Power Over Ethernet?


Has anyone looked into 802.3af or 802.3at Power Over Ethernet for use in tethers? This will get you up to gigabit Ethernet and 15 or 30 (60 and more are evolving) watts of power.

Note that this is not the "Voltage on the spare pairs" thing that some folks call Power Over Ethernet, but a standard for putting 50-ish volts on the (spare and) data wires and using all four pairs for power and data (hence the gigabit with 60W options). Turns out heating in the wire is one of the issues with the higher-power versions, which will probably be less of an issue in this application. 8*)

You can get gel-filled "direct burial" CAT5 wire, but it might be easiest to use a flexible (patch cable) CAT5 wire and replace it as needed. 100-foot CAT5 cables are $20-ish, and if the underwater connections are RJ45s filled with silicone grease it might be waterproof enough to work for enough dives to make it a consumable.

Anyone know what the buoyancy of CAT5 patch wire is?


One could also just use a remote sense DC power supply to deliver a desired supply voltage at whatever the current limit for your tether is. This would simplify the submersible by placing the regulation circuits topside, and remote sense supplies are pretty common.


Sure, but the advantage of the PoE solution is that the voltage is 50-ish volts, which keeps the current down, so you can draw (on the order of) 4X as much power as if (say) your ROV is running at 12V.

And the 'splitters' are COTS, so cheap per watt compared to a remote sensing power supply with large offset abilities.


I just did a posting on PoE in the Open ROV electronics section. I don't know if you folks have pursued this idea any further.



PoE spec is for ~48V. It's 48V, because "Telco power" is 48V. that's the voltage at which all the hardware at the telephone switching office runs at. That's the voltage that all Corporate Phone Systems run at. You can get some specialized rack mounted network gear that runs off 48V as well.

10BT only requires cat 3, 2 pair.

100BT requires 100MHz cat5, 2 pair.

gigabit requires 100MHz cat5e, 4 pair.

I cannot guarantee that the wiring will work correctly once you submerge it in water. I do not have a spool of cable and a bandwidth tester to see how the signal is degraded by wet cabling.

If you do buy "regular" network cable, be sure to buy a spool of the *PATCH* cable, not riser cable. riser is solid, whereas PATCH cables (or station cables) are stranded.

"Passive" POE puts power over the 2 pairs that does NOT have data on it. It is possible to run power on the data pairs, but I haven't tried doing so. It must be done in such a way that it won't interfere with the data signal.

48V is convenient, because of the telco industry: there are a LOT of 48V to misc voltage parts out there, especially in the surplus market. I have a bag full of 48-5V dc-dc converters I think I bought for $2 each.

The power limitation is partially heating, but most significantly voltage drop across the line. For every 100' of 26 AWG, expect 4 ohms of resistance in the cable. Double it because you need to go both there and back, and halve it because you use two wires for +V and two wires for -V. 48V @ 12W = 1/4A. 1/4A over 4 ohms is 1V drop. 1V / 48V = 2.1% power loss. Increase that to 1A for 48W, and you get 4V / 48V = 8.3% power loss and only 44V on the receiving end. Note that as you reduce your draw (say the batteries are fully charged and the rov is idle), the voltage WILL increase back to 48V. And as the cable length increases, your power loss increases too. Maximum distance on 100BT is 100m or ~300ft. so at 300ft, you get 12 ohms of resistance in the cable.


I spoke to Eric to see what his experience with the cable has been in the test dives he has done and he favors the 2-wire approach (light weight, flexible, etc.) but does not work with PoE approach. I think if we can make Cat5 2PR work that would be best for the electronics. We would have to do some test dives to verify.

I was looking at the Belden 72001E (cat 5e, solid, 24AWG), but as you pointed out this would not be as flexible as say the Belden 72002E (cat 5e, stranded, 26AWG PVC). There are different jacket options as well; PVC, FRNC and PUR that are shown in the Belden Industrial Ethernet Cabling Catalog. The 7202PU is described as "excellent flexibility, oil resistance, etc.). I will try to locate a distributor and possibly take a look at these. If anyone has experience with these or equivalent cables please let me know.


Sorry I am new to this thread. Has there been any update as to using the cat5e solid strand as a tether? Not for power but just for the two strand connection. Any luck in making this work ?



We have pretty much moved to the Ethernet over Powerline standard - Homeplug. Two strand cable is used with this approach. OpenROV sells both an inexpensive twisted pair cable (https://store.openrov.com/collections/components/products/extra-100m-tether) and a premium poly neutrally buoyant tether with a kevlar strand (https://store.openrov.com/collections/accessories/products/neutrally-buoyant-tether). The Homeplug adapters can run at 200Mbps half/duplex over this type of cable for 100 Meters.



Yes sir thank you. I understand what you are saying, I was just wondering if anyone has had any luck using a cat5e cable ?


Hi @chris11:

Plenty of people have used Cat-5 cable for ROV projects. BlueROV, for instance, still does.

The good thing with Cat-5 cable is that you don’t need the complication of homeplug adapters to convert to a 2-wire protocol. You pay for it, however, with a much bulkier cable.



Thank you for your response. I will admit, I am out of my element on this subject. I own a diving company and use this for nothing more than a simple video inspection. If it wasn’t for the step by step instruction on the build I would have been complexity lost. The reason I would like to use the cat5e is because of my need to lower and pick up the rov into and out the water at times of up to 15, by use of a cable. So if I was to use the cat5e cable would I simply connect two of its wires to the two wires on the rov and plug it into the homplug box that in turn connects to my computer via Ethernet connector and USB ?



If all you want is a stouter cable, then yes, you can just use Cat-5 cable, and use one of the twisted pairs as the tether, and don’t connect the others.

If you don’t need neutral buoyancy, another possibility is to use speaker wire (“zip cord”), which is cheap and easy to get, and is stronger than the tether supplied in the 2.8 kit. We have done many dives using zip cord as a “downline” leading to a clump weight, with a short piece of neutrally buoyant tether between the clump weight and the ROV.

The best solution is to use the neutrally buoyant tether that we sell in the store, and to connect the Kevlar strand that is in the tether to the ROV. This is kind of expensive, however, and may not be necessary for your use.