Analog input channel acting strange

My analog input 4-20mA channel are acting different. It sometimes reads a good mA value and sometimes doesn't. It happens only on 3 channels of the 6 that are being used. The rest three work just fine. However, if I put the meter around it I can always see a good mA value. I am confused to why the card wouldn't see it if the meter is seeing a good value. Could it be something in the grounding that may cause something this strange? It's a Honeywell C300 DCS control system.

what devices are wired to the 3 good inputs what are wired to the 3 bad?

Ie loop powered, device powered.

what do you mean by sometimes value isn't. can you see a good value on meter but DCS is fluctuating. Sounds like noise and meter may not be sensitive to pick it up or is filtering it.

I have seen it before on non isolated plc input cards where the value chases around the meter reading. Usually when an active tx is on the non isolated cards and I believe it has to do with difference in potential between device commons.

Best bet is to get isolated channel cards, however sometimes this is not what is spec'ed or pratical, on RSLogix the input filter frequency can be adjusted and has always mitigated the noise for me, albeit at the price of a slower update rate, which depending on the process may not be an issue.

What I mean is, if I see 12mA steady signal on the meter I don't register that on the DCS. It comes up as 'Not a number' meaning it doesn't see it. We changed out the card with a brand new one. It works for few hours then it stops.
The 3 devices are from 3 different actionpaks. The actionpak has the motor VDC as input and the output is 4-20mA to the module. It is very intermittent. I don't think it is anything to do with noise.

are you looking at the raw value coming into the dcs or is it going through some blocks first. Math/Scaling

I'm with labeledas on this -

There is some small liklihood of a math block affected by a conditional statement, but not likely The higher probability is a ground loop causing problems. A 4-20mA signal can 'ride' on common mode voltage.

Frequently the ground current adds to the mA current, then the ground loop drives the signal way off scale.

There are conditions where a handheld, battery powered milliampmeter will not 'see' the ground loop, because it is floating.

One way to test for ground loops is to put your meter on DCV and check from each signal lead to ground, then also check for ACV from each signal lead to ground. One expects to see the DC voltage that drives the current loop, but the (-) should be a low offset from zero. Common mode can come and go over time, depending on ground faults in the field.

Are the 'action pak' field modules isolated, so that a ground loop can't be created between the field device and the DCS
analog input card? You should get the spec sheets for those 'actionpaks' and confirm that each is isolated from input to
output and from its power supply.

DCS analog input cards vary in their ability to handle common mode. Any signal connected to a single ended, non-isolated
AI card can produce a common mode signal which affects any of the other signals as well. Are there other devices connected to this particular AI card?

I have never seen an isolated converter module fail so that the isolation failed and non-isolated signal passes through. Typically when they fail, they fail to pass any signal through. But a field wiring problem, like a unwanted connection to ground on the DCS side of the signal could be eliminating the common mode that the isolator is producing.

Thanks guys. So this will probably sound stupid but there are jumpers on the card assembly that were set for differential. I am going to change them out to single ended and start back up, but I think that was the issue. Will update this thread with sufficient run hours without issue once done.

I suspect not. Differential is the better sampling technique of the two. Differential tends to have better common mode rejection than single ended, which has none.

In some case, the input wiring needs to be changed for single-ended.