MPI to RS485 pin connection


Thread Starter


How to setup communication between an S7-300 PLC with MPI 9-pin port and an ELPRO 405U Radio modem with a 2-pin RS485 port?

I am looking for a detailed pin-to-pin connection diagram.

Daniel Chartier


MPI uses a RS-485 physical layer, so use standard RS-485 connections with a shielded twisted-pair cable:

3----3 (Tx/B)
5----5 (Gnd/Shield)
8----8 (Rx/A)

Or you can use Profibus connectors and cable (recommended if there is any great distance between the partners, because of the impedance matching circuits in the connectors). Physical connection is not the only thing you must validate for this to work: does the radio modem support MPI communication protocol? if not, you must change your project definition; you will probably need to connect to a serial module in the PLC, likely a CP341 with RS-485 interface (part no. 6ES7 341-iCH01-0AE0) program the hayes modem strings in Dbs, and use the integrated ASCII blocks to send commands to the modem.

In a project we did a very long time ago, we used the following: (look at the 2nd diagram)

Hope this helps,
Daniel Chartier
MPI uses 485 as the electrical specification but the protocol is proprietary to Siemens. So unless the Elpro uses the MPI protocol I don't think you will find the two devices communicating.