Modbus RTU Configuration Commands

To configure Modbus RTU communication in the controller, follow these steps:

1. Create a Modbus RTU object:

   require('luamodbus') 
   mb = luamodbus.rtu()

2. Open Modbus RTU connection:

   • Set the communication parameters (baud rate, parity, data bits, stop bits, and duplex mode).

   • Example (19200 baud rate, even parity, 8 data bits, 1 stop bit, half duplex):

     mb:open('/dev/RS485', 19200, 'E', 8, 1, 'H')
     mb:connect()

3. Terminal name:

   • The terminal name is /dev/RS485.

4. Supported baud rates:

   • Choose from the following baud rates:

     • 300 bit/s

     • 600 bit/s

     • 1200 bit/s

     • 2400 bit/s

     • 4800 bit/s

     • 9600 bit/s

     • 19200 bit/s

     • 38400 bit/s

     • 57600 bit/s

     • 115200 bit/s

     • 230400 bit/s

5. Parity:

   • Set the parity mode:

     • “N” (None)

     • “E” (Even)

     • “O” (Odd)

6. Data bits and stop bits:

   • Data bits: Choose from 5, 6, 7, or 8.

   • Stop bits: Choose either 1 or 2.

   • Duplex mode:

     • “H” (Half duplex)

     • “F” (Full duplex, not supported in RS-485)

7. Baud rate considerations:

   • The baud rate affects communication distance:

     • 9600 bit/sec: Max distance for 1-15 Modbus RTU devices is 1200 meters.

     • 19200 bit/sec: Max distance is 900 meters (typical with Belden 3105A cables).

       Baudrate setting	Maximum communication distance for 1 to 15 Modbus RTU devices (Typical with Belden 3105A cables)
       9600 bit/sec	1200 m
       19200 bit/sec	900 m

8. Parity explanation:

   • Parity checks for successful transmission.

   • Odd parity: Odd number of 1s.

   • Even parity: Even number of 1s.

   • Both gateway and meter must match (odd, even, or none).

9. Delay between frames:

   • Some devices need extra time after a response.

   • Use the delay command (e.g., os.sleep(1.5)).

10. Set extension address:

    • Set slave address (e.g., mb:setslave(123)).

11. Read registers:

    • Read from address 1000 and store the value:

      value = mb:readregisters(1000)

12. Close Modbus connection:

    mb:close()

13. Timeout intervals:

    • Adjust timeout intervals as needed:

      • Byte timeout: 

        mb:getbytetimeout()

        mb:setbytetimeout(timeout)

      • Response timeout: 

        mb:getresponsetimeout()

        mb:setresponsetimeout(timeout)

      • Receive timeout (extension mode only): 

        mb:getreceivetimeout()

        mb:setreceivetimeout(timeout)

14. Example initialization:

    • Initialize Modbus on first script execution:

      if not mb then
           require('luamodbus')
           mb = luamodbus.rtu()
           mb:open('/dev/RS485', 38400, 'E', 8, 1, 'H')
           mb:connect()
       end
       mb:setslave(30)
       mb:flush()