Black Box Sounder The Garmin GSD 21 adds sounder capabilities to your Garmin chartplotter. The box connects through NMEA/ Serial or CANet and is compatible with: GPSMAP 276/278, 292, 2106c/2110c, or any Garmin with NMEA/ Serial connections. Fast sounder display scrolling with UltraScroll technology, delivering excellent target and bottom definition. 500 watts analogue power with dual frequency transducer, 1500' maximum depth.