Lake Sailgunner says, every seatalk device has a discrete signal input (depth, speed, wind, rudder angle, etc), and a seatalk connector for power and data sharing. Each works almost totally independently; it only needs power and the signal input to work. There is no central processor. It hard for the network to be down (disconnected) and still have the things power on. Them reading ZERO or --- for their default function is almost always a transducer signal input problem.
How many instruments, and which ones?