USB to PC Adapter for Siemens S7-300/400


Thread Starter

Drew Faulkner

Has anyone had any success talking to a Siemens S7 PLC via USB hub+serial adapter?

I have a working RS232-MPI/DP adapter, but a laptop with a blown serial port. Adding a USB hub with built-in serial and parallel ports works for
serial comms to everything but the Siemens PLC.

Siemens tech support are non-committal - 'It might work.... can't guarantee it'.

It was all working before the laptop port died, and the laptop repair is big bucks, so I'd rather like a solution that makes financial sense. The Siemens PCMCIA card is a bit too expensive.


Drew Faulkner
Istari Systems Ltd
[email protected]

Mark Hensley

Hi Drew

The S7MPI adapter may work over a USB based serial port but I have found that the adapter is very sensitive to the timing of the messages it receives during the configuration of the adapter. We have an OPC server for the S7 MPI adapter that works fine when it is directly connected to a PC serial port. If I try to access the MPI adapter over an Ethernet terminal server like Digi or Lantronix the timing is just off enough that our server can't initialize the adapter. I haven't tried it over USB. I do have an Edgeport 4 port USB serial adapter that I have tried with a number of our drivers but I haven't tried it with our S7MPI driver. In working with the USB adapter I was able to run all 4 ports at 19200 baud with very good performance results. The biggest loss of performance occurred on our Idec driver when talking to a FA2 device. The protocol for this device is a byte per byte echo between the driver and the PLC. In this mode of operation it would appear that the large number of single byte transfers over the USB bus to
the Edgeport adapter had an almost 50% performance impact on the Idec driver versus using a native PC serial port. Given that the MPI adapter does appear to be a little sensitive to timing, you may see an occasional startup issue when using the MPI adapter. Once the MPI adapter is configured it is less sensitive to timing within the actual MPI messages.

Mark Hensley
Kepware Inc.
81 Bridge St.
Yarmouth Maine 04096
Did you check the RS232 signals output voltage of your USB hub? Perhaps they're just lower than EIA RS232 standards...Siemens devices usually require a strict compliance with electrical standards.

Luca Gallina

Drew Faulkner

Yep - checked and found to be fine. Siemens themselves have said to me that the Step 7 software will not work through a USB to RS232 converter. They are however developing a specific USB adapter for S7 programming. No release date though.

It seems they must be pretty 'tight' on talking to the serial port - no virtual ports are supported, so I assume the driver isn't very flexible.


Drew Faulkner

Ah, that's for PPI (S7-200); I need to talk MPI to S7-300/400

I'd already read that article - thanks anyway.