I am playing with a Xilinx Spartan-3A development (XC3S50A-TQ144C) and then I tried to implement RS-232 serial port by following the guide here: http://www.fpga4fun.com/SerialInterface.html
I bought a DB-9 port and connect RxD,TxD,GND of DB-9 port to three pin of my development board. (Of coz GND is connect to the GND of my board)
I tried to send a signal of one character every second from the pin to TxD. I checked the signal with a scope. The signal should be correct.
But I cannot see any character shown on my terminal by reading from /dev/ttyS0 (Serial port on Linux).
The scope show that the signal is sent using +2.5V and -2.5V respectively. I think the problem is that serial port should be sent with at least +5V / -5V. How can I modify the board (or construct an amplifier circuit) to send signal in higher voltage?
I have a photo capture of my scope here: