I'm using a DigiX (Arduino Due based 3.3V) board to connect to a GPRS/GSM SIM908 breakout board. The SIM908 EVB board has the following pins on its RS232:RS232 pinout

I've got all three UART TX lines hooked up to the RX Lines (and RX to TX) on my Arduino but the board keeps resetting, which I would guess is some sort of voltage issue (I turned the board off immediately). However there are three MAX3232s on the underside of the board, which I thought would have made it so I could just connect the lines directly to the Arduino. Does anyone know why this might be happening or how to properly connect these two devices?

Additional info:

Pinout of the DigiX.

Schematic of the DigiX.

(The below links are in Chinese):

Here is a SIM908 Hardware guide.

The full schematic of the breakout board.


1 Answer 1


Both the DigiX and the SIM908 are natively 3.3V boards. They should be able to connect to each other directly.

For some reason, the SIM908 breakout seems to only offer RS232 level serial outputs for the GPRS and GPS. On U3 (the SIM908 module itself) Pins 68/71 offer 3.3V levels for the GPRS (GPRS-TXD and GPRS-RXD) and 15/16 offer 3.3V levels for the GPS (GPS-TXD and GPS-RXD).

These are converted to RS232 levels by the MAX3232 and presented on J3 and J13. I can't see any point where the 3.3V signals are presented on a connector.

You could either:

  • Breakout the 3.3V signals onto connectors by modifying the SIM908 breakout board.
  • Build a level converter to take the 3.3V signals from the DigiX to RS232 and then connect them as before.

You may have damaged the DigiX board connecting RS232 levels directly to it however. The ARM based Arduinos are far less tolerant of voltage abuse than the ATmega based Arduinos.

  • Thanks for the answer. I dont want to modify the SIM908 board so I'm going to go with building or buying a level converter. I'm assuming I can go with 3x something like these ebay.com/itm/… Or even building my own sodoityourself.com/max232-serial-level-converter just without the actual serial port? Will I need to have one of these for each RX and RX line (so 6 total)? Commented Feb 13, 2014 at 10:43
  • Both of those solutions would work. They are bi-directional and have two TX/RX pairs each, so you would only need 2 boards (1.5, sort of). Commented Feb 13, 2014 at 11:00

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