ABSTRACT: MODEMCTR.CPP 
   
   Modemctr is a sample loopback program that allows the user to toggle the
   modem control signals on a UART based serial communications card.
   It is set up to use COM1. When 'R' is pressed, the Request-to-send (RTS)
   control register is toggled, changing the status of the clear-to-send (CTS)
   and ring-indicator (RI) registers. When 'D' is pressed, the
   Data-terminal-ready (DTR) register is toggled, changing the status of the
   data-set-ready (DSR) and receive-line-signal-detect (RLSD) registers. Hitting
   the Esc key exits the program. 

   Below is a list of the connections necessary for the loopback cable.

 DB-9 RS-232:

      1 to 4 to 6 (DCD to DTR to DSR)
      7 to 8 to 9 (RTS to CTS to RI)

 DB-9 RS-422:

      6 to 9 (RTS+ to CTS+)
      7 to 8 (RTS- to CTS-)

      Note: the DB-9 RS-422 interface does not support the DCD, DSR, or RI signals,
      and the DB-25 RS-422 is not listed because is does not support any
      modem control signals.

 DB-25 RS-232:

      4 to 5 to 22 (RTS to CTS to RI)
      6 to 8 to 20 (DCD to DTR to DSR)

 DB-25 RS-530:

      5 to 4 (RTS- to CTS-)
      13 to 19 (RTS+ to CTS+)
      10 to 22 to 23 (DCD+ to DSR+ to DTR+)
      6 to 8 to 20 (DSR- to DCD- to DTR-)

      Note: the RS-530 interface does not support the RI signal.