This article is intended to provide an hack guide to the Siemens MC35 GSM terminal. Siemens MC35 is a common GSM/GPRS terminal that can provide wireless network connectivity to embedded applications.
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.

Disclaimer: all data reported below are just results of empiric tests. I don't accept any responsibility in case of faults or damages. Please be careful when connecting power plug. A wrong voltage or inverted polarity can irremediably damage your device.

The 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 but they had absolutely unfair prices (it's not the money itself, but i can't manage to spend a lot of money for a stupid cable) so i decided to make it by myself.
After a few inspections I realized that only 3 of the 6 RJ-12 pins were connected.

VDD (8 to 30 V) Pin 1 and 4
GND Pin 6

Now, here got the first trouble. RJ-12 jacks are really difficoult to be found on the market. RJ-11 jack, on the other hand, are really common (and cheap of course) but lack of external 1 and 6 pins (they only have 4 centered pins).
The solution came immediately: 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 can now 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 unmelt and detach the plug itself and near components.

Serial communication
Communication with MC35 terminal is achieved with the 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. However i'll report in this page a little brief of most common AT commands.

• Dialing a number: ATD012345; (please 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<CR>Message Text<CR>Ctrl+Z
• Shutdown: AT+SMSO