I am trying to repair my side scan sonar towfish which has a failed ADSL communications board.
The sonar is powered via 140VDC from the topside unit, transmitted down a 100m coaxial tow cable. I’ve read that a pair of Fathom X boards can be used to transmit data over a line that is energized with AC or DC power, and I’ve ordered a pair of the boards to give it a shot.
My question is: How do you wire the Fathom X boards to either end of the 140VDC cable? Are all of the necessary components built in to the Fathom X boards, or is some kind of external isolation circuitry required?
Is it really as simple as connecting the two homeplug wires directly to either end of the the tow cable, as shown in this diagram?
I haven’t personally used the Fathom-Xs like this, but electrically speaking a DC power supply tries to maintain a constant voltage, and a power-using device generally likes to receive a constant voltage, so you may want a low pass filter of some sort on each end to isolate the DC components, so that the power supply doesn’t try to cancel out the HomePlug signals from the Fathom-X, and so that the device doesn’t receive a noisy power supply.
As Rusty mentioned, the Fathom-X boards will do their own rejection of the DC component to separate out the signal for their internal use, so you only need to worry about rejection of the high frequency signal component acting as noise for your DC devices (although you’ll also need to make sure that any filtering you add has minimal impact on the signal side of the filter).
Just tested, and I can confirm this configuration works great! I wired my two Fathom-X boards to either end of the tow cable in the configuration shown in that diagram, and they connected immediately and started passing data.
I soldered the two tether leads directly to the pins coming out the back of the coaxial cable connectors in the towfish and topside box. When powered up, the tether has 140V of DC running down it. FathomX handled it just fine.
Now I can hopefully squeeze a few more years out of this excellent old sonar system. Thanks for the help!