A Modbus poll timeout error occurs when a Modbus master device sends a request to a slave device, but the slave device fails to respond within the specified timeout period. The master device waits for a certain amount of time for the slave to respond, and if the response is not received within that timeframe, it assumes the slave is not responding and generates a timeout error.
Use a serial monitor (like RealTerm or Hercules) to sniff the line. Alternatively, use a known-good USB-to-RS485 converter connected directly to the slave with a short cable (bypassing long runs). If the timeout disappears, your field wiring is faulty. modbus poll timeout error
Follow this systematic approach: