Analog Output Card not Outputting?

[sthays10]

Hope someone has experience with this. We've recently upgraded a system at a remote site. This is a AB SLC 5/05 processor with two 10-slot chassis. On the second chassis, we have a 1746-NO8I analog output card, and are using two outputs to set the speeds of two VFDs. Everything worked after installation, and has worked for 2 weeks.

The NO8Is have two outputs wired to a pair of VFDs for speed reference. The VFDs are currently giving us an error that there is no input signal.

Right now, the module status shows green, but the channel statuses are not lit.

I read the manual for the NO8I, and there really isn't much of a troubleshooting guide. We're going to be troubleshooting this remotely with an electrician who is not really familiar with PLCs in general, but has a computer he can hook up to the PLC.

Anyone have any experience with these cards or have a theory why the card would abruptly stop working?

Thanks!
Steve

 

[widelto]

Are you using an external power supply or using power from slc backplane ??

 

[danatomega]

I would look at the wiring first.
Then I would re-examine the troubleshooting guide on page 6-5 of the manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um026_-en-p.pdf

it says if channel status lights are off that the channels are not enabled.

Also from page 6-4
I/O error codes appear in word S:6 of the SLC 500 processor status
file. The first two digits of the error code identify the slot (in
hexadecimal) experiencing the error. The last two digits identify the
I/O error code (in hexadecimal).
The error codes that apply to the 1746-NO8 module include (in
hexadecimal):​

•​

​

​

​

50 to 5E (I/O errors)​

•​

​

​

​

71 (watchdog error)​

• 90 to 94 (I/O errors)

 

[James Mcquade]

if things worked ok for 2 weeks, you must ask yourself what changed?

wiring, power supplies, output cards, interface boards to the drives, plc code?

regards,
james

 

[sthays10]

Are you using an external power supply or using power from slc backplane ??

We are using power from the SLC backplane.

 

[sthays10]

if things worked ok for 2 weeks, you must ask yourself what changed?

wiring, power supplies, output cards, interface boards to the drives, plc code?

That's the thing - nothing should have changed. No computer was hooked up to modify the code, no wires were changed (we have a pretty strict policy and culture of modifications only to written procedures, and there were none). The error occured on a weekend when no one was around. The problem was discovered this morning when our operators arrived.

 

[sthays10]

I would look at the wiring first.
Then I would re-examine the troubleshooting guide on page 6-5 of the manual:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um026_-en-p.pdf

it says if channel status lights are off that the channels are not enabled.

The wiring is where we are going to start, and thank you for the link to the manual. I must have been looking only at the installation manual, it didn't have the troubleshooting section of the one you linked to.

I will have my electrician double check the module LED and channel LED status again to ensure they are still off.


​

 

[Helliana]

Have you checked the configuration of the card in the PLC program. You have to configure your output table for your card to your needed parameters. I'd take the wires off the card and check the output directly on the card with a multimeter (be sure your multimeter's current fuse is not blown). If you get an output there, then it is wiring.

 

[sthays10]

Have you checked the configuration of the card in the PLC program. You have to configure your output table for your card to your needed parameters. I'd take the wires off the card and check the output directly on the card with a multimeter (be sure your multimeter's current fuse is not blown). If you get an output there, then it is wiring.

Thanks for the reply - this is one of the first steps I plan on taking after I have them verify the channel LED statuses.

This may seem like a basic question, but my coworker and I have a disagreement over it: when you measure the current output of an analog output card, do you need to measure it with a shunt resistor (250 ohm) in series? I know that some of our devices require that shunt resistor to properly calibrate, and the resistor is needed when using a HART meter, but I didn't think it was required when measuring the output of an analog card. Coworker disagreed.

 

[oldnerd]

Also check your field devices.

 

[sthays10]

Found an additional, *small* problem. The processor is faulted.

When my electrician (again, not PLC savvy) emailed me about the VFD fault codes and the Analog Output LED statuses, he didn't mention the processor was faulted. He also didn't mention that the HMI was displayed transmitter values that were obviously wrong (ie, pumps were showing running, pressure indicated pumps were running, VFD speed feedback was showing 33hz).

So I'm now going to investigate the processor and why it's faulted. Thanks for all the helpful replies, I'll post back with what the problem was.

 

[James Mcquade]

its probably a minor error fault - math overflow/underflow.


regards,
james

 

[widelto]

Before resetting PLC, be sure to write down error code. If caused by program you can debug it, and avoid it happens once again.

 

[sthays10]

its probably a minor error fault - math overflow/underflow.

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

Before resetting PLC, be sure to write down error code. If caused by program you can debug it, and avoid it happens once again.

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

 

[danw]

FYI, when you measure the current output of an analog output card, do you need to measure it with a shunt resistor (250 ohm) in series?

It shouldn't matter. The milliampmeter needs to be in series, and if the shunt resistor is in a 'healthy' loop circuit it makes no difference because a viable healthy analog output will drive the current through both the resistor and the meter in a healthy circuit. A healthy circuit is one without ground loops or excessive resistance (more than the typical 500 or 800 ohms the loop is designed to drive).

Since it might not be clear whether the loop is 'healthy, I typically disconnect any wiring/load initially to check the current from AO, just in case the far end has a problem where excessive resistance has been inserted into the circuit.

Then I'll check the current with the rest of the circuit connected.