Wiring Fieldpoint AO and AI

[kbdas]

Can someone help me with this wiring problem?
I need to control a bank of six 4-20ma pressure control valves. The valves have 4 wires each: a) 24VDC b) 4-20ma setpoint signal c) common (24VDC and setpoint signal) d) 4-20ma output signal. Using an FP-AO-200 I can easily write a setpoint to a valve using (b) and (c). The problem lies in reading the "actual" pressure value from connection (d), the 4-20ma output signal. This output is NOT referenced to the common but to the 24V supply line. In other words, I can read the current output signal easily using a multimeter connected between the (a) terminal and (d) terminal. I cannot figure a way to do this with my FP-AI-111 since all the commons on this module are tied together. Am I missing someth
ing here?

[Aaron_G]

The key to the answer is realizing that each FieldPoint I/O module is isolated. Thus the common of the FP-AI-111 and the common of the FP-AO-200 (as measured with respect to the C or Com terminals of each terminal based) are not electrically connected unless you have physically inserted a wire linking the terminals.

Thus you can wire the (a) and (d) terminals to the FP-AI-111 pins Com and Iin1 respectively, while wiring pins (b) and (c) to the FP-AO-200 pins Iout1 and Com respectively. The downside is that any other measurements going into the FP-AI-111 are now referenced to the 24 Volt signal. However, if your other signals are floating or can be referenced to the 24 Volt level, you are fine. If the other signals can not be referenced to the 24 Volt level, t
han you need to use an additional AI module to measure that one signal.

Regards,
Aaron