Communicate sim900 with DE2

Hi everybody,

I am doing a projects that use GPS module sim900 to communicate with FPGA DE2.

The module sim900 has 1 UART interface to command out, simply it has only 3 lines RXD and TXD and ground.

I designed a system-on chip based ARM and some interfaces (of course it has UART as well), and then loading design into DE2 kit.

I connected 3 lines of UART interface from sim900 with 3 GPIO pins (UART in my design used GPIO from board), but not sure why I cannot get any signal back after turning on the sim900 circuit.

I also test separately sim900 + PC and my design + PC and they are working normaly.

Anybody has experience on this case. Many thanks...

Regards,