John,
Paul and Thomas are correct. The Fathom-X board only uses two wires. I can share a little more about our future plans for those wires.
We’re working on a way to send power through the tether, providing the BlueROV2 with nearly unlimited endurance. In doing that, we need as much copper as possible to send the power without a large voltage drop through the tether. To do that, we will utilize all 4 pairs for both power and data. For example, pair 1 & 2 will be power and pair 3 & 4 will be ground. The Fathom-X module can carry data over active power lines, allowing us to utilize all of the wires for both power and data.
If you aren’t interested in power over the tether, then the extra wires are perfect for adding additional sensors, analog video streams, or other connections.
-Rusty