Tether ends


I am just finishing the assembly of the first of three Bluerov2s that I have bought and have a question that I hope you will pardon for its stupidity or the fact it may have been discussed and explained before.

The kits bought all have the advanced electronics package with the Fathom X Tether interface set. After connecting the blue and white wires to the fathom x ether interface board, I am still left with 6 wires (3 twisted pairs). what do the extra wires connect to? at the topside end, is it the same procedure or are you required to cap it and plug it into you laptop that way or do you have the second fathom x board wired and also have the 3 twisted pairs just loose at that end?

If you are using the Fathom-X tether boards, you will only use one twisted pair for communication between topside and ROV. The other three twisted pair are un-used (which means they are available for other purposes).

You need to connect the topside Fathom-X board to the tether using the same twisted pair that you connected to the ROV Fathom-X board. The topside Fathom-X board is then connected to your computer using a Cat5e network cable.

Hi John

Its right, you need only two wires. The rest of 6 wires, you can use for other solutions, like sensors etc.

I will use two wires for a water alarm module.

Regards Thomas


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, what voltage are you considering? AC or DC?


The Fathom-X board is only designed to work with DC voltages on the lines. We’re planning to use 48 VDC to minimize safety issues. This will be a hybrid system with a battery included for burst power requirements.

Technically, the Fathom-X board can handle much higher voltages, but you have to use extreme caution when working with that type of equipment.




Power over tether while still having an on board battery for surges would be great!

One of my concerns is for wreck photo mapping, I’m going to need lots of light (so 6-8 lights total, 4 forward and 2-4 down) I think about the power draw while the ROV is stabilizing from ocean surges and the dive time is going to be limited. If I can hook an invertor/transformer system to a 120amp battery on the surface, that would be awesome! Any ETA on that? (no pressure -lolz :wink: )




You’ll definitely need a lot of power for long mapping missions with that many lights! We’re only planning to get around 90 Watts through the tether connection.

I don’t think we’ll ever have an AC power setup for the BlueROV due to the safety issues. If you look at other small ROVs on the market there are few, if any, that use AC power. That’s usually reserved for much larger ROVs.

If you are interested in sustained operation I would look into high-voltage switching converters to send the power at a much higher voltage. I would also look into line insulation monitors and ground fault interruptors for safety!



So I’m guessing you’ll be sending about 1.8amps down the tether line?

If we have that option, coupled with higher capacity batteries, how much bottom time would you estimate with 6 - 8 lights? I’m not sure the draw of the all of the hardware, so have no estimate. If I could get 3-4 hours bottom time, that would be really great.


Thanks for your responses to my query about the tether ends. I am intrigued by the X Fathom boards capability in terms of allowing just a single twisted pair to do the work and offering a cheap and light weight solution for a long tether. That has particular appeal to me as I intend to take one of my Bluerov2s when I fly. Does anyone know of single twisted pair cable of the quality that is inside the Fathom tether? It looks to have a very stiff and strong sheath and I had considered stripping out one of my Fathom tethers to obtain it but I am sure there is a cheaper and less wasteful option available. I would be grateful if anyone could point me in the right direction.


You might want to consider this OpenROV single twisted pair cable.




It’s a bit too early to guess on endurance and it will depend significantly on other conditions such as current speed in the water.

We’ll actually be send closer to 4-5 amps, but the voltage drop reduces the usable power at the end.


Dear Thomas,

one more question about Bluerov2, I bag your pardon if this topic has been already discussed elsewhere.
You said that the remaining tether wires can be used for other solutions, like sensors etc. Can you explain how can I use these wires?

Can I just plug two other fathoms to the cable ends and send data to another pi or arduino through an internet shield?
Thank you in advance for your attention.

Hi @anelize,

There are a few options for connecting sensors and accessories through the extra twisted pairs. If you have a device that communicates with serial, then you can use an RS485 or RS422 converter to convert those serial signals to long distance “differential” signals that can go through the spare twisted pairs.

If you have a device that uses Ethernet, like a Raspberry Pi, then you can add another Fathom-X or, alternatively, add a small networking switch to split the Ethernet signal from the existing Fathom-X.

What are you trying to connect?


Thank you. I was very confused with the system, you helped a lot.