Int 14 Fn 1A  - Digiboard Digichannel PC/x - Special Character Flag/count  [S]

   AH = 1Ah
   BX = subfunction
       00h return pointer to special character flag byte
       01h return pointer to special character counter word
   DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)

Return: ES:BX -> special character flag or counter

Notes: flag is FFh if one or more special characters are in the receive
     buffer; it is 00h and the counter is invalid if no special characters
     are in the receive buffer
   counter (if valid) contains the number of characters in the receive
     buffer up to and including the last-received special character