INT 14 - Digiboard DigiCHANNEL PC/X* - READ STRING
        AH = 0Fh
        CX = number of characters to read
        ES:BX -> buffer
        DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)
Return: AX = number of characters read
        ZF clear if successful
        ZF set on error (line status or wrong number of characters)