Slightly OT: Disconnecting Ethernet

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I have an application in which I need to disconnect an ethernet channel
most of the time unless there is a special set of circumstances.  I have
a processor to control the disconnection, and access to the ethernet
(CAT5) cable and its conductors.

The application is in a pressure environment and that precludes me from
using the obvious relays on the 4 cores of the cable.  I have no software
access to the routers on either end of the cable, so I can't simply
program them to turn off the channel.  So, I'm looking for some way
of disabling the channel by only accessing the transmitted signals.

To put some context:  We have a subsea application in which two separate
ethernet circuits are used to transmit data between the surface and
the equipment subsea.  In normal use, one circuit is used for bidirectional
control and monitoring of the subsea systems, and the other is a dedicated,
one-way high-speed data link used for monitoring.  In order to minimise
the jitter and latencies on that link there will be only one data channel
open and transmission will be as fast as possible with no other data on
the link.  Normally the two networks are totally unconnected at both
the topside and bottom ends.

However, it's all too possible that one of the ethernet connections
becomes broken.  In this case we want to be able to bridge the
two independant networks onto the remaining wire, albeit with some
degradation in the link speeds.  On a surface system, this would
mean that we simply unplug the bottom end ethernet cable from its
router and plug in into the other network, and do the same thing
at the top end, so that both networks share the same bit of wire for
the uplink.  Everything should then work as before.

However, we can't find any volunteers to replug the networks in the subsea
end of the link, so we need to make the switch automatically.  That'd
be easy enough in a 1-bar environment, because we'd just use changeover
relays in the cable.  However the whole system's running in an oil-
filled box at whatever the pressure is at depth, which could be up
to 600MSW (or 60 atm).  So, I need to use solid state stuff to do
the switching.  I don't think that I can just stick semiconductor
switches in the circuit either because they have quite a large on-
resistance (although I've not tried it).

I was wondering if anyone here had any ideas?

Trev

--
Trevor Barton
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
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Re: Slightly OT: Disconnecting Ethernet

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bidirectional
dedicated,

Probably you could use semiconductor switches - many FETs have
on-resistences well below 1 Ohm.

The signal on a twisted-pair Ethernet is a little less than 1 V on 100 to
150 Ohm impedance. The network is transformer-isolated from the endpoint
electronics. IMHO, the greatest problem may be isolating the control signals
of the FET switches. If there are no significant other voltages to block,
the rest should be pretty straightforward.

HTH

Tauno Voipio
tauno voipio @ iki fi



Re: Slightly OT: Disconnecting Ethernet


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I think that you could do it with FET switches, which will easily go sub-ohm
for about 30 cents.  These are designed for switching power, but they should
work here.

But, also, I haven't thought it though, but it just seems like you could do
this with an Ethernet switch chip in the undersea unit.  Broadcom sells
these, for maybe $10.  Open op a commercial switch/hub and you'll see it in
there.  It could possibly be configured to do what you want as well.



Re: Slightly OT: Disconnecting Ethernet
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subsea
sub-ohm
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should

Yes, but these powermosfets all have internal (reverse) diodes. The fets
that don't often have Rds-on >25 ohm.

[snip]

--
Thanks,
Frank Bemelman
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Re: Slightly OT: Disconnecting Ethernet
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I think Fairchild or ADI may have had an app note or a flier on using
analox MUXes for routing Ethernet signals in things like laptops
docks/mini-docs.

Their idea was something along the lines of a user might want to have an
Ethernet port on the laptop itself, but also have the dock wired to
Ethernet so when they reach "home base" they could dock the laptop and
not have to mess with hooking up connections.

The point however is that they were using analog muxes with BW upto
350MHz for switching Ethernet signals...you could probably do the same
to connect/disconnect your device.

-- Jay.


Re: Slightly OT: Disconnecting Ethernet
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Fantastic, the url is

<http://www.fairchildsemi.com/markets/notebook/docking.html>

and it's pretty much what I want!  Thanks very much.

Trev

--
Trevor Barton
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
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Re: Slightly OT: Disconnecting Ethernet
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And just to follow up my post there are "lots" of analogue switches that
threaten to be able to do ethernet switching in their app notes.  They
all suggest the switch side of the magnetics, which is probably OK for
me, but I'm sure that with a bit of though it'd be easy enough to do
on the isolated side of the magnetics with appropriate isolation.

Analog ADG774 for example.

Duh, I don't know why I never though of analogue switches myself,
it's bleedin' obvious when someone points it out to you!  Thanks
everyone for the input!

Cheers,
Trev

--
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
Tel: +44 (113) 275 1339, Fax +44 (113) 224 9827
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Re: Slightly OT: Disconnecting Ethernet
Trev,

says...
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You're very welcome!

Please let us know how it turns out.

Best regards.
-- Jay


Re: Slightly OT: Disconnecting Ethernet
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You can use 2 FET's back-to-back. This will conduct in both directions
if driven high enough and not conduct at all when switched off.

