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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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Could be a serial connection,
an older DE9 connection.

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You may hear these
referred to as loopbacks,

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or bouncebacks, or null connections.

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Whatever you want to call 'em,

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I usually just refer to it as a loopback,

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and the most common one is

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the ethernet RJ-45 loopback connector.

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This guy has two ports,

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and the first is the male
port, which has the RJ-45 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,
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 could do this with just about

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any regular switching device,

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and that link light should come up

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once it registers that
guy, 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 me see that here.

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You got one, two, 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,

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a four-port device that is a
multi-function network device,

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it may not function properly,

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'cause effectively what this guy is,

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is a crossover connection.

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Pins one to three and two
to to six are crossed.

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So it's kind of like a crossover cable.

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It's not a crossover cable, but it's like,

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it does the same sort 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

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inside 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, you know,
if you have a rack-mount,

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24 port switch, 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, so you test it.

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And just plug in the guy,

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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,

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and if so, what port it is connected to.

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Now, the better way to do
that is to use, you know,

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a tone generator, 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 want to
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

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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 I'll show that now.

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Okay, so here we have the back

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of the AV editor computer we built.

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Here is our RJ-45 port,
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, or
the wired network connection

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is not giving us a link light.

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We can narrow down the problem

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by plugging in the loopback adapter,

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and see if it generates
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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Connect that to that side,

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and you can do that when you're connecting

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to the PC, or to a switch.

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And then you'd use the
other end of the patch cable

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to connect into the port,
whatever that port may be.

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But we can just connect the
RJ-45 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 RJ-45 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

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after 20 seconds or so
if it's a working port,

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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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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,

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or you might use 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 what the scope of the problem is.

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So a little bit about the RJ-45 loopback.

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You can use that to
find particular cables,

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and what ports they are
connected to on a switch,

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and identify if the RJ-45
connection on the computer

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is working properly.

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So that's it for this sub-lesson.