Tunnel – Reading and writing MODBUS records on an RS232 device using MQTT communication

Scenario details:

• A special screen with RS232 serial input is available to communicate with using modbus protocol. It is possible to display characters on the screen by writing in its modbus registries through the RS232 serial port
• We can control the screen remotely via 4G with an MTX modem with MTX-Tunnel firmware
• To avoid connectivity problems (SIMs with private IPs) MQTT protocol will be used, so the modem will automatically connect to the configured MQTT broker
• Once the modem is connected to the broker, the modem will periodically send its status (IP, coverage, etc) to a MQTT topic
• In order to write and read the modbus registries on the screen, the commands AT ^ MTXTUNNEL = GETMODBUS, … and AT ^ MTXTUNNEL = SETMODBUS will be sent to the modem via MQTT

Solution: MTX-Tunnel firmware + MTX-Java-IoT/MTX-Java-T/MTX-Java-T2

Config.txt configuration file:

Details:

• Remember what modem will replace the [IMEI] tags with their IMEI (unique identifier)
• The modem will subscribe to the MQTT topic “[IMEI]/AT1”, so all the AT commands sent to the MQTT topic will be received and executed by the modem
• The responses to the executed AT commands will be sent to the MQTT topic: “[IMEI]/ATR”
• Let’s say the screen allows to visualize 10 characters, and the modbus registries on the screen are from @10 to @19. Let’s suppose it is enough to write the ASCII code in those registries so they are visualized. The screen will have the modbus address @7
  If we want to write the word “HELLO” on the screen, which corresponds to the ASCII: 72, 79, 76 and 65, we will need to write those values in the modbus position 10, 11, 12 and 13, so we will send the following command via MQTT to the topic [IMEI]/AT1:
  AT^MTXTUNNEL=SETMODBUS,7;10;72;79;76;65
  This command writes in the modbus device with address @7, from register number @10, the values 72, 79, 76 and 65
• If we want to read the modbus registries on the screen, we use the AT ^ MTXTUNNEL = GETMODBUS command. If we want to read the same modbus registries from the previous
  section, we send the following AT command via MQTT:
  AT^MTXTUNNEL=GETMODBUS,7;10;3;4
  This command reads the modbus device with address @7, from register number @10, using the modbus 3 command and will read 4 registers. The result of the AT command the modem will send to the “[IMEI]/ATR” topic will be:
  AT^MTXTUNNEL=GETMODBUS,7;10;3;4
  72,79,76,65
  OK