WEBVTT

0:00:07.180000 --> 0:00:11.540000
 This video is part of the CCT routing
 and switching curriculum and it

0:00:11.540000 --> 0:00:13.360000
 comes from section four
 of that curriculum.

0:00:13.360000 --> 0:00:18.200000
 The title of this video is identifying
 null modem cables.

0:00:18.200000 --> 0:00:23.380000
 My name is Keith Bogart and I will be
 your instructor for this session.

0:00:23.380000 --> 0:00:25.940000
 So in this particular session we're
 going to be covering a single bullet

0:00:25.940000 --> 0:00:30.560000
 point from section four which is bullet
 point number four dot eleven which

0:00:30.560000 --> 0:00:34.580000
 is identifying null modem cables
 and their application.

0:00:34.580000 --> 0:00:39.660000
 So what is a null modem cable and
 what problem is solved by that?

0:00:39.660000 --> 0:00:43.820000
 So what we're talking about here
 is another type of serial cable.

0:00:43.820000 --> 0:00:47.100000
 Okay, this is not an RJ45 cable,
 this is a serial cable.

0:00:47.100000 --> 0:00:49.620000
 But what makes it different from some
 of the other serial cables we looked

0:00:49.620000 --> 0:00:55.500000
 at before? Well, when I was doing my
 video just a little while ago on

0:00:55.500000 --> 0:01:00.300000
 how to connect a modem to a serial
 port on like a laptop or router, we

0:01:00.300000 --> 0:01:05.280000
 were talking about connecting a DTE
 device which was your router or your

0:01:05.280000 --> 0:01:08.360000
 laptop to a DCE device.

0:01:08.360000 --> 0:01:11.620000
 And so that used what was called
 a straight through cable.

0:01:11.620000 --> 0:01:17.240000
 What if I want to connect two DTE devices
 together or two DCE devices

0:01:17.240000 --> 0:01:22.080000
 together? A straight through cable,
 a straight through RS-232 cable will

0:01:22.080000 --> 0:01:23.320000
 not work for that.

0:01:23.320000 --> 0:01:27.220000
 That's where I would need one
 of these, a null modem cable.

0:01:27.220000 --> 0:01:30.940000
 Now it's also called a crossover cable
 because as you can imagine, think

0:01:30.940000 --> 0:01:33.040000
 about Ethernet cabling, right?

0:01:33.040000 --> 0:01:36.820000
 If I'm connecting two routers together
 via Ethernet or two switches together

0:01:36.820000 --> 0:01:38.820000
 via Ethernet, what do I need?

0:01:38.820000 --> 0:01:40.200000
 I need a crossover cable.

0:01:40.200000 --> 0:01:43.220000
 I need the transmit to go to the receive
 and the transmit to go to the

0:01:43.220000 --> 0:01:45.640000
 receive. Two like devices.

0:01:45.640000 --> 0:01:50.000000
 Same idea here except we're talking
 about serial cables, not Ethernet

0:01:50.000000 --> 0:01:55.180000
 cables. So you can see that with a null
 modem cable, the signals are going

0:01:55.180000 --> 0:01:56.480000
 to be crossed over
 from each other.

0:01:56.480000 --> 0:01:59.940000
 And I'll show you what the pinout looks
 like for that in just one second.

0:01:59.940000 --> 0:02:03.860000
 So a null modem cable is also called
 a serial crossover cable.

0:02:03.860000 --> 0:02:07.640000
 Now there's a big difference between
 a serial, between a cable that's

0:02:07.640000 --> 0:02:10.980000
 what's called a rolled cable
 versus a crossover cable.

0:02:10.980000 --> 0:02:13.620000
 I've mentioned this a few times but
 it's very important to get in your

0:02:13.620000 --> 0:02:18.120000
 head. A rolled cable simply means that
 all the pins on one side are exactly

0:02:18.120000 --> 0:02:20.660000
 the opposite on the other side.

0:02:20.660000 --> 0:02:23.280000
 One goes to eight,
 two goes to seven.

0:02:23.280000 --> 0:02:26.020000
 That's not a null modem cable.

0:02:26.020000 --> 0:02:27.480000
 That's a rolled cable.

0:02:27.480000 --> 0:02:31.060000
 A crossover cable has a very distinct
 pinout pattern which I'll show you

0:02:31.060000 --> 0:02:35.260000
 in a second which is
 not a rolled cable.

0:02:35.260000 --> 0:02:37.420000
 So when would you have to
 use something like this?

0:02:37.420000 --> 0:02:45.160000
 Well, back in the day when PCs had serial
 ports and they didn't have Ethernet

0:02:45.160000 --> 0:02:48.100000
 ports, I can't even imagine how
 old that would have been.

0:02:48.100000 --> 0:02:51.520000
 But for example, if you want to connect
 one PC to another PC and do gaming

0:02:51.520000 --> 0:02:55.520000
 between those two PCs, this would
 be one example of doing that.

0:02:55.520000 --> 0:02:58.380000
 Connecting a cable between the two serial
 ports but it would have to be

0:02:58.380000 --> 0:03:01.480000
 a crossover cable,
 a null modem cable.

