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Fathom Tether Limitations


(Matt) #1

HI Everyone

My question is about interference on the tether UTPs.

I’m looking to push what I can install on the BlueROV but I’m unsure on how the Fathom tether can perform when fully used. With it being un shielded I’m wanting to know what everyone’s experience is with the likes of a Sonar combined with additional cameras and payload data links at the same time.
Can the UTP of the pair operate together with Ardusub link as well as Cameras on another FathomX and S on another pair, with say RS485 on another running data and sonar data (2 wire Ethernet) on another? (Just an example there)

What is everyone’s experience with what signal types WILL and WILL NOT work together at the same time? What types of signals, and frequencies, will happily work together on this tether? Anyone tried shielding techniques?

This is leading towards weather or not I change from a Fathom tether to something else. What are other popular choices for tethers out there at the moment?

Thanks everyone for any information, I know there are a lot of you out there who have experimented with various setups. I’m curious to hear your experiences with this…

~Matt


(Jacob) #2

We have operated numerous different RS485 devices alongside the Fathom X link without issue.

Depending on the bandwidth that you require, it may be unnecessary to run two ethernet links up the same tether. You can just attach multiple devices to an ethernet switch, or two fathom x in parallel on the ROV in order to add devices to the existing network, and have everything on the network communicate over the single twisted pair.

I do not know of anyone that has run multiple fathom x links over multiple twisted pairs in the tether, this might create crosstalk problems, but like I said, this can be avoided probably anyway with a different network setup.


(Matt) #3

Thanks Jacob

My main concern is a Sonar and the bandwidth its going to require. The sonar likely for us can run on a 2 wire ethernet connection but will be high bandwidth. Im worried the frequency for that link would create interference if I put it on another pair, but it would be to much data to add to the existing ethernet link.

How have sonars you tested been connected? Were they only running RS485? Iv been told the data rate for the sonar I want is to high for 485 comms so options here are fairly limited I think. What sonars have you seen working so far?


(Jacob) #4

I’m not sure of the exact sonar models, they were connected by rs485.

If you can specify a number for the bandwidth then we can definitively say whether or not you need another link. Otherwise this is all speculation.

That said, I doubt you will need an additional link, there is plenty of bandwidth to spare.


(Matt) #5

Currently the top choice is the Oculus M750d.
Anyone know the max bandwidth of this sonar? I know it needs a 100Base-T but the question is really how much of it does it use?

Anyone tried it on a BlueROV yet?


(Jacob) #6

From the user manual section 2.6.3, unfortunately they do not give an explicit bandwidth requirement:

If Oculus data is being sent over a bandwidth-limited network connection with other sensor/camera data (such
as a DSL extender, of WiFi) connectivity issues or dropped data packets may be observed.
In such cases, the network bandwidth available to Oculus can be decreased using the “Speed Limit” control, and
the sonar update rate will be reduced until the data output falls below the specified value.


(Matt) #7

Wow how did i miss that in the manual.
Thanks for that info.

So basically if I want full rate on the sonar then its best to dedicate its own pair then would be my assumption. However that would create some cross talk for sure if i threw a second ethernet pair on the Fathom tether. Ill be adding cameras so those other UTP are going to get very busy chatting away.

Putting the Oculus straight into a Fathom X would get comms on 2 wires?, but what about interference on the other UTP which also have comms? Any suggestions for noise and crosstalk supression?

And we are talking about 300m + of tether. So thats also a major factor here.


(Jacob) #8

I do not think that this is well-founded based on the information presented here.

A pair of the Fathom X tether interface boards will create an Ethernet link across a single twisted pair, that is correct. I personally do not have any further experience on this matter. If you try it, please let us know how it goes.


Fathom-X performance drops
(Matt) #9

OK great. Thats for all the advice Jacob.