Siemens S5 Interrupts

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?

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,

(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.

mk42 said:

(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.

Click to expand...

I agree.

First, thanks for replying!

Now, to the matter at hand:

mk42 said:

(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.

Click to expand...

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...

gclshortt said:

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,

Click to expand...

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