PLC - PC communication using a Radio modem


Thread Starter



Does anyone have any experiense to connect a PLC (in my case S7-226XM) to a PC using a radio modem? Any suggestion of a radio modem that will work (idle times etc)?

Olof Leander
Our recent experience has been good. The current crop of radio modems that use frequency hopping spread spectrum (FHSS) work very well for line-of-sight operation, and in our case, even with some obstructions in the line-of-sight.

The radio modems just replace copper wire. RS-485 in, RS-485 out, or ethernet in, ethernet out, it's just bits and bytes or packets.

In the US 900Mhz and 2.4GHz is used for unlicensed operation. But that varies country by country.

I can't open any process control trade journal without finding wireless ads. Check out your trade journals.

I have used freewave modems to connect a network of AB ControlLogix and SLCs with no real problems. Just make sure that they are line of sight!
Many of the Spread spectrum radios on the market will work, they do have to have a suitable RS-232 or RS-485 port on them. There are timing issues to be resolved, but the settings for one situation may not work for another. Contact Siemens tech support, they can help you out. I know we have used cell modems without any special timing changes, but using a SS radio may require a bit more tuning.
Hi Olof,
I used MDS Radio modem and RS485-RS232 converter to make connection between Siemen S-7 series (on profibus platform)to MDS radio. For PC or RS232 connection using TXD RXD and ground connect together CTS-RTS.

Hi Olof,

There are a few of them out there. If you want to do monitor and control at protocol level, the best way to go is with ELPRO modem. I found that you dont have to throttle the modem to suit the PLC. The one i have is a Ethernet Modbus/TCP model and i can get back IO points as well as convert my Profibus over the radio back to my PC ~2 miles away. Its a slick idea and provides a whole lot of flexibility in my factory.

I forget the website, but remember that they have a whole lot on there. Google for "Elpro 905U-G". You should find it.

Hope this helps,
Rich Walton
dear sir,
we had tried with siemens S7 200 using lotus wireless modem.

RX/TX commnication has been done with this wireless modem.
First decide whether using RS232/RS485

The modem connected to PC must be configured Half Duplex ,Slave

The modem connected to PLC must be configured Half duplex, slave

The SCADA/HMI/Utility on PC must use Master Driver depending upon PLC