Tunnel – Reading and periodic sending to an MQTT broker of the MODBUS registers of a PLC. DUAL SIM configuration due to loss of connectivity of an operator.

Scenario details:

• We have a Modbus RTU PLC. This PLC has in its internal memory a series of variables / registers (for example, a temperature and 3 counters, …) which must be read and periodically sent to an MQTT broker
• Therefore, the MTX-Tunnel must periodically interrogate the PLC through a serial port every 15 minutes to read these registers. The registers to be read are: for the temperature register nº20, and the counters are in registers 21, 22 and 23 respectively
• The MTX-Tunnel must send the value of the registers after each reading to an MQTT broker using a JSON object, but it must be able, in case of 2g / 3g / 4g communications failure, to store in flash memory up to 1500 readings that it will send when communications are restored
• For more security in communications, the modem must have DUAL SIM. In other words, the modem must have 2 SIM cards from 2 different telephone operators. The modem will need to change SIMs as long as it cannot get an IP address for more than 120 seconds

Solution: MTX-Tunnel firmware + MTX-IOT-S [4-N]

Config.txt configuration file:

Details:

• In this example a modem is used using the RS485 port, but the RS232 port could be used without problems (to use the RS232 port the parameter MTX_invertedCom: on should be entered)
• The summary of this example is as follows: the modem periodically reads, every 15 minutes, a series of ModBus registers from the PLC and sends them via a JSON object to an MQTT broker (to the topic specified in the LOGGER_mqttTopic parameter). In case of not being able to send the record (because there is no coverage at that time or the server is down) it stores the data in memory to send them later. Using MQTT commands it is also possible to connect to the equipment directly and consult / change the PLC registers in real time (to do this, search in this manual for the commands AT ^ MTXTunnel = getmodbus and AT ^ MTXTUNNEL = setmodbus)
• The JSON object sent to the MQTT broker is encoded as follows, as an example:
  {“IMEI”:353234028103206,”P”:”ID00001”,”TYPE”:”MODB”,”A”:1,”TS”:”20/08/12 08:31:44”,”ST”:20,”V1”:23,”V2”:275,”V3”:274,”V4”:32765}
• The modem has DUAL SIM (DUALSIM_mode: dual) configured. This will cause the modem to boot using the primary SIM card, which in the case of the MTX-IOT-S [4-N] modem is the external SIM card. In case coverage is lost and it is not possible to register on the operator network for 60 seconds (DUALSIM_timeout), the modem will switch to the secondary SIM card