I'm no engineer, but I'll take a look.

Greetings Smart Masses,
Thank you for adding me to the community.
I've become part of an unusual phenomenon involving an ABB drive and a IA1-D-VI analog input unit. Typically in a series circuit current stays the same and voltage drop across resistors changes based on E=I*R. I know this and as long as the total resistance is less than what the transmitter is rated for you'll read the same current anywhere along the circuit. However I have two devices displaying rpm's based on 4-20mA output of the drive and only one device is displaying the correct speed. The correct display has a resistance of 250 ohms. The other device is reading -1.8 mA. It has a resistance of 50 ohms.

How do I correct this?
Assuming that the input is configured properly and wired properly, then one has to consider the difficulty in driving the 4-20mA loop current through two devices in series, particularly with 4 wire devices that are AC powered, is that the each device's input signal is eventually referenced to a signal ground and sometimes that signal ground is at a different potential than either the source ground and/or the other device source ground. That difference in ground potentials is a common mode voltage which can
- create a ground loop with current added or subtracted to the loop current.
- drive the loop signal off scale or create an offset, positive or negative, to the input signal. Whether the input signal drives upscale or downscale depends on the polarity of the ground potentials.

The negative current could be a ground loop or it could be the input polarity is reversed so the IR drop at the input is 'negative'. Given that it is not a minimum 4mA, I suspect that it is a ground loop. The one device that reads correctly likely has an isolated input that does not 'see' the common mode voltage.

The conventional solution to ground loops and common mode is either to use a signal splitter which normally have isolated outputs or to use a current loop repeater/isolator for the signal feeding the problematic device.
Thanks for the reply. I know I have the polarity of the loop wired correctly. So, I'm thinking the problem is with how the drive is grounded in comparison to how the loop is grounded. The only problem there is the only "grounding" of the loop is the cable (shield/drain) wire is connected to the "shield" terminal in the drive and nothing else. I'm using shielded twisted pairs for each current loop from the drive.

What makes this circuit confusing is that it's wired exactly like another circuit displaying torque to the operator and that circuit is working fine.
Shield ground?
Your shield should be grounded at one end only (unless it's Profibus and your site grounding meets Siemens' expectation of equipotentiality, see note below), precisely to avoid carrying ground loop currents, because as we all know, potential (voltage) differences are what drives electrons (current). Is your shield grounded at only one end?

The ground potential differences I'm talking about is the ground that the analog inputs (plural) eventually reference in the input circuitry, not the shield ground, although the signal ground might well be the frame ground which is presumably connected to the bonded earth ground. It's a whole bunch to sort out.

You have the evidence of a ground loop in front of you. The question is "how much time and how much effort and how money are you able to spend to resolve it?"

My anecdotal evidence for my approach:
I have seen exactly one documented instance of an HVAC blogger who actually tracked down the source of a common mode problem that turned out to be a neutral/ground wiring issue in the plant lighting that affected his low signals. He mentioned that it cost him a day or so of troubleshooting and he thought it worthwhile to blog it. But that's one instance I have read of in reading control blogs and fora for almost 2 decades.

Siemens has a note at the beginning of their instrumentation user manuals that explains that sites where instrumentation is to installed needs to be "equipotenial", a code word meaning that electrical grounding needs to be constructed so that all ground have the same potential. Nice try, Siemens, but welcome to the real world. When I was kid, an EE friend of the family spent the better part of a decade (one week on-site, one week at home) working out grounding issues on the frozen ground or ice cap for the USA's NORAD ICBM missile radar sites in northern Canada (his job was so defense critical that the state department intervened to get his work visa approved). The family friend could not publicly document his efforts because of security issues, other than mentioning in passing to my Dad the kind of work he did.

All that said to show that I'm going buy a 4-20mA loop splitter whose outputs are isolated, install it and be done with it.

You're dealing with a 3 phase drive and some other devices. Be my guest, go chase the ground potential difference between these devices and, please, report back how you succeeded. I love to read success stories.

To each his own.
If it were me, I’d test the bad boy AI with a battery powered simulator/calibrator, which being battery powered is floating with respect to all grounds, in order to make sure the AI is functional with a non-grounded signal source. If the AI is good. then I’d fix the ground loop issue with an isolator. An isolator will not fix a bad AI.
My shield is only grounded on the drive's PE terminal. This drains to the equipment ground. The drive is a ten horse ABB ACS800. The analog out that is "driving" the circuit is rated at equal to 700 or less ohms. I've connected a process calibration tool to the display I'm having trouble with and it worked. It's just not working while connected to the drive.
My only comment to the "equipotential" grounding is thus:
The drive and the other electrical equipment are grounded on the same ground bus, so there may be as much as a millivolt of potential difference, but I wouldn't think it would be much. Also, I've worked with noisy circuits in the past where the noise has actually caused erratic behavior in servo valves. Unfortunately this new job didn't come with a Scopemeter.