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The following is a useful example of how to easily read RTU Modbus devices via GPRS. There are two ways to complete this task using the GPRS modem MTX65i + MTXTunnel.

The first and classic way is to use the MTX65i + MTXTunnel as a GPRS-serial gateway. This means that when you try to get a reading from a Modbus device, the server connects to the MTX modem’s IP and sends Modbus frames directly to the computer. The frames are read through the MTX modem which acts as a GPRS-serial gateway.

The second way, that I will discuss today, is more suitable when large quantities of computers are involved. Basically, a new feature from the new version of the MTXTunnel v7.14 is used. This new function allows the autonomous reading of Modbus devices connected to your serial port and it also forwards these readings on to the Web Server via JSON object.

This is the scenario:

• We have a PLC Modbus RTU. The PLC has a number of readings/records in its internal memory (e.g. a temperature and three meters) which must be periodically read and sent to a Web Server.
• Every 15 minutes, the MTXTunnel must obtain these readings from the PLC and it does this via the serial port. Record number 20 is the temperature reading while records 21, 22 and 23 are the respective meters.
• After taking each reading, the MTXTunnel needs to send the values to a Web Server via HTTP GET using a JSON object. However in case there is a GPRS communication failure, it has to be able to store up to 1500 readings in its flash memory so that it can send them as soon as communication is restored.
• You need to be able to access the MTXTunnel at any time in order to be able to read real-time PLC records as well as being able to write in them and modify PLC configuration records. To resolve this, set the MTXTunnel up like this:

Some details from this example to take into account:

1. This example uses an MTX65i with Modbus RS232 PLC communication, but it could work with an RS485 as well. Therefore you could use the MTX65IND2 model (with built-in RS485 communication).
2. The following is a summary of this example:
   The modem periodically reads (every 15 minutes) a series of Modbus readings from the PLC and sends them via JSON object to a Web server (to the URL specified in the LOGGER_server parameter). If you can’t send the reading (due to no GPRS network coverage or if the server is down), it stores the data to memory in order to send it later. With Telnet it’s possible to connect to the device directly so you can consult/change the PLC’s records in real time. For this you need to search for the following commands in this manual: AT^MTXTunnel=getmodbus y AT^MTXTUNNEL=setmodbus
3. The JSON object, that is sent to the specific URL in the LOGGER_server parameter, is encoded in the following way as an example:
   {“IMEI”:353234028103206,”P”:”ID00001″,”A”:1,”TS”:”20/08/12 08:31:44″,”V1″:23,”V2″:275,”V3″:274,”V4″:32765}
   This means that the Web Server receives a JSON object with the modem’s IMEI (IMEI), the password field (P) that we can also use to identify the computer if we don’t want to use the IMEI, the device’s Modbus address (A), the time stamp (TS) which is when the Modbus data was read and V1, V2,… along with every single one of the readings taken.

It’s that easy!!!