Cheap SIM900 board from ebay


I (unfortunatly) cheaped out on a SIM900 board from ebay from some chinese merchant:

Now I've got 2 problems: - The module can not connect to the network (I tried 2 sims which worked well when used in a smartphone) - I can not communicate via AT - commands. I tried nearly every tutorial on the internet, but getting a serial communication up and running was not possible (therefore I can not diagnose why it won't connect to the network).

All I know is that theres a MAX2323 level converter mounted on the backside. The pins are labelled RXD and TXD, not RX and TX. Unfortunatly the merchant cannot supply any datasheets...

Does anyone know this specific board?

That module may have RS232 I/O if there is a MAX232 IC onboard. The RS232 voltage levels are different and inverted from TTL serial.

Try this website. It has a schematic and manuals on the Download link.

Thanks everyone,
I guess the schematic is a solid base to check the module via a multimeter.
I’ll report if I got it working.