Kantronics KPC–3 Plus User Manual page 92

GPS Equipment (or weather station) Requirements
To set up a packet GPS station that can be tracked, you'll need the following
equipment:
a transceiver and antenna,
A Kantronics TNC that supports the NMEA 0183 interface standard.
a GPS unit with an NMEA interface and portable or external antenna. More than
50 vendors produce GPS units and most offer NMEA data interfacing which is
essential.
Over time, a number of software programs will be developed by amateurs to use packet
and GPS. One shareware program that has emerged already is the Automatic Packet
Reporting System (APRS), written by Bob Bruninga. An APRS system may consist of a
GPS unit, a computer, and (optionally) a packet unit. In APRS, NMEA data, from a GPS
unit, is passed to the computer, and it is the computer's job to display the information
and, if desired, transmit that location via a packet unit using unproto packets.
In some applications, you may wish to keep your PC or terminal attached to the TNC for
packet operations, yet switch serial port connection to the GPS unit periodically—just to
update your location. This arrangement is handy when only one COM port is available
on your PC. This can be accomplished with an APRS-HSP cable and APRS software.
Cabling a GPS Unit to a Kantronics TNC
The NMEA standard specifies that the signals in the GPS receiver data port follow the
EIA-422 standard.
Technical note: Many GPS vendors simply use "TTL" voltage levels (+5 V and 0 V), but
use the same polarity as RS232. These units are capable of driving a Kantronics TNC
RS232 serial data port input pin. However, some GPS vendors invert the sense of this
signal (true TTL), and these units will not directly interface with a TNC.
Warning: The Kantronics TNC serial ports are RS232; that is, they provide for signals
that swing plus and minus approximately 8 V. Check your GPS manual before
connecting this port to your GPS device to make sure that it is capable of handling
these voltages. Kantronics is not responsible for damage wherein its TNCs are attached
to other devices; see the Kantronics Limited Warranty.
To connect the data port of a GPS unit to your TNC, you'll need a three-wire cable. A
shielded cable is preferred. Refer to your GPS manual to determine which pins are used
for signal ground, data output to an RS232 device, and data input from an RS232
device (such as the TNC). At least two of these wires must be connected at both ends
of the cable:
connect the signal ground wire of the GPS data port to pin 7 of the TNC serial
port. This wire may be identified as ―NMEA-‖ by some GPS vendors.
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