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- [Instructor] Let's talk
about loopback plugs.

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There are loopback testing devices

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for just about any type of
communications connection.

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It could be a serial connection,
an older DE-9 connection.

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You may hear these referred to
as loopbacks or bounce backs

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or null connections.

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Whatever you wanna call 'em.

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I usually just refer to it as a loopback,

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and the most common one

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is the ethernet RJ45 loopback connector.

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This guy has two ports and
the first is the male port,

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which has the RJ45 plug,

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and then the other side
has the female port.

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So you can connect this
in a variety of ways.

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And what we do with this,

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the reason we use this,

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is really to find one of two things.

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One, we can find if a particular cable

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is connected to a switch.

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So, let's say there's a jack on the wall

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and you don't know if it's
connected to a switch or not.

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You could plug this into the jack

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and then that port will
light up on the switch.

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So you can check it that way.

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You could also test a patch cable.

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If the patch cable's connected
directly to a switch,

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you could do that from this end.

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And so, to show this, we'll connect this

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to a basic four port switch here.

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And I'll make a connection
to one of the ports now.

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And the lights are pretty dim,

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but you can see here,
there's a power light.

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And right now it doesn't have a connection

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but it usually takes about
20 seconds or so to show up.

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And there it goes.

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There's our power connection.

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We have a link light for
that particular port.

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If I was to switch the port,

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again it'll take about 20
seconds to register that guy.

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And you can do this

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with just about any
regular switching device.

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And that link light should come up

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once it registers that guy.

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So there you go.

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And it shows the port that
it is actually connected to.

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We see here, it's connected to port five,

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and let's see that here.

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You have one through
one through five here,

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and then it'll show that link light.

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Regardless of what switch
you use, this will work.

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But if you have one of those
SOHO routers, four port device

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that is a multifunction network device,

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it may not function
properly 'cause effectively

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what this guy is is a
crossover connection.

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Pins one to three and
two to six are crossed.

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So it's kinda like a crossover cable.

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It's not a crossover cable,

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but it's like it does
the same type of thing.

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But instead of the crossover
cable connecting like devices

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this bounces the signal back

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because those wires are crossed inside

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the single individual plug.

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It's not like they're
crossed across a cable.

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So, that's the loopback plug.

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So again, this may or may not
work for your SOHO router.

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How often are you
actually gonna need that?

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This is more for if you have
a rack mount 24 port switch,

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48 port switch, or that type of thing.

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Now, sometimes these fail.

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If you have an issue with one of these,

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you can use a basic
patch tester to test it.

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And just plug in the guy
and then run your test.

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And you should see one and three lit up

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because pin one is crossed
to three on that plug,

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and two and six should light up

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because pin two is crossed to six.

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That's the essence of
a crossover connection.

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And then they cross the
others for good measure.

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Four to seven and five to eight.

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And that's good because
you can actually use this

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for more than just ethernet.

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You can use this for other
types of networking connections

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as well that use those
other pairs of pins.

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And so it cycles through
and there you have it.

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That's your loopback adapter.

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So it's a nice little device to find out

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if the cable's connected
to a switch and if so,

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what port it is connected to.

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Now, there's the better
way to do that is to use

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a tone generator.

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If you have a ton of cables

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and you're not sure where the cable is,

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and if you plug this in and
you don't get a link light,

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then you're gonna wanna
use a tone and probe kit,

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or use your cable certifier
that has tone built in

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to check for that guy.

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Now, that was the first way.

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The second thing you can do
with this is test the computer.

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You can test the network card

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to see if it's actually functioning.

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And now we'll show that now.

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Okay, so here we have
the back of the AV Editor

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computer we built.

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Here is our RJ45 port.

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So we can test that.

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Let's say that we're not sure
that it's working properly

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and we either don't have
a network connection,

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a wired network connection,

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or the wired network
connection is not giving us

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a link light.

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We can narrow down the
problem by plugging in

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the loopback adapter
and see if it generates

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that link light for us.

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Now again, when you plug it in,

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it takes about 20 seconds
usually to register.

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And so we have our loopback adapter here.

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And by the way, you don't have
to just connect with this.

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You can use a patch cable if you want,

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and connect that to that side.

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And you can do that when
you're connecting to the PC

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or to a switch, and then
you'd use the other end

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of the patch cable to
connect into the port.

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Whatever that port may be.

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But we could just connect the
RJ45 loopback into the port.

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And so we'll do that now.

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And we'll give it about 20
seconds and it should light up.

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And so 20 seconds passed, and
it was just about 20 seconds,

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and we got that link light.

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So, we know that the network
card is working physically.

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We know we're getting
electricity to the card.

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The card is working properly,

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and it's bouncing signal
back and forth from the card,

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from the network adapter
into this RJ45 loopback.

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And I keep saying card, but
this is really integrated

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into the motherboard,
but the network adapter.

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So whatever it is, a network
adapter or a switch port,

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that'll link up and give you
a link light after 20 seconds

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or so if it's a working port,
if the connection is good.

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So, that's a nice little tool to use

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to test the physical connection,
physical connectivity

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and check those LEDs.

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Make sure that you have that connectivity.

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You know, then we could say, okay,

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we were connecting a network cable,

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but we weren't getting a link on the PC.

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We've ruled out that it's
the PC's network adapter.

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So now we can go further and
test that network connection.

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And you might use this, or you might use

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your tone and probe kit,

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or you might use your cable certifier.

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You know, it all depends on
how far you have to go with it.

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And you know, what the
scope of the problem is.

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So a little bit about the RJ45 loopback.

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You can use that to find particular cables

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and what ports they're
connected to on a switch,

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and identify if the RJ45
connection on the computer

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is working properly.

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So that's it for this sub lesson.