Int 14 Fn 12  - Digiboard Digichannel Pc/x* - Get Transmit Buffer Free Sp  [S]

   AH = 12h
   DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)

Return: AX = number of bytes free

See Also: AH=0Ah"Digiboard",AH=14h"Digiboard"