4-20MmA Wiring question

I've been tasked with hooking up / wiring a Watson Marlow qdos30 universal pump. Attached is a screen shot of the wiring diagram. I need to use pin 1 run/stop and pin 3 4-20mA. I'm a little confused since both connections share the same GND (0V). The run/stop will be coming from a 1756-OB32 getting power from a 24v power supply and the 4-20 from a 1756-OF8. I guess I don't understand how it will work since the 1756-OF8 is getting its 24v from the chassis backplane. How can both these connections to the pump reference the same 0V pin4???

Thanks

because zero volts is just the common for potentially several power sources...

Ok thanks
I guess the keyword is "referenced to pin4" for the run/stop signal, it's not actually sinking?

Yes, it's similar to a system that needs a 5 VDC, 12 VDC, +-10 VDC, 24 VDC, and 48 VDC power supplies and all the 0 VDC wires are tied to a common (often earth ground) point. No harm in that (until it causes you problems).

Your 1756-OF8 module probably does not come out of the box with the -24 VDC connected to ground. Check the user manual before you do connect that. The ControlLogix 1756-OF8 has non-isolated channels that have the RTN terminal connected together internally.

Using different voltage supplies on other non-isolated modules has been known to cause interference problems at times. There is a 1756-OF6CI Isolated Module available. That would help to prevent signal interference between the voltage levels.

If you do tie the RTN terminals to a common ground, make sure that that ground terminal measures 0 volts between it and the other remote -24 VDC power supply terminal. Some ground systems are worse than others, and that could cause a problem.