thankyou Zoran, this makes sense to me
This is the post (which could be totally wrong) that has confused me with advice to connect the garmin black -ve to the Furuno 620 green (rd-b receive mate for the rd-a data in) and no mention of connecting the black wires between Garmin 720 and Furuno 620.
http://www.ausfish.com.au/vforum/archive/index.php/t-58418.html
Regards,
Steve