Ethernet breaker

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

I'm looking for a device which could act as an ethernet breaker switch
based on the application or loss of power to that device. On one hand
we could accomplish what we need with a low-cost switch and two cables.
However, in our application and environment this would introduce
several new problems. What I really need is a two port fast ethernet
repeater that I could cut power to and break the link. Of course, such
a thing doesn't exist (or does it?).

Any suggestions on a part that could fill this role? We could probably
design and build a mechanical switch to break the link, but I'm trying
to find something we could buy before going down that road.

Thanks in advance for your suggestions.

-Rich

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

Thanks for the replies. Maybe I should describe the actual
application.

We have a machine with multiple PC based operator interfaces. During
machine servicing it is sometimes necessary to disable control of the
machine at all but one interface PC. We are looking for a way to
disable control that is a) not software based (we already have numerous
s/w interlocks built in to the system and b) doesn't involve removing
power to the PC. What we'd like to do is simply disable the data line
to the PC.

Two easily implemented methods are currently under consideration.

1) Use a managed ethernet switch at the center of the control system
and disable individual switch ports as needed. The problems are that
switch silicon
(arguably or provably) has a higher failure rate than say a
mechanical/electrical switch and to the user/safety inspector it still
looks and feels like a software switch.

2) Install a low-density ethernet (branch) switch between each PC and
the central (core) ethernet switch with an electrical relay controlling
power delivery to those branch switches. The relay states can then be
selected with a traditional mechanical knob switch. The problems here
are that we just greatly increase the number of potential failure
points in the control system and introduced a whole bunch more unused
ethernet ports which we are trying to limit for other safety reasons
(e.g. factory staff plugging in computers without having to follow
procedure).

My imaginary solution is a fairly simple, two port device that would
sit inline on the PC ethernet link and when it lost power would break
the link. Do you know of such a device?

Thanks again-
Rich

PS I'm using the groups.google interface for the first time. Apologies
if it does anything rude like top post or full-quote.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <1126791575.591318.240920@o13g2000cwo.googlegroups.com>,
Rich Noonan <rnoonan@gmail.com> wrote:
>I'm looking for a device which could act as an ethernet breaker switch
>based on the application or loss of power to that device. On one hand
>we could accomplish what we need with a low-cost switch and two cables.
> However, in our application and environment this would introduce
>several new problems. What I really need is a two port fast ethernet
>repeater that I could cut power to and break the link. Of course, such
>a thing doesn't exist (or does it?).
>
>Any suggestions on a part that could fill this role? We could probably
>design and build a mechanical switch to break the link, but I'm trying
>to find something we could buy before going down that road.
>
>Thanks in advance for your suggestions.
>

Several companies make boxes that can switch 120VAC frm an RS232
connection or TCP/IP via ethernet. You can just cut the power to the
switch.

See http://en.wikipedia.org/wiki/Kludge

ISTM if you lose power to "that device" it obviates the need to turn
off the bits. It's no longer listening.

Black Box Corp (www.blackbox.com) specializes in odd gadgets like
this.

Any managed hub or switch can control the ports via snmp protocol from
an network management application running on one or more systems. I'm
sure snmp commands can be scripted from a language like PHP but that's
above my pay grade.


--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <1126794928.866211.26990@z14g2000cwz.googlegroups.com>,
Rich Noonan <rnoonan@gmail.com> wrote:
>Thanks for the replies. Maybe I should describe the actual
>application.
>
>We have a machine with multiple PC based operator interfaces. During
>machine servicing it is sometimes necessary to disable control of the
>machine at all but one interface PC. We are looking for a way to
>disable control that is a) not software based (we already have numerous
>s/w interlocks built in to the system and b) doesn't involve removing
>power to the PC. What we'd like to do is simply disable the data line
>to the PC.
>
>Two easily implemented methods are currently under consideration.
>
>1) Use a managed ethernet switch at the center of the control system
>and disable individual switch ports as needed. The problems are that
>switch silicon
>(arguably or provably) has a higher failure rate than say a
>mechanical/electrical switch and to the user/safety inspector it still
>looks and feels like a software switch.
>


Are you dealing with life-critical systems or otherwise covered by
OSHA?

