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Using GPS Babel to convert Humminbird waypoints to Lowrance format
Free download at http://www.gpsbabel.org/

First download your waypoints from your Humminbird locator onto an Sd card. Note that...

Linking two units through NMEA 0183 to share a GPS module
This does not allow waypoint sharing
http://www.bassboatcentral.com/boardstuff3/MLee1.jpg

After you have the units wired as per diagram...