Ethernet wiring for a new ROV

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ROV212
Posts: 6
Joined: Aug 17th, 2011, 12:35 pm
Location: New York City

Ethernet wiring for a new ROV

Post by ROV212 »

Hello everyone. Haven't been on here in a while, but I'm back now, and I'm looking for a little help on something.

In my first ROV project, my tether contained DC cables for four thrusters and one ethernet cable for video signal and camera power. Needless to say, this tether was thick and not very flexible. This time around, I want to use one ethernet cable to carry everything. My current set-up is three thrusters and one camera. The camera power will be carried on the ROV itself, and I will have a 12V DC battery topside.

Three thrusters: one DC+ and one DC- each, x 3 = 6 leads; Video + and Video -, two more, total 8 leads. There are 4 twisted pairs in ethernet, that's 8 wires. So, my first question is, can an ethernet cable handle this combination of power and data? Everything I've read seems to indicate yes. but I've never worked in-depth with ethernet cables like this before...which leads me to my next question. If the ethernet cable can run these 8 lines with no problem, then I need to know HOW to make these connections. I'm assuming I need baluns here; do I need one per power line and one for video? (At both ends, that makes 8 baluns...pricey) Or is there a way to wire directly from DC lines to the ethernet cable, in the form of some kind of wiring block ("homemade" balun?)

I think I've provided all the info necessary for an answer, but if anybody out there has further questions, please hit me up.

Thanks everybody!
dna1990
Posts: 48
Joined: Mar 14th, 2013, 6:36 pm

Re: Ethernet wiring for a new ROV

Post by dna1990 »

I don't know the science and numbers...but putting motor-level current across a long stretch of 24AWG sounds iffy to me. Most of the kids use 18AWG for the experimental beginner ROVs.

Is this a fairly short (50ft) tether, or looking to go much longer/deeper? The voltage drop will only make the amp issue more of a challenge on the small wires.


That being said, I know a land based application where I run 12VDC @ 1.2 amps over 1100ft of CAT5. Single pair, not even doubled up. But that is a long way from 3-4 amps on a typical small thruster. What size motors you talking about? A little cam-only observation ROV would probably be perfect on CAT5.


I would also test this before fully committing, with regards to video interference. Even with twisted pair - the video feed could easily pick up noise when you fire the motors. You only need a pair of baluns on the video signal wire.

The best way to physically wire this IMO, would be to strip the CAT5 jacket and wire each pair to either the motor or the switch. I would not fiddle with the typical RJ45 connector, if that is what you were asking. The only way I would say keep the RJ45, is if you plan to disconnect the tether from the ROV with some like this: http://bulgin.co.uk/Products/Buccaneer/ ... ernet.html
ROV212
Posts: 6
Joined: Aug 17th, 2011, 12:35 pm
Location: New York City

Re: Ethernet wiring for a new ROV

Post by ROV212 »

Thanks for the answers! First of all, yes, this will be a rather short tether....50 feet is indeed what I was considering. I'm using 500gph bilge pump cartridges as my thrusters. In response to what you wrote, I'm thinking that even if I used one ethernet line per thruster (plus one for the video), the four of them in one tether certainly wouldn't be as thick and stiff as my previous one, which was four household-style cords (one for each thruster) and one ethernet video/power cable. In that configuration, there was no noise from the thrusters in the video feed, so that may be the way to go.

The ship will be cam-only, no arms or pincers or anything fancy. Two dive lights attached via clamps will provide (self-powered) illumination. But this beast will be carrying four to five digital cameras (each with their respective on-board power and data storage). A CCTV camera runs video tap to the surface.

Lastly, the connections. The CCTV camera does have its own balun, but I will still need one topside for connection to my monitor. And I was hoping that I'd be able to twist the motor wires together with the leads from the ethernet cable. This whole project came about because I'd been to the Buccaneer website and saw their waterproof connectors. The idea of making a ship with a detachable cable seemed very appealing to me!
dna1990
Posts: 48
Joined: Mar 14th, 2013, 6:36 pm

Re: Ethernet wiring for a new ROV

Post by dna1990 »

If looking at the Buccaneer for RJ45, note they have several models with different terminations on the back. The photos are not always clear as to which is which.

The IDC model has the standard punchdown you would see on a Ethernet jack, and would be easy to then take 24AWG style wires from there and connect to whatever is in the drybox. If you get the PC or 90 models, you will need to solder this to a board of some design, noting that so far, I have been unable to find a nicely matched breakout board (premade) to fit the PC model. And the IDC is much more expensive.

It can be a pain for transportation, but if this is your only unit - I would consider skipping a watertight connector and just direct wire everything...then epoxy a casing around those. Otherwise you will still need to split out the thruster connections and camera most likely in a drybox - then deal with the connections from the drybox back out to the thrusters and camera. If you go the permanent direct connection/epoxy route - leave some slack esp in the camera cable from the camera to that junction. So that if you ever build another ROV, you can cut that cam off and still have enough wire to re-connect to something else.
ROV212
Posts: 6
Joined: Aug 17th, 2011, 12:35 pm
Location: New York City

Re: Ethernet wiring for a new ROV

Post by ROV212 »

I'm right there with you on that front...already have a drybox built onto the frame! At the moment I think it might be a bit small, but replacing it will be relatively easy.

Because I'm using PVC tubes for the frame, the thruster wires are running inside the tubes to a watertight connector. I've drilled holes in the bottom of the drybox just big enough to work the watertight connectors into, then silicone-glued the connectors on both the inside and outside of the box. (I bought spare connectors just in case I needed to change the box to something larger. The Container Store [http://www.containerstore.com/welcome.htm] has tons of boxes to choose from, some already watertight, others that need to be water-proofed.)

Also, my navcam is in a PVC tube with a screw-in plug. I need to get a gasket of some kind to fit into threaded end. As with the detachable cables, I'd like to keep this ROV from turning obsolete.

How expensive are those Buccaneer IDC connectors? Basically, I pick and choose where on this ROV I want to spend more money, and I felt that giving this ship fully detachable cabling might be worth the extra money.
dna1990
Posts: 48
Joined: Mar 14th, 2013, 6:36 pm

Re: Ethernet wiring for a new ROV

Post by dna1990 »

If memory serves, the PC board solder kind were like $9 and the IDC were $30. I think I got mine from Mouser.
It was enough for me to look harder at connecting the solder kind.


Test your box with the tether hooked up, at depth several times before adding expensive electronics. Pressure can make quick work of a homemade seal. Better to find out first how well it does.
Also if it were to flood, you just anchored your ROV with switching that much air for water, so better have a secure pull rope that won't stress the tether...

Some here have noted that not all sealants work well with all box materials, like the hard acrylic of a clear otterbox. Don't know the ones that do work well together, but again...test.



The above is from experience, talking to a nearby high school team week before last. They had a good otterbox, but the place where they home made a connection out to the motor wires failed at 13'. It had tested perfect for several dives before that in 7' of water. Stopped their day and almost ruined six Jag motor controllers, but they came back to life after careful rinse and dry.
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ROVEnthusiast
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Re: Ethernet wiring for a new ROV

Post by ROVEnthusiast »

You could use a relay system like on the SeaFox retrofit with one Ethernet cable sending the signals to the relays and some 16 or 18 gauge speaker wire for power to the relays and therefore the motors with little power drop.
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