If you add up the all the extra electrical connections, power sources
etc for a relay-based system in use on any scale I'd argue that a
managed hub with a web interface is *much* more reliable and in the
unlikely case that it does fail it will do so in *much* less
interesting ways and be trivial to spare and fix.





--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

William P. N. Smith wrote:
> "Rich Noonan" <rnoonan@gmail.com> wrote:
> >I'm looking for a device which could act as an ethernet breaker switch
> >based on the application or loss of power to that device.
>
> How about removing the power to a 2-port switch? Would that solve
> your problem?

This would be a decent solution b/c then I wouldn't be compelled to
pour epoxy into unused switch ports :-( Have you seen 2 port ethernet
switches? Can you reference a manufacturer?

> If this is something you have to do one at a time, can you unplug
> cables or flip hardware switches? I've seen CAT5 mechanical
> switches..

Can't unplug. HW switches would be great! Please tell me more if you
can.

Thanks-
Rich

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <1126791575.591318.240920@o13g2000cwo.googlegroups.com>,
Rich Noonan <rnoonan@gmail.com> wrote:
:I'm looking for a device which could act as an ethernet breaker switch
:based on the application or loss of power to that device. On one hand
:we could accomplish what we need with a low-cost switch and two cables.
: However, in our application and environment this would introduce
:several new problems. What I really need is a two port fast ethernet
:repeater that I could cut power to and break the link. Of course, such
:a thing doesn't exist (or does it?).

Your reference to "several new problems" makes it difficult for us to
know what would be acceptable and what would not.


I don't know whether this will help you, but recently I have been
investigating remote-controllable serial ports and remote
controllable power switches. For my particular application, I was
looking at ones that can be reached via ethernet. Especially for
the power switches I also found devices that can be controlled via
serial lines -- up to 2000 feet for one of the devices.

Typical remote control units have 1, 8, or 16 ports, with 4 and 32
port units not too hard to find, and with 2 port units available but
not easy to find.

The choices are differentiated on their remote access method (serial /
ethernet); their security (ssh?); their modularity; their flexibility
(several are uLinux with SDKs); their number of ports; their physical
size; the amperage the power controls handle; and, of course,
upon their cost.

Links to several manufacturers can be found at
http://www.voip-info.org/tiki-print.php?page=Remote+Console+and+Power+Control

In addition to those, I suggest checking out http://www.synaccess-net.com/
(Note: their web page has not yet caught up to several recent security
enhancements.)

I have several other units bookmarked; if you want a bookmark dump,
send me an email note.
--
Entropy is the logarithm of probability -- Boltzmann

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

Rich Noonan wrote:
> 1) Use a managed ethernet switch at the center of the control system
> and disable individual switch ports as needed. The problems are that
> switch silicon
> (arguably or provably) has a higher failure rate than say a
> mechanical/electrical switch and to the user/safety inspector it still
> looks and feels like a software switch.

I have seen something that reminds me of what you're asking for, except
it's completely the opposite!

Years ago I had some external (to a webserver) hardware SSL
accelerators. They were designed to go inline with a webserver's
Ethernet cable, and do the SSL work whenever traffic appeared on
TCP:443.

These boxes had 2 RJ45 Ethernet connectors on the front panel. These
jacks were connected to *both* the ethernet transceivers of the
hardware in the box *and* to each other through some DPDT relays (i
guess).

The idea was that *even* if power to your SSL box was cut, the
webserver would stay online.

So, yes, it's completely the opposite of what you're looking for, but
it makes the point that running ethernet through some relays might be
okay

/chris

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

"Rich Noonan" <rnoonan@gmail.com> wrote:
>I'm looking for a device which could act as an ethernet breaker switch
>based on the application or loss of power to that device.

How about removing the power to a 2-port switch? Would that solve
your problem?

If this is something you have to do one at a time, can you unplug
cables or flip hardware switches? I've seen CAT5 mechanical
switches..

