Error 4 in Modbus Communication: This error typically represents an issue with the modbus slave or server device. It means the PLC rejected the request or could not process it.

Possible Causes and Solutions:

1. Invalid Register Address Ensure the register address you're trying to read exists.

Some devices require zero-based addressing (e.g., register 0 instead of 1).

Example:

python Copy code read_result = client.read_holding_registers(0, 1) # Try register 0 2. Wrong Register Type Confirm if the register is a holding register, input register, or other type. If it's not a holding register, use a function appropriate for your PLC: python Copy code read_result = client.read_input_registers(0, 1) # For input registers 3. Incorrect Function Code Verify that your PLC supports the function code used by read_holding_registers. 4. PLC Configuration Ensure that the PLC is configured correctly to allow reading/writing to the requested registers. Verify PLC's security settings and Modbus access permissions. 5. Test with Minimal Configuration Try reading a simple register directly with minimal configuration to isolate the issue:

python Copy code read_result = client.read_holding_registers(0, 1) if read_result: print(f"Register Value: {read_result[0]}") else: print(f"Failed to read register. Error: {client.last_error}") Let me know if further debugging steps are needed or additional error details appear!