&S0      &S0 = DSR (Data Set Ready) is always true.  This line
          tells the computer the modem is powered on and ready to
          go.  This is the normal use of DSR.
          &S1 = DSR only goes true when the modem has made
          connection with a remote modem.  This is a non-standard
          use of DSR.  Normally the modem uses the DCD (Data
          Carrier Detect) wire to tell the computer this fact.