Dear: My project used Atmel AT89S52,and I need two UART channels. So,I need software implementation of asynchronous serial I/O. Who can tell me how to do?
You will find an example here: http://www.programmersheaven.com/zone5/cat27/2369.htm I have not tried it myself. Things get more complex (but more efficient) if you implement a soft UART using interrupts. Of course a transmit only will be much simpler than fully bidirectional.