For the OP:
If the oil is totally non-conductive, it might be possible to immerse
non-sealed relays in it completely. And how presure resistant are reed
relays? The glass tubes are soo small that they might withstand a lot.

--
Stef


Re: Slightly OT: Disconnecting Ethernet
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Yes, you can use relays and they work for a while, but they are not
very reliable long-term.  We have done it in the past with normal
plastic encapsulated relays by drilling small holes in the plastic
case or removing it altogether, but they soon become unreliable.

I think that there are a couple of problems.  It'd difficult to
ensure that the oil is always clean in the environment they are
used in, the equipment can be pulled apart on deck in a force 9 gale for
diagnostic work and reassembled  in less than ideal conditions.
Also, I suspect on low power relays the oil ends up gunging up the
workings and probably attacks the plastic parts at least at the
interface level which makes tham sticky, and they don't have a
sufficiently large pull-in force to overcome that.  They work
to start with but after a few weeks or a year or so they just
stop working.

I don't know about reed relays.  We use light bulbs in pressurised
gas up to about 18 bar subsea, the small 12V halogen bulbs and they
are OK it seems but they are replaced regularly (after only a few
10s of hours use).  I *guess* reed relays are probably OK, especially
if they have secondary encapsulation (eg epoxy or something) but it's
be only a guess and I'd shy away from it if at all possible!

Anyway, I'm looking at the Fairchild device now that seems to be
made for just my purpose!

Cheers,
Trev.

--
Trevor Barton
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
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Re: Slightly OT: Disconnecting Ethernet
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... snip ...
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Why can't you drive both links through a virtual driver, and do
the switching entirely in software?

--
Chuck F ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: Slightly OT: Disconnecting Ethernet
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'Cause although I have a processor, I'd need 4 ethernet ports (two
to connect to the bottom end units, and two to connect to each of the
uplinks).  That's problematic.

Trev


--
Trevor Barton
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
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Re: Slightly [different] Disconnecting Ethernet problem
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I have been pondering (though not very seriously) a slightly similair
problem for a while now. Seeing as the topic is on the table I thought I may
as well ask and see what gets suggested :

I want to be able to have two CAT5 cables that are normally independent of
each other coming into my device. E.g. in a VOIP telephone with a two
(three) port ethernet switch. However, when power is lost on the device, I
want the two cables to be bridged, and still support high speed comms :

    cable from hub <-> device with 2-port ethernet switch <-> PC

but when power to device goes down, or other fault :

    cable from hub <-> [low resistance through-connection requiring no
power] <-> PC

obviously one would have to take into account resultant cable lengths,
particulalry given that the analogue switch probably has worse impedance
problems than a normal twisted cable .

The "requiring no power" (i.e normally closed)  part is the biggest problem
to me....

Any suggestions ?



Re: Slightly [different] Disconnecting Ethernet problem

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It shouldn't be.  A simple mechanical relay with "opener" switches
should do that for you.  I.e. a relay switch that is closed by
default, and opened by applying current to the magnet.  May not be the
most reliable or long-term stable solution in the world, and it's
everyone's guess what happens if you try to transmit 1000baseT
(gigabit ethernet) over it --- but simple it definitely is.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: Slightly [different] Disconnecting Ethernet problem
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A NC relay?  If you've no special environmental concerns a normally
closed relay connected to your power supply will do that, or to a
digital output of you want to more intelligently turn it on (eg after
your device has booted).  I doubt the effect on the circuit impedances
would be more notifable than an RJ45 connector/wall connector/crap
homemade wiring behind the trunking.

Trev

--
Trevor Barton
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
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Re: Slightly OT: Disconnecting Ethernet
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Rather than switching the Ethernet, why not switch the MII interfaces
to the PHYs?  These are digital signals and can be easily switched with
normal buffers, transceivers, or quickswitch parts.

If you were planning to use integrated MAC/PHY parts, you will have to
buy separate PHY chips, but they aren't too expensive.

Much better than using relays, FETs, etc. on the analog side, IMHO.

Is there some reason why *software* can't control which Ethernet is
used?

Re: Slightly OT: Disconnecting Ethernet
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Yes, it's less to do with the cost than the board acerage.  I need 8
channels on one side and probably the same number of the other.
Although, of course, the magnetics are fairly large.

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Not any electronic reason, but a practical one; I can save hours and
hours of software development time by removing the need for someone to
learn how to drive a switch through its serial interface by reducing the
control to a single digital output.  By the time someone has trawled
through and understood the datasheet, written the code to drive whatever
the serial interface is, debugged and tested it, we'll have used about
a week of software effort.  Now, use the frig factor someone posted
on here a while ago (add one and increase to the next time unit) that
maps to about two months of software development.  If I wanted to do
anything more complicated, I agree, the software route is the correct
way to go, but to just switch an input on or off it's overkill.

Trev

--
Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
Tel: +44 (113) 275 1339, Fax +44 (113) 224 9827
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