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <dgc27d$n5i$1@panix5.panix.com>, Al Dykes <adykes@panix.com> wrote:
:If you add up the all the extra electrical connections, power sources
:etc for a relay-based system in use on any scale I'd argue that a
:managed hub with a web interface is *much* more reliable and in the
:unlikely case that it does fail it will do so in *much* less
:interesting ways and be trivial to spare and fix.

Ummm... where are you going to find a managed *hub* with a web interface?

There were some managed hubs, but they weren't common at the best of times,
and they predated widespread use of web interfaces. They used serial, or
telnet + snmp, or custom GUIs.

These days, it is apparently difficult to find true hubs; according to
some of the groups I follow, devices being marketed as hubs have often
been switched over by the manufacturer to actually be switches under the
cover.
--
Any sufficiently old bug becomes a feature.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <dgc65v$j6n$1@canopus.cc.umanitoba.ca>,
Walter Roberson <roberson@ibd.nrc-cnrc.gc.ca> wrote:
>In article <dgc27d$n5i$1@panix5.panix.com>, Al Dykes <adykes@panix.com> wrote:
>:If you add up the all the extra electrical connections, power sources
>:etc for a relay-based system in use on any scale I'd argue that a
>:managed hub with a web interface is *much* more reliable and in the
>:unlikely case that it does fail it will do so in *much* less
>:interesting ways and be trivial to spare and fix.
>
>Ummm... where are you going to find a managed *hub* with a web interface?
>
>There were some managed hubs, but they weren't common at the best of times,
>and they predated widespread use of web interfaces. They used serial, or
>telnet + snmp, or custom GUIs.
>
>These days, it is apparently difficult to find true hubs; according to
>some of the groups I follow, devices being marketed as hubs have often
>been switched over by the manufacturer to actually be switches under the
>cover.
>--
>Any sufficiently old bug becomes a feature.


Somehow I think we are playing in different leagues. CISCO, HP and a
pile of other venders make managed hubs and switches. The HP procurve
stuff (except for the bottom end) has built-in WEB management
interface. I'm not familiar with CISCO.

Look for an HP Procurve box on ebay and then check HP.com to find the
manual and read it to see what it's management capability is.
Procurve has a lifetime warranty so it's hard to go wrong. I'm partial
to the HP 4000 switch.

I repeat my statement; a business grade hub is going to be more
reliable than a pile of relays, cables and wall wort power supplies.



--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <1126794928.866211.26990@z14g2000cwz.googlegroups.com>,
Rich Noonan <rnoonan@gmail.com> wrote:
:My imaginary solution is a fairly simple, two port device that would
:sit inline on the PC ethernet link and when it lost power would break
:the link. Do you know of such a device?

There are many "broadband routers" on the market, a number of which
can be set to turn off NAT.
--
"No one has the right to destroy another person's belief by
demanding empirical evidence." -- Ann Landers

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

googlegroups@marget.com wrote:

(snip)

> So, yes, it's completely the opposite of what you're looking for, but
> it makes the point that running ethernet through some relays might be
> okay

I would think a simple relay would be more reliable than a relay
switching a complex of electronics.

I think you can just short across each pair to turn it off. That
is probably more reliable than opening the circuit. (Probably one
pair is enough if you decide which one.) I am pretty
sure it won't hurt anything, but you might want to verify that
first. I think it will also have less effect on the impedance
characteristics of the line, though it might be better not to try
it at gigabit levels anyway.

-- glen

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <dgc8ds$t1e$1@panix5.panix.com>, Al Dykes <adykes@panix.com> wrote:
:Somehow I think we are playing in different leagues. CISCO, HP and a
ile of other venders make managed hubs and switches. The HP procurve
:stuff (except for the bottom end) has built-in WEB management
:interface. I'm not familiar with CISCO.

I found the Procurve Hubs on HP's site, and you are right that they
do have a Web interface. On the other hand, checking around to stores,
I'm finding them available as Used, or "1 in stock" (at wildly different
prices.)

