Hi Nick,
This would be dependent upon the Garmin GPS unit being set to output NMEA GPS sentences as ArcPad doesn't support Garmin's proprietary GPS format. Once connected and GPS on and running, open ArcPad and select GPS > GPS Preferences > GPS tab and run Find GPS utility (black binoculars) to search for the connected GPS. Please review the ArcPad Help for more information.
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Cheers,
Denise