The wiring puts a single, 4-20mA current signal through a 250 ohm resistor. The 1-5Vdc voltage drop across that resistor is connected to each of two 1756-I16 AI cards.
The manual for the 1756-I16 card shows a single-ended wiring scheme for voltage signals.
This may, or may not be the wiring in question.
If it is, how could channel A on input card A see a different value than channel B on input card B ?
They can't. Both channels get exactly the same signal, a voltage paralleled from a common source, 4-20mA signal.
Could common mode offsets account for the Ch A vs Ch B difference?
While single ended AI cards are notorious for having common mode problems, manifested as an offset on each channel when multiple devices at different ground potentials are connected to a single ended card, that is apparently not the case here.
Common mode problems on single-ended cards affect all channels on a given card, because they all share a common ground. The complaint is typically, "the first three channels worked fine until I connected signal 4 and now all 4 channels are driven off-scale."
That is not the case here. And it is unlikely, since 2-wire transmitters are typically ungrounded (floating), so even if one side of the power supply (p/s) is grounded, the ground potential difference is limited to the difference between the p/s and the AI (RTN terminal) grounds. This assumes all the transmitters share the same power supply.
Furthermore, 3 signals, each split to dual channels, have worked without a problem for some period of time. as singled ended voltage signals.
I therefore conclude that a difference due to true differences in the signal source is unlikely.
However, a difference in signal values might be accounted for in the
sampling of signal values for Channels A and B.
The signal in question is flow signal, which has a significant amount of noise/variation:
"I would say about 300 to 500 per second the flow vary."
"If I watch the PLC AIs channels live/online, I can see the difference fluctuate up to 200 quickly. Its flow reading and I know it vary, but shouldn't vary by that much."
The signals which are not problematic are apparently signals that have less variation than the flow signal:
"No, the other transmitters are configured to alarm with about a 10 difference because its temperature and pressure, and less fluctuation."
The less noisy signals do not see a deviation.
The noisy signal sees a deviation between Ch A and Ch B, even though the signals at the AI inputs are identical.
I'm an instrument guy, not an A-B guy, so I do not know how the input values are sampled in time, with respect to one another.
But I suspect that there is enough time lag between the sampling of Ch A and the sampling of Ch B that the difference is caused by the inherent noise/variation in the flow signal.
Ch A is sampled at T0 (time zero) with a value of 12444 units.
Ch B is sampled (arbitrarily) 50mS later by which time the flow signal is now 12645 units, only a 1.3% change, but enough to trigger an alarm.
An A-B guy will have to comment on how channels on two different AI cards are sampled and what parameters (scan time, scan order, whatever) affect that sampling and what a likely time skew between AI card A and AI card B samples could be.
It might be that filtering/damping/averaging the flow signal at the flowmeter so that non-simultaneous sampling will not see as great a difference as is now seen.
