MFJ GPS INTERFACE INFO:

The MFJ-1278B and MFJ-1278BT includes GPS code for sending one or two NMEA 
sentences to the unproto address.  When placed in NMEA mode with a non-zero 
value in the NMEABCN parameter, the TNC sends the most recently received 
NMEA sentences that were received from the attached GPS.  No other outgoing 
packets will occur except ID, BText, CTEXT, and mailbox data.

NMEA is ASCII at 4800 baud, 8 bits, no parity, and 1 stop bit consisting of 
one or more types of sentences containing various types of data.  ie:

$GPRMC,034523,Y,02754.31,N,08023.03,W,1.5,035,031294,4W*23<CR><LF>

       034523 - - - - - - - - - - - - - - - - - - - - - - UTC time
              Y - - - - - - - - - - - - - - - - - - - - - Shows data is OK
                02754.31,N  - - - - - - - - - - - - - - - latitude
                           08023.03,W - - - - - - - - - - longitude
                                      1.5 - - - - - - - - groundspeed.
                                          035 - - - - - - heading degrees.
                                              031294  - - dd/mm/yy.
TNC NMEA Commands:

NMeabcn  n  Periodicity for the GPS packet in 10's of seconds.  If none of 
            the selected sentences were received since power up, no packet
            is sent.  N = 0 sets normal TNC operation.  NMEA mode is enabled 
            when non-zero and the TNC is powered up in the selected CONMODE.  CONMODE "must" 
            The TNC must be set to CONVERSE, SENDPAC to $0D, and CR to ON.

NMEAFLT1  ssssss    where sssss is the 5 character NMEA preamble.  Note that               
NMEAFLT2  ssssss    the leading $ is already assumed.
          ie:       cmd: NMEAFLT1  GPGGA
                    cmd: NMEAFLT2  GPZDA
                    cmd: NMEAFLT2 %      (this cancels the existing value)
Notes:   The TNC must be configured to 4800 baud to match the NMEA standard.
NMEA defines a balanced RS-422 interface.  Although this typically works
when plugged directly into an RS-232 port, such operation is NOT guaranteed.
