MODBUS Comunication with RS485 on ST-200


Thread Starter


I try to briefly explain my problem.

My goal is to communicate with the plc (S7200 CPU224) in order to read and modify variables during program execution.

I am in possession of an interface rs485, rs232 to Ethernet (USR-TCP232-24) of which I enclose a picture and the manual.
From what I understand the plc on db9 connector has two dedicated pins to rs485 (3 and 8), but it seems that the term refers only to the standard RS485 electric, while the communication protocol, I believe remains MPI.

To overcome this, I set the communication mode of the plc on port 0, as Modbus slave and tried to communicate with the plc to pc. Specifically referring to this tutorial:

By monitoring the virtual com, however, it seems that the plc always responds with a double 0 in binary.
Where am I wrong? Should I put the resistors 220 and 390 ohm output from db9 plc?

If I can not do it this way, what is the solution? And 'need for a power cable mpi rs232? Why can not I use the rs485?

USR-TCP232-24 Img:

USR-TCP232-24 Manual:!8thyWBoA!BXr2M_ZlzQaFfKsMZGrswhVsQc5GFQSG4-sdADJ-eWg

Thanks for your help and sorry for my English is not too good.
I'm not familiar with the hardware you are trying to use however I have done a lot with RS-485 in the past.

I think the key is to start with the baud rate as slow as possible and monitor the communications. For this I have used an old laptop with RS-232 port. Using the laptop with hyperterminal or similar terminal software tie the RXD of the port to RS-485 negative line then you can watch the request and response while setting the different parameters. The problem is different vendors use different terminology so it's often a matter of trial and error until you get a match.
Thanks for the advice. But I have the updates. I connected the interface directly with an RS232 serial cable, I configured the PLC as a Modbus slave and then I tried to connect to PC with the simulator Modbus Pol. The connection fails and the simulator gives me error saying that the answer is too short. By monitoring the com port, I observe that I receive in response to a binary/ascii always equal to what they send. What can this mean?

Unfortunately at the moment, I do not have a PC with a serial interface. if I can not solve, I shall buy a serial card for my desktop pc.

Anyway thanks again :)
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