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GPS Unit Initialization from the TNC
Some GPS receivers require reception of a special string of characters for initialization
before they will send any information. You can tell your TNC to send the initialization
string to your GPS unit by entering the required string using the GPSINIT command. For
instance, if your GPS receiver requires the string $PGRMO,GPGGA,1 to be sent, you
would give the command
GPSINIT $PGRMO,GPGGA,1
to your TNC. Upon power up, the TNC will then send that string via its serial port to the
GPS unit (when the TNC INTFACE command is set to GPS.)
GPS Operations
Starting the TNC in GPS Mode
Hint: Be sure to set ABAUD to that required by your GPS unit.
Once you have the TNC parameters set for GPS, you need to set the TNC to start in
GPS Mode on power up. To do this, set the INTFACE command to GPS. When the
TNC resets (either from a RESET command or by turning the TNC off and then on) it
will be in the GPS Mode and the GPSINIT string (if any) will be sent to the serial port.
Normally, you would set the INTFACE command to GPS, turn the TNC off, connect your
GPS unit, and then turn the GPS unit ON and then turn the TNC ON.
When operating in the GPS Mode, the TNC will automatically update the LT buffers
every time it receives the designated NMEA strings from the GPS receiver.
Exiting GPS Mode
If you want to exit the GPS Mode (perhaps to set new parameters), connect your
computer or terminal to the serial port of the TNC and type three <Ctrl+C> characters.
The TNC will reset and the INTFACE command will be set to TERMINAL.
Hint: You may have changed the ABAUD setting (usually to 4800) for the GPS mode. If
so, before you can use the three <Ctrl+C> characters to exit GPS mode, you must set
your RS232 baud in your computer to the same as the ABAUD setting in your TNC.
Other GPS Features
Slotted Beacons
The BLT command provides for the ability to specify not only the interval between
beacons, but a starting time for the beacons. This permits a ―slotted‖ system of beacons
by numerous stations. For instance, lets say your local area has 30 stations that each
should beacon their location every 30 minutes. A beacon would be transmitted every n
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