The KPC3 Plus with software ROM version 8.X or above is a good choice for APRS Packet. One nice feature is the fact that you can run it stand alone after installing a 9 volt battery inside the case. You can find some TNC's that are physically smaller however, if that is an issue in your installation.
There are lots of KPC3's out there, one can be found on the used market or swapfest table. Any KPC3 can be upgraded to the latest software ROM with full GPS support for about $40.00 from Kantronics. So buy a used one cheap... and upgrade it yourself.
NMEA Data from any source (your GPS unit) is sent at 4800 Baud. You must setup your TNC to accept serial data communications at 4800 baud so it can hear and understand the NMEA data from your GPS. This is done on a KPC3 Plus by doing a HARD RESET.
Remove the two screws that hold the cover on the TNC. Locate a two pin jumper labeled #11, it is near the 9 volt battery compartment. Use the black jumper to short out the two pins of jumper 11. Power on the TNC, then turn it off. Remove the jumper and put it on one pin only (just to hold it in place until needed again someday). Don't turn the TNC on yet. Connect the TNC to a computer.
If you're running Windows, run the Terminal or Hyperterminal program (found under Programs / Accessories / (Hyper)Terminal. Set up a New Connection named KPC3, under File / Properties select direct connect via the com port you have connected to the TNC. Press Configure, and set the Bits Per Second to 4800 (leave the rest as is). Save the file, you will now have a shortcut in the Programs / Accessories / (Hyper)Terminal folder named KPC3, just click on that in the future, so you will never have to do this setup again.
Now with the terminal program ready to go set for 4800 baud, turn on the TNC and type a * (shift 8). The TNC should respond with it's logon screen, if you can read the information (you don't get a string of garbage...) it's set for 4800 baud communications and your ready to program it.
If you're using your TNC as a mobile tracker, make sure your GPS is setup to send NMEA data (not some other format such as GARMIN) at 4800 baud. You only need two wires between the GPS and the 25 pin connector of the TNC, data and ground.
Data coming out of the GPS goes to pin 2 of the TNC. Connect pin 7 (gnd) of the TNC to the data ground of the GPS.
Here is the KPC3 program I use on my NODE station.
Here is the KPC3 program I use on my Tracker station.
Please see: Program Guide, TRACKERS, HOME RELAY and NODES.
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