How to use the Modbus master FB on serial line

A great help in the communication trobleshooting can be achieved by activating the SpyOn bit. By activating it, it’s possible by connecting to the CPU module by telnet (Port 23, User Admin, Password Admin) to activate the SpyData command. This command allows to spy all the data frames sent an received by the serial line, this is helpful in identify the communication problems.

As telnet client you can use our Toolly program, as you can see in this screenshot, it’s possible to spy all the Modbus packets exchanged on the serial line.

It’s a great satisfaction for us to read your comments about our SlimLine product, like you, many italian schools and universities are using it as training product. The availability of the LogicLab programming tool with the SimuLab as PLC simulator embedded on it, all completely free is a great opportunity to develop and test programs only by using the PC.

The full compliance with the international standard IEC61131-3 for programmable logic controllers makes LogicLab a good training for all the PLC world. As a proof of that, in italy a university professor has already written two books that treat the use of SlimLine product (See the picture), these books are published for schools and amateurs.

Now you are working with Modbus protocol and are using the FB we provide, but we have develop many FBs and in the last years we are pushing the TCP/IP communications with a lot of FB that manages DNS, NTP, FTP, HTTP protocols and we are specializing on the IoT market by developping a FB that manage the REST protocol and server side some PHP classes to receive data from the SlimLine and store them on a MySQL database (Theese classes work well on a LAMP system).

About the SimuLab, it’s not a full functional PLC simulator, so it’s not possible to test all the functions and FBs, on the simulator only the logic and analog I/Os are managed, so it’s not possible to simulate the serial connection.

About the libraries you have to migrate your project to the new format *.plcprj please refer to this topic (It’s in Italian I’ll translate).

Starting form XTarget_12 we have added some new FBs one manages the serial port, we encourage to use the new FB instead of the Sysfopen function, so I have modified the example (Download the ModbusMasterTest project). As you can see in the ladder program (Screenshot) the Modbus Master executes a 16#03 Read Holding Registers command from a slave device with node 1.

The slave device in the example is the same SlimLine Logic Lab Base IEC 61131-3 CPU, on which the two RS232 serial ports are connected between them (See picture). As you can see in the picture to connect between them the two serial ports the CBL054 and CBL055 adapters must be used. But as a slave sistem you can use another SlimLine module or any other device.

As visible in the green box of the screenshot, the MdbRegs an UINT array, is mapped in the DB100 of the SlimLine at offset 256. The DB100 is accessible by the Modbus protocol and it’s mapped at address 40000. So to read the MdbRegs the address 40000+(256/2) must be defined.

In the red boxes of the watch windows it’s possible to change the value in the MdbRegs array, and you will see that on the RxData array the values immediately changes. The SpyOn input is set to TRUE so in the Toolly screen by sending the SpyData command are reported all the modbus strings. The ModbusMasterTest program can be downloaded here.

The PLC In Pratica book as the ISBN Code 9788869280153.