Mike, my Z520 already had everything networked together. What graphs do you have installed? Mine had 2 Lowrance 520C's. If you've got the Lowrance graphs, you can go under the main menu and scroll down and find NMEA 2000. Once there you can view all the devices that are on the network including the GPS. If you see those other devices you are already networked. Chances are you do not have the network interface for the OB though. Here is the cable you will need for the HPDI:

http://www.boatfix.com/shop4/s...=7448