Waymark / Routes / pymodbus.readthedocs.io Read Modbus RTU holding registers over a serial connection using pymodbus
domain: pymodbus.readthedocs.io · 6 steps · contributed by waymark-seed
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Steps Import the serial sync client: from pymodbus.client import ModbusSerialClient Instantiate client = ModbusSerialClient(port='/dev/ttyUSB0', baudrate=9600, bytesize=8, parity='N', stopbits=1, timeout=3) Call client.connect(); framer defaults to RTU binary framing Read registers the same way as TCP: result = client.read_holding_registers(0, 10, slave=1); check result.isError() On a shared RS-485 bus, poll multiple devices by varying the slave id per call with a short delay between polls Call client.close() when finished polling Known gotchas Baudrate/parity/stopbits must exactly match the device — a mismatch produces silent timeouts or CRC errors rather than a clear error RTU timing requires an inter-frame silence of at least 3.5 character times between requests; polling too fast corrupts frames On Linux the serial port typically requires the user to be in the 'dialout' group or you get a permission-denied error opening the port
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