This article is a hack guide to the Siemens MC35 GSM terminal. Siemens MC35 is a common GSM/GPRS terminal that can provide embedded systems with GSM network connectivity. I bought one of this on an auction site at a deal price. Unfortunately I got no manuals, no instructions and no cables, just the bare terminal and the GSM antenna.
Power plug connector
Powering it up is the first problem. The MC35 terminal has an uncommon RJ-11 like jack (formerly an RJ-12, I suppose) for power supply. I found a lot of power cables for sale on EBay but they have absolutely ridiculously high prices (it's not the money itself, but I can't manage to spend 30-40 EUR for a stupid cable) so I decided to make it by myself and write this guide. After a few inspections, it turns out that only 3 of the 6 RJ-12 pins are connected on the PCB.
VDD: (8 to 30 V) Pin 1 and 4. GND: Pin 6
RJ-12 jacks are really difficult to be found on the market. RJ-11 jacks (i.e. phone jacks), on the other hand, are really common and cheap but lack the external pins 1 and 6 (they only have the four central pins). Hence, put the two things together and get the hack: just open the MC35 unit and short-circuit on the RJ-12 female connector pins 1 and 2 together and 5-6 together. As result of this you will be able to use a common RJ-11 jack and connect VDD to 1,3 pins (referred to 4 RJ-11 pinout) and GND to pin 4. Please note that soldering on a SMD circuit is a very delicate process. Don't insist too much with your solder on the connector or the heat will melt and detach the plug itself (and nearby components).
Communication with MC35 terminal is achieved with a standard RS-232 DB9 connector. The GSM will reply to standard AT command set plus a set of dedicated AT commands. You should easily find an AT reference manual for MC35 terminal. Here are the most common ones:
- Dialing a number: ATD012345;
(Note the final " ; " Without final ; a data call will be initiated and the modem will put itself in data mode. If you provide a final ; a voice call will be available and the terminal will remain in command mode)
- Answering a call: ATA
- Hanging-up a call: ATH
- Display product information: ATI
- Switch to PPP/Data mode: ATO
- Switch to Command mode: +++
- Read available operators: AT+COPS?
- Enter startup PIN: AT+CPIN=pin
- RSSI Signal quality: AT+CSQ (Reply value 0 to 99)
- Send an SMS: AT+CMGC=012345
Message Text Ctrl+Z
- Shutdown: AT+SMSO