Tether Update - Decision for Kickstarter Backers

In addition to managing the production of the kits, we’ve also been busy integrating the tremendous feedback of the community and continuing to test the ROV components in an effort to improve the project. As part of that testing (see the decision tree here), we’ve found that communication over the single twisted pair tether hasn’t been as consistent as we’d like at 100m. We began trying different scenarios and found that if we reduce the tether length to 50m, the connection is much more reliable and can support higher bandwidth communication.
There are a number of different options and directions to go from here. As co-developers, we want you to pursue the strategy that’s most relevant/exciting for you. As such, we want to give you the option of choosing which tether you want to include in your kit.
For most people, a 50m tether should work great (50m is the length of a 16 story building), but if you’d still like to get a good connection over a longer distance (at the cost of less agility in the water) we can send you 100m of CAT-5 Ethernet instead. Also, if digital communication is your specialty, and you’re interested in trying to get a signal to go further through single twisted pair, let us know, and can arrange to send you a longer length. Here’s a summary of our thinking and a description of the options:
Standard CAT-5 Ethernet:
Greater reliability to 100m
Higher bandwidth (100BaseTX instead of 10BaseT)
Possibilities for Power Over Ethernet development
Direct connection- does not require the Ethernet to single-twisted-pair adapters
4x as heavy in the water - will affect ROV buoyancy without tether flotation
2x as much water drag - will make ROV harder to drive quickly or in a current, especially if floatation is added
Less flexibility
A plug for detaching the tether from the ROV requires more pins (and has not been developed yet)
Single Twisted Pair
Sleek, low drag, and light weight allows ROV to move easily
Easy to repair and modify tether (only two wires to solder)
Consistent communication to 50m (longer is possible, and has been done, but it’s not as consistent)
Lower bandwidth (usually can only support 10BaseT)
General Recommendation
  • We recommend the 50m Single Twisted Pair tether for most people. This is the default tether we plan to ship with the kits. This tether will have minimal drag and weight in the water. If you want the Single-Twisted Pair, we’ll include 50m of Beldin 1353A tether as well as two Ethernet-to-single-twisted-pair baluns in your kit.*
  • We recommend using Standard CAT-5 Ethernet for software/electronics developers who prefer higher bandwidth connections and people who plan to use a full 100m of tether. If you want standard CAT-5 Ethernet, we’ll include about 100m of it in your kit in place of the twisted-pair-tether and Ethernet adapters.
*Important Note
We’re planning to redesign the off-the-shelf Ethernet-to-single-twisted-pair baluns in an effort to get more reliability over 100m. This won’t be done in time for the kit shipment. If you want, we can send you 100m of Beldin 1353A and you can participate in the development process.
What do you think?


If it worked to 50 and could be upgraded later, this is acceptable to me.

Working with the development will be fun.


Redesigning the Ethernet Balun is a good idea.

An alternate path to investigate would be finding a high-speed 2-wire modem (not the dial-up kind). Ethernet over AC Powerlines is in its 3rd generation or so and can achieve up to 85 Mbps over AC Powerlines. There are a number of chipset manufacturers and most have evaluation kits. By not implementing the AC Power isolation section and wiring the modems on each end to each other through the tether, we may be able to achieve a fast Ethernet speed and still use the light-weight single pair tether. It would mean some power draw on both ends of the tether, but I feel that it would be worth the hit.

Some of the chipset manufacturers are: Maxim (MAX298x), Coppergate (HomePlug AV CG2110), and Qualcom (AR7400). One or more of these companies may even agree to help for the positive public relations.


I’m in favor of a thin tether. The single twisted pair I saw used in Florida was great. Low profile to reduce drag is key as otherwise currents will just dominate (or necessitate bigger thrusters…bigger hull). ROVs and all uw robotic systems never get smaller so starting small is impt IMO. That said length is impt when you consider tether management and proper rode needed for maneuvers. I guess I’d like to start with stick time on the 50 m and then see about going to 100 m.


Looking at the decision tree, I am most intrigued by the fiber optic option, since it should provide speed, range and lightness of material. Some transceivers/media converters are pretty cheap. see for eg http://www.directron.com/mc100cm.html for ~$30.

Alas, having zero experience in fiber optics its just speculation how those things might work out (or not).

Its twisted pair for me for starters

I am also interested in longer than 50m (but we can start with 50m).

About bouyancy effects, what is the weight of the cable alternatives?
50m single twisted pair?
100m Cat 5?

Regarding Cat5, we're talking about regular 4-pair cable with (or without?) outer jacket?

What a about using a dual twisted pair, that should also support 100Mbit/s?

Something like this perhaps without the outer jacket:

Have reinforcing the tether been considered? (Not sure if it is necessary or not..).
Perhaps use thin multifilament braided fishing line braided with the cable to reinforce the tether?

I also like the fiberoptic idea, like the Jason ROV used to explore Titanic :) :



We're on the same page. We definitely discussed stripping down a Cat-5 line to dual twisted par without the jacket. We just haven't had the time to test it yet, and had to make a decision about the Kickstarter kits. You could definitely try it! That'd be awesome!

Fiber optic - we'd love to, but have always got hung up on the price points. Also, it's really easy to damage the cable.

I still think the most long-term potential is in the two-wire tether, but the more testing we do as a community the better.


I think there are two things that are crucial to the Open Rov success. Number one is the 100 meters. It’s the difference of a diver taking a camera down and opting to use a ROV. Fifty meters just isn’t enough. The second is the size of the tether. Small flexible is important, I don’t know what the answer is but I believe the 100 meters is crucial.


