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Today I am going to talk about a little Multiplex feature in Siemens modems. As you know, some of the current modems like the MTX65+G  (GPRS+GPS modem) and the imminent MTX-HC25+PLUS (UMTS+GPS) have a single communication serial port.

There may be some applications where we’d like to have more than one serial port.

Why would I want more than one serial port?

Well for example it could be to maintain a GPRS communication through a serial port, to monitor the coverage level through another serial port and to obtain GPS coordinates through the other serial port. So yes it is possible but we use Multiplex mode for this.

The Multiplex protocol (GSM 07.10 y 3G TS 27.010) enables an asynchronous serial interface to be split into three virtual channels. It’s basically a communications protocol that encapsulates communications between the host (e.g. PC/ microprocessor) and a corresponding device (modem) with three virtual serial ports.

How do you use it?

Siemens provides some drivers for Windows (XP/2000) to be able to use this feature. If you don’t want to use a PC with Windows and you want to use a microprocessor for example, you can install the Multiplex protocol. It’s quite complex but it can be done.

To use driver in Windows, all you have to do is install the Siemens WinMux application. When you run it a window like this will appear:

 All you have to do is have the modem physically connected to the PC’s COM port so that the driver will find it by clicking on the “Start Scan” button. Once the modem is connected it will suggest some port numbers as to where you should install the 3 virtual serial ports. As you can see in the figure above, I have installed it in COM24, COM25 and COM26.

After installing the driver, go to Start-> Control Panel-> System-> Hardware-> Device Manager:

  You will se that the “Serial Multiplex Driver” has been assigned to a COM, in my case COM1. If you right click on the installed driver’s properties you will be able to change the assigned COM, the speed and the virtual COM numbers.

One last thing, there’s a catch with the number assigned to each of the COM ports (it’s probably Windows’ fault) and the catch is that it doesn’t let you install it on any COM number. For example there are programs which don’t accept high COM numbers (COM24/COM25/COM26). If you want to change it, I have already registered to Windows you already know:

Start -> Run -> Manage

I searched for a VirtPort1 string and I changed the assigned COM for another one that interested me. It worked without any problems although I had to restart the computer after making a change before using it.

If we no try to open up to 3 HyperTerminal windows, each one associated with a virtual COM port, and we send AT commands you will see that it works without any problems.

I hope you found this interesting, see you next time.