This is an example of connecting Garmin handheld to Humminbird 798SI. Reason for doing this is becouse my internal GPS died, and I want to get GPS fix from Garmin.
Things you need:
- old Garmin PC cable
- humminbird AS HHGPS cable
Garmin pinout (male - device side):

pin 1 - data in (white wire)
pin 2 - data out (brown or yellow wire)
pin 3 - ground (black wire)
pin 4 - +12V (red wire)

Garmin pinout (female - cable side):

Humminbird AS HHGPS pinout :

To do :
- Insulate white and red wire on humminbird AS HHGPS cable

- Insulate white and red wire on garmin cable

- Solder black to black, and himminbird green to garmin yellow (or brown, depends on your cable)

- Insulate all

Now go to garmin device - settings/interface/IO format/ and choose NMEA in/NMEA out
- Connect cable to devices, and now you have garmin GPS fix transfered to your HB unit
