Not all Mac serial ports are capable of thise speed, but for those that are, it's a simple CONTROL call. See Inside Macintosh: Serial Manager for details.
OK thanks for that, I've had a look and am still have trouble, ho hum !
I'm calling the FN CONTROL like this,
Now Inside Mac says that after FN CONTROL it will return the baud rate that has been set in pbBlk.csParam, I'm checking this by,
pbBlk.ioRefNum% = -6 (Modem port)
pbBlk.csCode% = 13 (set new baud rate)
pbBlk.csParam& = 115200 (baud rate I want to set)
osErr% = FN CONTROL (@pbBlk)
BaudRate& = PEEK LONG(@pbBlk.csParam&)
However on baud rates from 300 up to 19200 there is not problem. But above that pbBlk.csParam returns gibberish. I'm testing this on a PPC 4400 which I know can run 115200.