Siemens S5 Interrupts

arthur.hauer

Member
Join Date
Nov 2016
Location
Paraná
Posts
34
Hello again!

Working on a S5-95 project (again).
Out of the blue, the PLC enters STOP mode. The malfunction, according to ISTACK, was caused by the analog input scaling FB (FB 250).
Disabling interrupts whenever either multiplication (FB 242) or division (FB 243) FBs are called seem to have solved the problem.
My question is: why does this solution work?
 
(warning, no actual useful S5 experience)

Maybe there is some kind of overflow causing the math instructions to fail? In an S7, that would trigger a programming interrupt. It could be a divide by 0 error, or a number too big/small after the math instruction.
 
(warning, no actual useful S5 experience)

Maybe there is some kind of overflow causing the math instructions to fail? In an S7, that would trigger a programming interrupt. It could be a divide by 0 error, or a number too big/small after the math instruction.

I agree.
 
First, thanks for replying!

Now, to the matter at hand:
(warning, no actual useful S5 experience)

Maybe there is some kind of overflow causing the math instructions to fail? In an S7, that would trigger a programming interrupt. It could be a divide by 0 error, or a number too big/small after the math instruction.

Both problematic blocks have overflow handling, and division block can handle division by 0. I don't believe that's the problem, but I might be wrong...

https://cache.industry.siemens.com/dl/files/319/1091319/att_6471/v1/6ES5_998-1MF23.pdf

"Arithmetic operations are executed independently of the RLO. The result is available in ACCU 1 for
further processing. The contents of ACCU 2 are not changed. "

Just a thought. Maybe you have to disable the interrupts because the arithmetic operations are also executed independently.

Regards,

I think this means the arithmetic operations occur without checking the RLO status. Something like unconditional execution. Even so, that's a possibility.
 

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