Engineering student on a placement. I’m helping the maintenance team wire 9 energy meters (Modbus RTU, 9600 8E1) to a Pi gateway that sends to Hermetry. The run is about 140 m along a cable tray next to some VFD motor cables. We use shielded twisted pair (Belden-type, one pair plus drain).
The gateway logs about 2-5 % CRC errors, more when the big extraction fans start. The electrician and the senior engineer disagree on the shield: one says connect it to earth at the panel, one says to the 0 V of the 24 V supply, and someone on a forum said both ends. I’d like to understand which is right and why, I need to write it up for my report.
Do you also have a separate 0 V / common wire between the meters and the gateway, or just A and B?
Just A and B and the shield. The shield is connected to the gateway’s GND terminal right now.
Isn’t earth and 0 V the same thing in the end?
Not at 140 m next to VFDs it isn’t.
Using the shield as your signal reference is the main problem. Noise current from the drives flows in it and straight into your receiver’s ground.
I sort this out on every substation job we do, and I treat signal, reference and shield as three separate conductors.
Signal: A and B on the twisted pair.
Reference: RS-485 needs a common mode reference. Add a third conductor (use a 2-pair cable, the second pair for 0 V) and connect the 0 V of every transceiver to it. Without it, the ground difference between the panel and a meter 140 m away eats into the receiver’s common-mode range, and VFD noise does the rest.
Shield: to protective earth, at the panel end, with a 360° clamp where the cable enters the cabinet, not a pigtail to a terminal. At the meter end either leave it open or connect it to earth through a 10 nF capacitor, which gives you HF shielding without a low-frequency ground loop. Both ends hard-bonded is fine only where the earthing system is meshed and equipotential, which next to a tray of VFD cables you shouldn’t assume.
Also check: 120 Ω at both ends of the line only, bias resistors once at the gateway, the tray separation from VFD cables (cross at 90° if you must), and no star wiring.
For the write-up, it helps to log the error rate. Have the gateway count CRC errors and timeouts per poll cycle and send them as telemetry, e.g. mb.crc_err and mb.timeouts, so you can show the before/after on a chart.
+1 to logging the error counters, we have an alarm on ours when crc errors go above 1 % for an hour.
We pulled a 2-pair cable last week, used the second pair for 0 V, moved the shield from GND to the earth bar with a clamp, and put a 10 nF to earth at the far end. CRC errors went from ~3 % to 0.02 % over 4 days, and nothing changes when the fans start. The before/after chart is in my report now. Thank you.
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