I am going to reply with the 50 meter 2 wire otion but I'd like to participate in the development process. I am may have a solution but I am to some people in my industry.




These are great points, and thank you very much for the links and part recommendations. I've already got a list of several brands of two-twisted-pair CAT-5 that I was going to try out, so I'll add the Hyperline to it as well. I may also get the Ethernet to Fiber converters to play around with, although they would have to be stripped down a bit to leave any room in the electronics tube.

The train of thought you guys have is right on. Just as the decision tree shows, there are many technical routes to go, but I think that for now, focusing on one and doing it well is the best approach (for me at least). My hope, of course, is that other people will try different branches of the tree and whichever ends up being the best negotiation of cost, capability, and practicality will prevail.

I wanted to share the philosophy I've had about tethers with you guys and hear your thoughts:

I think that the best architecture for tether is one where the ROV (or possibly also electronics on the top side) have sufficient capability to communicate through an extremely low quality/ bad characteristic wire.

There are many options for getting good performance out of a higher quality line- for instance, we could purchase custom made, neutrally buoyant ROV tether that has two twisted pairs in it, or we could use very thin coax such as Belden RG-179DT which would be more easy to get the range we want with, but both of these would cost somewhere on the order of $200 per 100m length. We already know that it is possible to communicate over bad wires- for instance, using Ethernet Over Powerline adapters, one could achieve on the order of 100mbps through thin, non-category wire, but the devises are large, expensive, and can be dangerous if used improperly.

I think that with the ingenuity of people in our community, we'll find a way of doing it with devises that are small, inexpensive, and very safe. Being able to use low quality wire will also make it easier for people to get tether materials locally, create make-shift replacements in the field, have multiple tethers for different deployment scenarios. For these reasons, I believe that leaving less to the wire is a good path for the long run.

For those of you who want to help tackle this problem, you rock. Let me arm you with some of the thing's you'll need to be aware of:

1. Impedance. Impedance defines how voltage and current impulses in a modulated line relate to each other in a given circuit based on that circuit's effective inductive, capacitive, and resistive properties. The characteristic impedance of a wire is effected by many things, including the positioning of one conductor relative to another (e.g. twist rate and coiling) and the dielectric between and surrounding those conductors (e.g. the insulation material being used, and the substance the wires and insulation are immersed in like air, water, and salt water). In our experience a 100 ohm impedance wire can drop to something like 95 ohms when placed in water. Also, it's a reason that a stripped CAT-5 wire with out the PVC jacket may not work as well. If the impedance of an electrical circuit (like our Ethernet-to-twisted-pair adapters) is not the same as the impedance of a wire, then some amount of signal will be rejected and losses will occur. Not all wires have the same characteristic impedance, so that's important to look that up before trying a wire out or you'll have bad results. If a wire is non-category wire, its impedance may very greatly even over sections of a given length.

2. Cross talk. Cross talk is a type of signal interference that happens as a result of EMF (Electro-Magnetic Frequency) coming from near-by wires that are also carrying a signal. A single tether going through water is unlikely to experience cross talk, but the portion of a tether that is coiled up on the surface is very likely to experience it. Ethernet uses twisted pairs (one twisted pair for transmit, and one twisted pair for receive) in order to reduce cross talk. TX and RX are done over two wires each so that 0's and 1's can be sent deferentially (see this link to understand differential signaling), and the pairs are twisted so that each of the two wires gets the same dose of any interference (making it possible to remove that interference deferentially). Twist rates in each of the four pairs in CAT-5 are often different to assure that no one wire in a set gets a greater dose then its counterpart by always coming close to the same wire in a different set.

3. Resistance. The thinner a wire is, the more resistance it will have. We generally want to use the thinnest wire possible to reduce weight and drag, but it should be noted that that means there will be more attenuation down the line. A 5v DC supply going through 100m of 18AWG copper wire with a 100mA load will experience a drop of around 8 or 9% (resulting in 4.5v at the other end), but the same scenario using 28AWG copper wire will experience a drop of nearly 90% (resulting in only about 500mV on the other end). Also, poor quality stranded wires often don't have continuous strands (strands may break internally) so effective resistance may be even worse then what is typical for that type of wire.

It seems to me that many of these issues can be dealt with using brawns rather then brains- that is, we can just create an amplifier that will boost the signal enough to counteract attenuation, but at some point, it might be good to optimize even further by introducing a tune-able impedance matching circuit (I picture a small set screw in the side of a little black box that can be adjusted to match whatever random tether is being used), or by moving away from the standard Ethernet signal all together and going to something like the IEEE 1901 standard that Homeplug devices use. Of course, going this rout may once again create issues of cost.

Whatever method is used, I think we'll know we've been successful when we can get a satisfying amount of bandwidth (on the order of 100mbps or more) and little lag (<10ms) on thin (around 28AWG), low quality, non-category wire (that would cost on the order of $0.25USD/m or less), with a system that is small, cheap, and safe.

There is clearly a lot of work to do here, and we're only on step one, but I think we'll be able to climb pretty high pretty fast. Let's start by getting these ROVs in the water!



Hi, I chose the 2 wire recommended option on the questionnaire, I do expect that this option is with the 100 meter Beldin. I could not find anywhere to inform you about this not even a comment section.


Hi Kjetil,

Sure thing. We were planning to send 50m, but we can send you 100m if you want to work with it. Email me if you want us to include that - david@openrov.com.


gee. I must have been tired when I wrote that :P

I may have a solution but I need to talk to a few friends in my industry as soon as I get back home, I am currently offshore. I have CI innovations and Schilling robotics people onboard if anyone has any questions yall want me to ask them.