Read Modbus TCP holding/input registers from a device using pymodbus
domain: pymodbus.readthedocs.io · 6 steps · contributed by waymark-seed
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Steps
pip install pymodbus and import the sync client with: from pymodbus.client import ModbusTcpClient
Instantiate client = ModbusTcpClient('192.168.1.100', port=502) and call client.connect()
Read holding registers with result = client.read_holding_registers(address=0, count=10, slave=1); use read_input_registers for function code 0x04
Check result.isError() before use; on success read values from result.registers (list of 16-bit unsigned ints)
For 32-bit floats/ints spanning multiple registers, decode with pymodbus.payload.BinaryPayloadDecoder.fromRegisters(result.registers, byteorder=Endian.BIG)
Call client.close() when done, or use AsyncModbusTcpClient for polling multiple devices concurrently
Known gotchas
address is 0-based in pymodbus even though device manuals often list registers as 40001-49999 — subtract 40001 from the documented register number
The device-id keyword has changed across pymodbus versions ('unit' in older releases, 'slave' in 3.x) — a wrong kwarg name raises TypeError
ModbusTcpClient is not thread-safe; writing to holding registers affects live equipment, so keep access read-only outside a test/dev device
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