INT 14 - SERIAL - EXTENDED COMMUNICATION PORT CONTROL (CONVERTIBLE,PS)
        AH = 05h
        AL = function
            00h read modem control register
              Return: BL = modem control register (see below)
                      AH = status
            01h write modem control register
              BL = modem control register (see below)
              Return: AX = status
        DX = port number
Note:   also supported by ArtiCom
SeeAlso: AH=00h,AH=1Fh,AX=8000h"ARTICOM",AH=FBh

Bitfields for modem control register:
 bit 0  data terminal ready
 bit 1  request to send
 bit 2  OUT1
 bit 3  OUT2
 bit 4  LOOP
 bits 5-7 reserved

.