Our test rigs have a custom board with a PIC24FJ128GA202 that should act as a Modbus RTU slave (address 7, 19200 8E1) so a gateway can poll four holding registers and report them to Hermetry. Transceiver is a MAX485, DE and /RE tied together on RB9.
From the gateway (a Pi with pymodbus, tested fine against a commercial meter on the same bus) I get timeouts only:
pymodbus.exceptions.ModbusIOException: Modbus Error: [Input/Output] No Response received from the remote slave/Unable to decode responseOn the scope I see the request arrive on the A/B pair and on the MAX485’s RO pin. The PIC never seems to react. My UART RX interrupt doesn’t fire. TX works: if I make the PIC send a test byte every second, the Pi sees it. I mapped U1RX to RP8 and U1TX to RP7 with PPS. I’ve been staring at this for two days.
Is RO actually reaching the PIC pin? Scope the PIC pin itself, not the MAX485 side.
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