Poll Modbus TCP devices from a gateway and forward readings to an MQTT broker

Steps

1. Identify the Modbus TCP device's IP address and port (default 502), its Unit ID (also called Slave ID or Server ID), and the register map (holding registers, input registers, coils, discrete inputs) from the device datasheet
2. Use a Modbus TCP client library (e.g., pymodbus in Python) to connect and read registers: connect to the device IP/port, then call read_holding_registers(address, count, unit=<unit_id>); interpret the raw 16-bit register values using the datasheet's data type and scale factor
3. Implement a polling loop with a configurable interval (e.g., every 5 seconds); handle connection errors and Modbus exception codes (e.g., 0x02 Illegal Data Address, 0x04 Slave Device Failure) with retry logic and exponential backoff
4. Normalize the polled values (apply scale factors, convert units) and serialize them to JSON; publish to the MQTT broker on a topic like factory/<site>/device/<id>/telemetry at QoS 1
5. If multiple Modbus devices share the same gateway, serialize requests to avoid collisions; Modbus TCP supports multiple concurrent TCP connections to different Unit IDs but a single device may not handle concurrent requests gracefully
6. Log Modbus response times and exception rates; persistent exception code 0x06 (Slave Device Busy) indicates the poll interval is too aggressive for the device's processing speed