Puzzle more than a problem. Two boards with ADM2582E isolated RS-485 transceivers. Bus side is A, B and nothing else, no ground or shield connected between them. 115200 baud, 2 m on the bench, Modbus RTU works perfectly.
The bus side of each ADM2582E has its own isolated ground (GND2) that isn’t connected to anything. A differential receiver needs its inputs inside its common-mode range relative to its own ground. So what is setting the common-mode voltage here? Next month this goes between two buildings, about 300 m, to a gateway that forwards to Hermetry, and I’d like to understand it before then.
On the bench both power supplies share mains earth, so the „isolated“ sides are probably closer to each other than you think through supply capacitance.
Wait, RS-485 doesn’t need a ground wire? I always ran three wires.
Do you have termination and bias on either end?
120 ohm at both ends, bias on one end only (the master).
Tyler has part of it. Each receiver input also has its own resistance to that side’s GND2 (the input impedance, around 96k for a 1/8 unit load device, more or less), and the bias network ties A and B toward one side’s isolated supply and ground. Each isolated side is floating, so it settles wherever those resistances and the leakage across the isolation barrier put it. The two GND2s drift toward each other through the bus wires themselves. On a 2 m bench with nothing injecting current, that’s close enough, so it works.
Over 300 m between buildings you get induced common-mode voltages and lightning transients. With nothing defining the common mode, one side can sit far outside the receiver’s range and the link gets flaky, or the isolation barrier takes surges it doesn’t need to.
Standard practice: run a third conductor as a signal reference, connect it to GND2 at each node through about 100 ohm to limit current if the grounds disagree, and earth the cable shield at one end only. Michael, that’s why you were taught three wires, it’s the right habit. Keep the bias on one end and termination at both ends as you have it.
Makes sense. Measured it: GND2 to GND2 drifted around 0.6 V on the bench, so plenty of margin there and none to count on outdoors. Ordered cable with an extra pair, reference through 100 ohm at each end, shield earthed at the gateway end. Thanks Ciaran, and Tyler for the supply capacitance hint.
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