Hi Simon,

2400 baud is the only accurate rate you will get out of a 12 MHz crystal. 4800 baud at 24 MHz is the next best thing. This is all based upon your requirement of a 12 MHz crystal. http://www.keil.com has a web site that calculates baud at different crystal frequencies. You will need to select one which has a 1 percent error rate to communicate with An IBM comm port.

Regards,

Charles Bannister