Analog Output Card not Outputting?

sthays10 said:

What would cause the math underflow or overflow errors? I've never really seen many PLC faults before.

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Any division by zero, or any other math operation that could cause an overflow or underflow.

Typically, for SLC Programs, as the very last rung of LAD-2, people will put in an unconditional latch of S:5/0 (the math overflow trap bit) to prevent the processor from hard faulting. It only hard faults if that bit is set at the beginning of the program scan.

Whenever unconditionally resetting the overflow trap, I like to include one more rung just above that, so that if S:5/0 is set, I latch another internal bit so that if I'm ever looking online at the program, I can see that an overflow has happened.

All of these fault bits are explained very well in the help files loaded with Logix 500.

sthays10 said:

What would cause the math underflow or overflow errors? I've never really seen many PLC faults before.

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A divide by zero or a result that is too large to be stored in the destination register. Add a rung at the end of the main program file that unconditionally sets bit S:5/0 if you don't want a math error to fault your processor.

Further info: Unfortunately, it wasn't a math overflow error. We got a 358h alarm, which is a non-recoverable specialty card fault. I had them reset it, and when they went back into run mode, it faulted again, this time giving the 355h alarm. Going to call AB tech support.

Thanks for the help so far. Will keep updating.

Followup: Contacted AB tech support. Analog input card (1746-NI8) is definitely bad. Unfortunately, we purchased it in November of 2012 even though we just installed it last year, so it's out of warranty. We're going to contact our local distributor to see how much it would be to repair it.

Thanks for the help!

sthays10 said:

What would cause the math underflow or overflow errors? I've never really seen many PLC faults before.



Great advice - I'll definitely have my electrician do that, and follow up to get to the root cause of the error.

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Most common cause is Divide by 0. Any divide math statements should always be preceded by a NEQ check that the divisor isn't zero.

FYI, for a SLC processor, it doesn't fault unless the overflow flag is set at the end of scan. It doesn't actually fault at the rung with the offending divide by zero. So, a common practice to avoid a faulted processor is in the last rung of the main routine put an OTU S:5/0.

Im in searching this 08 channel analog output card 1746-NO8I but not available in pakistan any alternate? (all 08 channel in use)