Ethernet over 500m (underwater)

[Moonwalker031]

Hi I am an electronics engineer and just subscribed to this forum. Its very informative - keep it up.

I have a project for building an ROV. Maximum depth is around 400m but I'm planning to use a cable of 500m for drag compenstion. My idea is to use IP camera on the ROV and send the commands via another IP. Now the problem is how to send the ethernet signal over 500m of cable. Cat5 is not possible for such lengths I guess.Fibre optic is ok but expensive and not very flexible.So I think Coax should be my choice.Well I found ethernet to coax converters which should work well with RG6 cable but I was hoping to use RG174 so that it would have the least drag possible underwater.

So has anyone worked with ethernet to coax converters using RG174? Or is there some other solution for sending ethernet using a thin wire?

 

[Moonwalker031]

Thanks again guys. I am still trying to decide what is the best way to go.
So here are some more questions.

Can I send composite video signal over RG174 cable of 500m length?
And if yes, is there a way to also send RS485 data (low baud) over the same cable?

 

[danw2]

Ethernet doesn't die at 100m, like it hits a brick wall. The 100m length limitation has something to do with propagation time for out and back and the collision detection algorithm used.

There's lots of anecdotal stories of doing ethernet over distances more than 100m. My nephew does 500m with a ethernet surveillance camera on a 10Mb connection. It works.

I'd be inclined to try it with ethernet over 500m, you won't have any VFD hash/noise to contend with . . .

One other thought, in commercial ethernet cable, there's 8 wires, 4 pairs, only two are used for each connection, but the other two pairs are there, idle as it were.

You might consider streaming your camera data on 2 pair, to avoid collisions with any other data; and run your control data (or whatever else there is) on the other 2 pair. It would take 2 ports and 'custom' connectors, but this is a hobby, right?

I've done RS-485 with CAT5, but only at distances <100m. I suspect it will work fine at 500m, but I'd use one of the extra pairs for a ground reference.

 

[Moonwalker031]

Thanks m8 for your explanation.
Rs485 would have no problem with limited slewrate and low baud for such lengths. I can also send composite video over 500m twisted pair cable using an amplifier. However I
Cat5 is a little thick for me since I need the least possible drag in the water.

So my options are-
RG174 - This is not good for ethernet but it should be good for composite video and maybe also for RS485. I need to find a way to encode both signals over the same cable if possible though. Also have a question if it can withstand 50bars of pressure without crushing compleatly.

Single twisted pair - Same as above but facing same encoding problem. Pressure should not be a problem here.

Plastic Fibre optic (possibly single). - Using twin fibers is ok for ethernet so would have no problems with encoding. I was hoping I can use single cable though to limit the outside diameter. Also have a question about maximum pressure it can withstand.

 

[biff44]

Why don't you try to call or email someone over at Woods Hole and ask what cable problems they encounter.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting