Your expectations are not too great.
Referencing page 5 of your Garmin installation
manual, your machine has NMEA 0183
connections (combined with the power pigtail).
uses RS422 protocol, which is compatible with RS232
for these purposes, which is the standard 9-pin serial
port or DP-9 on a computer.
If you don't have a serial
port, you can use USB with a serial to usb converter, sometimes called a serial emulator. They are like $10.
To transmit/receive NMEA
0183 to a computer, your will use Garmin wire 3 (Receive/Transmit (B) -), wire 4 (Transmit (A) +), and wire 5 (Receive (A) +) per manual page 5. Tie wire 3 into wire 2, the ground.
The DP-9's serial pins of interest are 2 (Receive), 3 (Transmit), and 5 (ground). Google
"RS232 pinout" for graphics (image search).
So, going from Garmin to DB-9:
3 goes to 5.
4 goes to 2.
5 goes to 3.
Again, make sure Garmin wire 3 is tied to ground (Garmin wire 2).
Most programs designed to digest NMEA
data will have instructions on how to allow the program to receive the data. It usually entails a setting within the program to allow communications
to COM1 (or whatever COM the serial port is tagged as).
for instance will receive/transmit NMJEA data to from a chartplotter
suing this method. I have also had it working with SeaClear.
If you call Garmin, the above is exactly what they will tell you, as some of their older units came with more detailed instructions, as RS232
interface was more common back in the day.
In case you don't have some old computer serial cord laying around, just go to your local electronics parts
store, and buy a male DB-9 solder connector, and a few feet of shielded 3-conductor cable. (I used telephone cable once and it worked fine, and sometimes an old length of traducer cable works.) If the cable has more than 3 conductors, just don't use all of them, and pick your 3 favorite colors.