Lurker, automation engineer. We have a Pi-based gateway in building A reading 8 energy meters over RS485. Five meters are in building A, three in building B, about 80 m of shielded twisted pair between them, 9600 baud, 120 ohm at both ends. The gateway sends to Hermetry, each meter is a child device.
The MAX485 on the gateway’s RS485 hat dies every 3 to 6 weeks. Swapped hats four times now. When it dies, the A side reads fine again after a swap, nothing else changes. Of course the meters just stop reporting and nobody notices until someone opens the dashboard.
Shield is grounded at both ends. The two buildings have separate supplies. What kills a transceiver this regularly?
Cheap clone chips? We had a batch of clone transceivers that were all bad.
Could it be lightning? 80 m outside sounds like an antenna.
I’m a facility manager, not an engineer, but we had exactly this with meters in our hotel’s annex, and our electrician explained it to me. Two buildings with separate supplies don’t sit at the same ground potential. Your RS485 signal is measured against the gateway’s ground, and the MAX485 only tolerates the bus being about -7 to +12 V away from its own ground. When something big switches in building B, the difference jumps past that and the chip dies. Shield grounded at both ends also lets current flow along the shield between the two grounds, which makes it worse. He measured several volts AC between the two grounds with a meter.
What fixed ours, per the electrician:
- An isolated RS485 transceiver on the gateway (ADM2587E or ISO3082 type, or a ready isolated module). The bus side floats with the bus, the Pi side stays on the Pi’s ground.
- Shield grounded at one end only, and a third wire as signal reference between the transceivers.
- Surge protection (TVS diodes made for RS485, like the SM712) at the building entry, which also covers Michael’s lightning point.
Since the change, two years without a dead chip.
And for « nobody notices »: our gateway script sends v1/gw/disconnect for a meter that stops answering three polls in a row, so the child goes offline in Hermetry, and the profile of the meters has an offline alarm. We get a message within 15 minutes.
Measured 9 V AC between the two building grounds, so that’s it. Isolated module ordered, shield now only at the gateway end. The disconnect after missed polls is in the script already, the offline alarm caught a meter we unplugged on purpose 15 minutes later.
ok not the chips then, good to know
- Vous devez être connecté pour répondre à ce sujet.
Rejoignez la discussion
Connectez-vous pour poser et répondre à des questions, aimer les messages utiles et marquer la réponse qui a résolu votre problème. Chaque contribution rapporte des points et des niveaux.