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 response

On 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.