ComGSM – not so easy as I thought

Unfortunately things haven’t gone quite as I had hoped, although I’m hoping this is more of a hiccup than anything else.

I have learned a few things about this GSM module:

Once you have installed a SIM you need to power cycle it before it will register

There are several LEDs on it – one of which is for power, and other for signal. This blinks very slowly before the SIM is registered, and then rapidly once it is registered.

I am not quite sure whether the pins are labelled properly

Having done a lot of fiddling around I have not been able to get any response out of the GSM unit by connecting it to the alarm panel. It should be showing a signal strength which it reads out from the module but it’s not showing. Similarly I can’t get it to send any kind of messages so there’s obviously something up.

My instinct is that it is not communicating at all, and there is something wrong with the interface between the panel and the module. The next thing to do is to try and interface with the module itself. I didn’t have anything suitable so looking on ebay I found this:

 usb (2)

I liked this partly because it was cheap (£2.70) and also because it came with a short cable as shown to connect up the individual pins. It has arrived, and so the next thing to do is hook it up to the GSM module and see if I can get any communication going. This will let me confirm that the pinout is correct, and also what the baud rate etc should be because the alarm panel is expecting 9600 8-N-1.

I’ll post again once I’ve had a chance to test it.

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3 thoughts on “ComGSM – not so easy as I thought”

  1. Hi, did you have any luck with the DIY ComGSM? I’m in the same situation where the official ComGSM seems very expensive and is in a separate box, so was thinking of doing something with an Arduino and GSM module via Serial (to translation commands from Serial to that suited to the GSM module). On reading your post it seems it might be happy talking to any GSM module via Serial although of course the AT modem commands may vary between a generic mobile part and the official ComGSM, meaning it will not be sending the correct AT commands.

  2. Hi, I read further up that you had success using the SIM800L, I’ve now ordered this module and have some DC-DC converters already to hand. The SIM800L requires a maximum of 4.4 Volt, so to drop the voltage and allow these boards to work with 5 volt they have two diodes in series, whilst this is a cheap way to drop some voltage it’s not linear or ideal, so I’m going to short those diodes out and will supply 4 volt from the DC-DC converter so it gets a more stable voltage.

  3. Yes the SIM800L seems to be the best bet, there are clearly some subtle differences in command sets and also what these boards will tolerate. I’ve heard some reports of different behaviour between boards as well but since they are so cheap it’s best to experiment. I certainly had no problems with mine, worked first time whereas the first one I got (a Siemens) didn’t work at all.

    Didn’t realise about the diodes, that’s very interesting. Will be interested to hear how you get on.

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