Raymarine use the sea talk system as means to connect all and any raymarine devices on a boat to one other. It is basically a common backbone that has leads coming off it (Spurs). The backbone needs its own 12v power supply from one of the spurs which can be taken from a spare breaker or fuse on board. So in your case a spur will go to the sounder and dsm and the other will be a 12vdc supply. Not sure if you have the backbone kit with it but some marine stores sell them.  If I have confused you more feel free to pm or call.