USB-serial converter (modprobe pl2303):

usb 4-1: new full speed USB device using uhci_hcd and address 4
usb 4-1: New USB device found, idVendor=067b, idProduct=2303
usb 4-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
usb 4-1: Product: USB-Serial Controller
usb 4-1: Manufacturer: Prolific Technology Inc.
usb 4-1: configuration #1 chosen from 1 choice
pl2303 4-1:1.0: pl2303 converter detected
usb 4-1: pl2303 converter now attached to ttyUSB0

Download tracks: (data format needs to be set to garmin)

gpsbabel -t -i garmin -f /dev/ttyUSB0 -o gpx -F tracks.gpx

Erase maps:

sendmap20 -t/dev/ttyUSB0 -e

Upload maps:

sendmap20 -t/dev/ttyUSB0 map1.img map2.img ...

... or use qlandkarte/qlandkartegt

Plot maps/profiles at http://www.gpsvisualizer.com/. Download/create foreign maps from OpenStreetMap.