:Look for an HP Procurve box on ebay

If you have to buy it on eBay now, then it it isn't a good choice
for devices intended to be easily replaceable.
--
Daylight is a trademark of OSRAM SYLVANIA INC.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <dgc99i$n7s$1@canopus.cc.umanitoba.ca>,
Walter Roberson <roberson@ibd.nrc-cnrc.gc.ca> wrote:
>In article <dgc8ds$t1e$1@panix5.panix.com>, Al Dykes <adykes@panix.com> wrote:
>:Somehow I think we are playing in different leagues. CISCO, HP and a
>ile of other venders make managed hubs and switches. The HP procurve
>:stuff (except for the bottom end) has built-in WEB management
>:interface. I'm not familiar with CISCO.
>
>I found the Procurve Hubs on HP's site, and you are right that they
>do have a Web interface. On the other hand, checking around to stores,
>I'm finding them available as Used, or "1 in stock" (at wildly different
>prices.)
>
>:Look for an HP Procurve box on ebay
>


Buy two or three, there *real* cheap.

HP will fix them forever.



--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

"Rich Noonan" <rnoonan@gmail.com> wrote:
>William P. N. Smith wrote:
>> How about removing the power to a 2-port switch? Would that solve
>> your problem?
>
>This would be a decent solution b/c then I wouldn't be compelled to
>pour epoxy into unused switch ports :-( Have you seen 2 port ethernet
>switches? Can you reference a manufacturer?

Hmm, Linksys BEFSR11 looks like a product that's still extant, and you
can probably put it in static routing or DMZ mode if you need to...
Under $50 at Amazon, probably cheaper elsewhere.

>Can't unplug. HW switches would be great! Please tell me more if you
>can.

I'm not home right now, though even if I were I'm not sure I could put
my hand on the printout. Of course, in this case, google isn't your
friend, as "cat5 switch" gets you lots of electronic 10/100 switches,
but no mechanical ones.

I do know they exist! 8*)

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

A long time ago, at a trade show far away...

There was a company called LanHopper that made managable (via console or
out of band SNMP) mechanical ethernet "switches". I put switches in
quotes as they were not what we consider switches today. The were full of
relays. You could hook one port to any other port, or just physically
disconnect a port.

Sounds like what the OP is looking for. Have no idea where they went or
what happened to them.

--
Frank Stutzman
Bonanza N494B "Hula Girl"
Hood River, OR

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <08vji1hofodmd9cl6cq5io83ttsfvur2as@4ax.com>,
William P. N. Smith <> wrote:
>"Rich Noonan" <rnoonan@gmail.com> wrote:
>>William P. N. Smith wrote:
>>> How about removing the power to a 2-port switch? Would that solve
>>> your problem?
>>
>>This would be a decent solution b/c then I wouldn't be compelled to
>>pour epoxy into unused switch ports :-( Have you seen 2 port ethernet
>>switches? Can you reference a manufacturer?
>
>Hmm, Linksys BEFSR11 looks like a product that's still extant, and you
>can probably put it in static routing or DMZ mode if you need to...
>Under $50 at Amazon, probably cheaper elsewhere.
>
>>Can't unplug. HW switches would be great! Please tell me more if you
>>can.
>
>I'm not home right now, though even if I were I'm not sure I could put
>my hand on the printout. Of course, in this case, google isn't your
>friend, as "cat5 switch" gets you lots of electronic 10/100 switches,
>but no mechanical ones.
>
>I do know they exist! 8*)


Check blackbox.com
--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <1126811324.599443.75360@f14g2000cwb.googlegroups.com>,
Rich Noonan <rnoonan@gmail.com> wrote:
>William P. N. Smith wrote:

>> How about removing the power to a 2-port switch? Would that solve
>> your problem?

>This would be a decent solution b/c then I wouldn't be compelled to
>pour epoxy into unused switch ports :-( Have you seen 2 port ethernet
>switches? Can you reference a manufacturer?

http://www.drbott.com/prod/db.lasso?code=4331-FS02

$US10
--
Look out, there are llamas!

