S200, CPU224XP, configuring MODBUS on..

Hi pals..

You see I am attempting to configure a small S7-200 PLC i.e. cpu224XP which ha 2 ports to communicate with the PC as well as to read RTUs on Modbus as well.

At present, my client is having the PLC(programmed earlier by someone else) read temperature data from 4 TICs which have Modbus ports. Now he wants to provide a PC connection, with a VB application acting as SCADA. As I understand, the S7-200s have a freeport, which seems to be configured as a Modbus master.

My problem is.. How can now the PC act as master. Merely altering the Freeport's configuration to Modbus slave(PLC is now a RTU) may result in the PLC being unable to read the temperature measuring instruments, as slaves cannot poll other slaves! Moreover, the temperature data is required in the PLC's logic. It is not sufficient merely to bring it to the PC's application.

Then what are the pitfalls of using the second port? Suppose I were to configure this one as Modbus slave, what happens next? can it connect to MicroWin?. I plan to use a generic 3rd party Modbus DDE server. So will the port work next time round with the Siemens' MPI/PPI->PC adapter?


Will greatly appreciate any workable suggestions

Raj S. Iyer

I haven't work with S7-200 some years and cann't give you complete answer. But why you want to use Modbus? Modbus isn't native for Siemens. Probably Modbus master realized in your PLC is "hand-made". If you will realize Modbus slave at second port, you will not able to connect through Modbus and Microwin protocol simultaneously.
Better to look to software PC Access from Siemens.

Thank Gambrinus... Which country is Riga?


Just now, that is today I have solved the problem. It was a modbus slave after all!. I must share my experiences.

It is impossible to configure the Freeport on S7-200s as a Modbus Master.
To configure it as a slave, you must have the Library function of Modbus init. In that Function block you have to give the line parameters such as baudrate, parity etc.

While downloading the program, the PLC's port is in PPI mode. Once in run mode it behaves as a Modbus RS-485 port. Only Pins 3 & 8 are needed. These are the D+ & D- signals respectively. I used a MOXa converter as well as an i-dam convertor. The IDAM had auto baud detect and in the MOXA, I had to configure it. However in both cases, the communication took place easily. It was detected on MODSCAN and used as Modbus Topic in Wonderware's Modbus DDE server. With this server, we could easily see PLC data in EXCEL as well as VB.

Thanks once again, my friend, your suggestions are most welcome.May you have a happy day!!!


Raj S. Iyer