Schneider EGX300 RS485 signal ground for PM9C
I've got one Schneider EGX300 (and also one EGX100) lying around. I'd like to utilize them to get the data from a bunch of PM9C energy meters via RS485.
All PM9C are 230V - powered.
The EGX300/EGX100 would be PoE - powered.
I'm a 100% noob in RS485, so maybe my question is trivial. But I find the docs somewhat self-contradictory regarding the "0V" contact in this case, so it would be nice if someone would clarify this to me.
According to the "PowerLogic System Catalogue 2011" EGX100 and PM9C were explicitly advertised as compatible (btw, it's a bit weird to have "PM9" instead of "PM9C" or at least "PM9P" on the right, I guess that's just a typo):
According to the PM9C manual, RS485 got the following contacts:
- D0, A/A'
- D1, B,B'
- 0V, Common
The drawing from the PM9C installation guide shows a separate cable shied (wrapped around 3 wires, including "common" aka "0V"). The shield is not connected to the "0V" / "Common" :
According to the EGX300-Installation-Guide, on the EGX300 side, RS485 (in the half-duplex configuration) got the following contacts:
- TX+ (unused in half-duplex)
- TX- (unused in half-duplex)
- RX+
- RX-
- Shield
There's no explicit "0V" / signal ground.
I'm not sure if the "Vdc-" (contact 6, the "input power when not using PoE") could be treated like "0V".
The recommended cable is "Belden 9841", which is a single twisted pair (shielded).
According to the "Modbus™ Communications System for MicroLogic™ A, P, and H Trip Units Instruction Bulletin", "0V" is just "Vdc-". In a scenario with a shared DC power source. It does not mention ether PoE or multiple separate DC power sources. Also, it's about 4-wire scenario and a different product.
According to the most detailed (yet somewhat unofficial) PM9C-related info I found (the "Resolving PM9C Schneider Power Meter Modbus Communication" by Claire Rousseau):
>Rules for SG wiring:
- Connect PM9C SG directly to the Modbus master's RS-485 common/SG terminal.
- Do not connect SG to protective earth (PE) at the PM9C end. The meter's SG is a logic reference, not a safety ground.
- Do not create a multi-point ground by also bonding SG to PE at the master — this forms a ground loop that injects 50/60 Hz noise into the data lines and may damage the transceivers.
- Use twisted pair for A/B and a third conductor for SG within the same overall cable jacket to keep the reference close to the data pair.
Why does my PM9C not respond even with correct baud, parity, and address?
The most common cause is a missing signal-common (SG) wire. The PM9C's RS-485 transceiver requires SG connected to the master's RS-485 common terminal. Without it, the common-mode voltage drifts outside the EIA-485 receiver range and the slave ignores all frames. Wire SG and re-test before suspecting the meter itself.
So... What's is the correct approach?
- Use "Belden 9841" (or equivalent "1 twisted pair + shield").
- connect the cable shield to "0V" on PM9C
- Use "Belden 9842" (or equivalent "2 twisted pairs + shield")
- connect wire-1 from the second twisted pair to "0V" on PM9C
- connect wire-1 from the second twisted pair to "Shield" on EGX300
- connect the cable shield to "Shield" on EGX300 as well
- connect the cable shield to external protective earth (once, at any point)
- Use "Belden 9842" (or equivalent "2 twisted pairs + shield")
- connect wire-1 from the second twisted pair to "0V" on PM9C
- connect wire-1 from the second twisted pair to "Vdc-" (contact 6) on EGX300
- connect the cable shield to "Shield" on EGX300.
- connect the cable shield to external protective earth (once, at any point).
- ....
The EGX300 terminals, just for reference (from EGX300 - User Guide.pdf):
