An AB 1746-NO8I analog output module will output 4-20mA correctly but after 30 minutes or so the output will steadily drop until no more than 11mA will be output. All 8 channels affected. Replaced PLC power supply. Replaced rack. Module is new. External 24VDC loop power. Switced to internal power and rewired output, same problem. AB techsupport is stumped and so are we. Output is linear but max out is 11mA.

 

Try disconnecting your analog output wiring and simply install 250ohm resistors across the 4-20mA terminals on the analog output card. Then with a voltmeter, periodically take readings across the resistors. You should read 1-5Vdc across those resistors.

If the any of the outputs clamp at 2.75Vdc (ie; 11mA * 250ohms), then I would think you have a fault internal to the plc or analog output card.

If the output card works fine with only resistors installed, then the fault might be in the wiring, external devices, or due to grounding issues. If this is the case, what are you wiring to? You may need 4-20mA isolators on your analog output card.

Kind Regards,
Donald Lupo ([email protected])

 

Check instrumentation ground (nor more than 2 V) & use signal isolator (In good make e.g. acromag)

 

I agree with Mr. Lupo's test.

Since you have powered the loops with both an external loop power supply and internal power supply and gotten the same results, the likely fault is in the PLC (module/backplane), unless your field loads are changing resistance over time.

His prescribed test will indicate whether known fixed resistance loads will produce the same result as your field loads.

Let us know what happens.

David

 

I may have seen a similar problem with the 4 analog output version (1476-NO4I). We powered it internally. Used a new 1746-P1 rack power supply in a 7 slot chassis.

In our case, I put it in and checked the 4-20 mA which seemed to be fine. Later on our customer mentioned that he could not get full pressure from the device that we are feeding. I haven't been able to check if the mA output is still at 20 mA for 100% output but this sounds to be similar enough to warrant more investigation.

Any resolution will be posted here (with a KB # if Rockwell is involved at our end).

Regards,
Russ

 

I am stumped too.

Try this. Move the card to a different slot in the rack. Set the output to a fixed value and feed it to a 250 Ohm resistor and see what it does.

This sounds like some kind of wierd software bug.

 

Hi Fred,

First I would suggest you to check the Registry Values in PLC for the correspoding AOs. If these values correspond to 11 mA, then the problem is coming from the Soft end. Then you can proceed with the tests proposed by our friends'.

Good Luck!!!

Ramu

 

For an answer to this it would be necessary to have full details of everything connected to the loop. I have seen these symptoms before, and it has always come down to a voltage drop cause by too high a load at 20mA.

 

It turned out to be a bad card. I guess bad out of the box. Thanks for all the suggestions.