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

William P. N. Smith wrote:
> "Rich Noonan" <rnoonan@gmail.com> wrote:
>
>>I'm looking for a device which could act as an ethernet breaker switch
>>based on the application or loss of power to that device.
>
>
> How about removing the power to a 2-port switch? Would that solve
> your problem?
>
> If this is something you have to do one at a time, can you unplug
> cables or flip hardware switches? I've seen CAT5 mechanical
> switches..

Do you have space in/near the machine controller where you can stash a
4-port switch without the danger of people plugging extra equipment in?

If yes, something like this would be possible: put in a 4-port switch
connecting machine, one operator interface, and a cable to the upstream
port on the next switch. Next switch connects to all other operator
interfaces and whatever people want to plug in. To limit control, cut
power to the second switch, or put a hardware/keylock switch in the
cable between the two switches.

J.

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

glen herrmannsfeldt wrote:

> I would think a simple relay would be more reliable than a relay
> switching a complex of electronics.

Don't forget, you'll need a relay that can handle the signal, without
disturbing the impedance too much.

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

James Knott wrote:

> glen herrmannsfeldt wrote:

>>I would think a simple relay would be more reliable than a relay
>>switching a complex of electronics.

> Don't forget, you'll need a relay that can handle the signal, without
> disturbing the impedance too much.

Yes, that is why I thought it might be better to short the line.

A double pole relay with the armature of the two poles shorted,
and then the two wires of the appropriate pair to the fixed contacts.
The fixed contacts are much shorter in most relays, and so should have
less effect on the signal. Still, it might be that 100baseTX is as
far as I might want to go with it. The cable can be arranged so that
the twisted pair comes right to the contacts without a long stub,
other than the contacts themselves.

At least you don't need a very expensive coaxial relay!

-- glen

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

Thanks all for the input. For those concerned with my electrical
skills, I assure you I have almost none! I'm a software/systems eng.
at a fairly large company with access to top notch EE and ME folks.
However, like many these days our management is all about the buy and
very down on the build solutions. However, if we do go the build
direction you've given me some great bits of data to start the ball
rolling.

So, I did end up finding something fairly close to our needs. The big
question will be pricing, but it looks to be a cool device for our type
of application.
http://www2.automation.siemens.com/net/html_76/produkte/040_ind_sec_scalance_s.htm
During a discussion of this whole problem I was asked about embedded,
data inspection type security in the equipment and this Siemens device
claims to address some of that. However, most importantly to me is
that it has 2 x 10/100B-TX ports and a standard industrial type
package.

But if that doesn't work out you've got me started with some design
points to pass along for an in-house switch. Thanks again!

-Rich

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

Al Dykes wrote:
> I asked before, You aren't doing anything safety-critical or covered
> by OSHA, are you?

Yes, it's safety critical and when the equipment is installed in the US
I'm sure it's covered by OSHA. But it's covered by much more stringent
industry regs which is what I'm helping respond to.

As an example, OSHA defines high-voltage as >50V. When fully
operational, areas of our equipment have >3KV running through. It can
arc almost 3 inches and you can be injured just pointing in the wrong
direction. OSHA regs, if any relevant ones exist, would not adequately
cover this.

So, that's why when a tech has defeated interlocks on the safety doors
to troubleshoot a problem in a high-voltage area it's critical that
they be solely in control of the machine. With that much danger at
play you might think they would just go through the trouble of
unplugging the ethernet as needed, but this can be challenging both
physically and procedurally. A blend of automation, direct human
interface, and careful procedure is the standard solution.

-Rich

 

[Guest]

Archived from groups: comp.dcom.lans.ethernet (More info?)

"Rich Noonan" <rnoonan@gmail.com> writes:

> I'm looking for a device which could act as an ethernet breaker switch
> based on the application or loss of power to that device. On one hand
> we could accomplish what we need with a low-cost switch and two cables.
> However, in our application and environment this would introduce
> several new problems. What I really need is a two port fast ethernet
> repeater that I could cut power to and break the link. Of course, such
> a thing doesn't exist (or does it?).
>
> Any suggestions on a part that could fill this role? We could probably
> design and build a mechanical switch to break the link, but I'm trying
> to find something we could buy before going down that road.

