dec 21, 2012

maths are oke, however for next loops with exits and strange return instructions can cause problems with the stack pointer.

James Mcquade · Dec 21, 2012

The only thing that i would caution you about is the math overflow / underflow bit.

in an ab plc, if that bit is set, it will fault the processor.

i have the habit of unlatching the math bit at the end of the program.

regards,
james

Lancie1 · Dec 21, 2012

You didn't mention the type of PLC, but I think in general the answer is the same for all.

I would bet that it is an Allen Bradley MicroLogix 1400, as in Leon's previous post:

http://www.plctalk.net/qanda/showthread.php?p=516602#post516602

jac · Dec 21, 2012

James I've only ever seen that fault a SLC and you can solve that by placing a unconditioned OTU S:5/0 rung at the end of your LAD2 file.

OkiePC · Dec 21, 2012

Better yet, know the victim of thy trap:

http://www.plctalk.net/qanda/showthread.php?postid=14307#post14307

using Math instruction alot in program, will it harm ?

leonlai

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