INT 14 - Digiboard DigiCHANNEL PC/X* - CCB COMMAND
        AH = 16h
        AL = CCB command number (see below)
        BL = byte 2
        BH = byte 3
        CL = byte 1 (for all channel functions except 4Eh and 4Fh)
        DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)
Return: AH = status
            00h successful
            FFh error

Values for CCB command number:
 40h Set Receive Mid Water Mark
 41h Set Receive High Water Mark
 42h Flush Receive Buffer
 43h Flush Transmit Buffer
 44h Transmit Pause
 45h Transmit Resume
 46h Set Interrupt to Host Mask
 47h Set Baud, Data, Stop and Parity
 48h Send Break
 49h Set Modem Lines
 4Ah Set Break Count
 4Bh Set Handshake
 4Ch Set Xon/Xoff Characters
 4Dh Set Transmit Mid Water Mark
 4Eh IRQ Polling Timer to Host
 4Fh Buffer Set All
 50h Port On
 51h Port Off
 52h Receive Pause
 53h Special Character Interrupt
 54h RS-422 Enable