No ready made part recommendation.
Basicly you could build what you need by combining an Ethernet
switch and an relay connected to PC serial/parallel port
(in case you have PC nearby that runs the application).

Just connect the switch between the power source and the switch.
The computer control power to switch on/off.
If your switch uses low voltage power supply (like those small
"wall wart" powered switches), then just adding a relay to one of
the power wires is preaty easy and safe. If you know how to safely
build mains powered things (and you have necessary qualifications
to do that) then circuit for controlling mains power to device is
another option.

You can find some information on interfaceing relay to serial port at
http://www.epanorama.net/circuits/rspower.html

Another interresting RS-232 port hack:
Marko Mäkelä's electronics projects: RS-232 Solid State Relay
http://www.funet.fi/~msmakela/electronics/relay/

Parallel port interfacing made easy
http://www.epanorama.net/circuits/parallel_output.html


There are also commercial products and electronics kits
for PC interfacing.

And there are companies that make remotely manageable
power switchers that can take controllign signal
from almost everywhere (serial port, Ethernet,
telephone line, cellular phone SMS text message etc..)


besides using switch and turn it's power on/off you can use
the relay directly to cut the Ethernet signals..
A relay that cuts the connection on all the four signal carrying
wires (applied to 10Base-T and 100Base-TX) shoudl do the job.
Keep in mind in the circuit design not to cause too much
impedance mismatches and crosstalk for the signals.
Many normal signal relay contacts should carry Ethernet signals
acceptably.
Other way to cut the connection woudl be to short the signal
wire pair (one relay contact on TX pair and other on RX pair).
Short circuit should stop the Ethernet signals from getting
to other end, so should look same to cards that line is cut.

Those are just few of my ideas for this.


tomi031cv








--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

 

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Archived from groups: comp.dcom.lans.ethernet (More info?)

"Rich Noonan" <rnoonan@gmail.com> writes:

> I'm looking for a device which could act as an ethernet breaker switch
> based on the application or loss of power to that device. On one hand
> we could accomplish what we need with a low-cost switch and two cables.
> However, in our application and environment this would introduce
> several new problems. What I really need is a two port fast ethernet
> repeater that I could cut power to and break the link. Of course, such
> a thing doesn't exist (or does it?).
>
> Any suggestions on a part that could fill this role? We could probably
> design and build a mechanical switch to break the link, but I'm trying
> to find something we could buy before going down that road.

No ready made part recommendation.
Basicly you could build what you need by combining an Ethernet
switch and an relay connected to PC serial/parallel port
(in case you have PC nearby that runs the application).

Just connect the switch between the power source and the switch.
The computer control power to switch on/off.
If your switch uses low voltage power supply (like those small
"wall wart" powered switches), then just adding a relay to one of
the power wires is preaty easy and safe. If you know how to safely
build mains powered things (and you have necessary qualifications
to do that) then circuit for controlling mains power to device is
another option.

You can find some information on interfaceing relay to serial port at
http://www.epanorama.net/circuits/rspower.html

Another interresting RS-232 port hack:
Marko Mäkelä's electronics projects: RS-232 Solid State Relay
http://www.funet.fi/~msmakela/electronics/relay/

Parallel port interfacing made easy
http://www.epanorama.net/circuits/parallel_output.html


There are also commercial products and electronics kits
for PC interfacing.

And there are companies that make remotely manageable
power switchers that can take controllign signal
from almost everywhere (serial port, Ethernet,
telephone line, cellular phone SMS text message etc..)


besides using switch and turn it's power on/off you can use
the relay directly to cut the Ethernet signals..
A relay that cuts the connection on all the four signal carrying
wires (applied to 10Base-T and 100Base-TX) shoudl do the job.
Keep in mind in the circuit design not to cause too much
impedance mismatches and crosstalk for the signals.
Many normal signal relay contacts should carry Ethernet signals
acceptably.
Other way to cut the connection would be to short the signal
wire pair (one relay contact on TX pair and other on RX pair).
Short circuit should stop the Ethernet signals from getting
to other end, so should look same to cards that line is cut.


--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/