0:03:01.480000 --> 0:03:07.300000
 Another more practical example is most
 Cisco devices, the console port

0:03:07.300000 --> 0:03:10.860000
 is an RJ45 jack but
 not all of them.

0:03:10.860000 --> 0:03:14.420000
 Some devices, for example, right here
 we see a content delivery engine.

0:03:14.420000 --> 0:03:18.700000
 This has a console port
 which is a DB9 port.

0:03:18.700000 --> 0:03:23.040000
 So in order to connect your laptop
 to that port, you're going to need

0:03:23.040000 --> 0:03:29.360000
 a null modem cable because this is DTE
 going to DTE and this case, that

0:03:29.360000 --> 0:03:34.020000
 regular flat blue cable that they
 give you is not going to work.

0:03:34.020000 --> 0:03:35.220000
 So here we see what
 they look like.

0:03:35.220000 --> 0:03:36.380000
 Here's the actual pinouts.

0:03:36.380000 --> 0:03:39.600000
 Like I mentioned in the previous video,
 the main thing here is not the

0:03:39.600000 --> 0:03:44.260000
 connector. Hopefully the connector on
 the cable will say null modem but

0:03:44.260000 --> 0:03:48.380000
 it might not. The main thing is you
 have to know what the pinout is to

0:03:48.380000 --> 0:03:49.840000
 truly make it null modem.

0:03:49.840000 --> 0:03:51.960000
 When we can see here,
 it's pretty simple.

0:03:51.960000 --> 0:03:55.940000
 A basic null modem cable just
 consists of three wires.

0:03:55.940000 --> 0:04:00.900000
 So pin five of ground is going to pin
 five of ground on the other side.

0:04:00.900000 --> 0:04:06.400000
 It's mostly the receiving transmit that's
 crossed over with each other.

0:04:06.400000 --> 0:04:09.880000
 So here pin two which is received on one
 side is going to pin three transpond

0:04:09.880000 --> 0:04:13.720000
 on the other and you can see pin three
 on one side is going to pin two

0:04:13.720000 --> 0:04:14.240000
 on the other side.

0:04:14.240000 --> 0:04:19.020000
 So that's the main thing, three to
 two, three to two and five to five.

0:04:19.020000 --> 0:04:22.120000
 So that's our simple
 null modem cable.

0:04:22.120000 --> 0:04:26.480000
 Now if I was connecting like for example
 two modems together to DCE devices

0:04:26.480000 --> 0:04:30.580000
 that also relied on additional signaling,
 control signaling like data

0:04:30.580000 --> 0:04:35.340000
 set ready and ready to send, you know
 RTS, CTS and all that other stuff,

0:04:35.340000 --> 0:04:38.400000
 those are some additional pins that
 would have to be crossed over.

0:04:38.400000 --> 0:04:41.340000
 So you can see there is a different kind
 of a null modem cable that includes

0:04:41.340000 --> 0:04:47.620000
 hand shaking. Main thing, when you see
 a serial interface like a DB nine

0:04:47.620000 --> 0:04:52.760000
 interface or a DB 25 interface, you
 don't necessarily know whether that

0:04:52.760000 --> 0:04:58.160000
 device is pinned out
 for DTE or DCE.

0:04:58.160000 --> 0:05:01.820000
 Just by looking at that, you don't
 know if it's considering pin two to

0:05:01.820000 --> 0:05:05.880000
 be received or if it's considering
 pin two to be transmit, you need to

0:05:05.880000 --> 0:05:06.660000
 look in the documentation.

0:05:06.660000 --> 0:05:15.180000
 Now all laptops are DTE and all modems
 are DCE and all routers are DTE.

0:05:15.180000 --> 0:05:18.120000
 But there's a lot of other stuff out
 there like printers and other things

0:05:18.120000 --> 0:05:23.660000
 that may be pinned out in
 as a DCE or a DTE device.

0:05:23.660000 --> 0:05:26.220000
 And so you really need to know
 what I'm talking about here.

0:05:26.220000 --> 0:05:32.100000
 If it's you have to know, am I going
 DTE to DTE or is it DTE to DCE?

0:05:32.100000 --> 0:05:34.440000
 They take different cable types.

0:05:34.440000 --> 0:05:36.460000
 So what do they look like?

0:05:36.460000 --> 0:05:39.800000
 Hopefully it'll be labeled like
 we see the one here on the left.

0:05:39.800000 --> 0:05:41.280000
 It says null modem.

0:05:41.280000 --> 0:05:43.880000
 Here we actually have
 a null modem adapter.

0:05:43.880000 --> 0:05:49.140000
 Last point I want to impress upon
 you before I close out this video.

0:05:49.140000 --> 0:05:56.640000
 I've noticed a lot of websites that
 show the Cisco flat blue satin cable

0:05:56.640000 --> 0:06:00.240000
 as a null modem cable and they
 actually list this image.

0:06:00.240000 --> 0:06:03.620000
 That is technically incorrect.

0:06:03.620000 --> 0:06:05.800000
 That is not a null modem cable.

0:06:05.800000 --> 0:06:08.420000
 That is a rolled cable.

0:06:08.420000 --> 0:06:12.400000
 Remember there's a difference
 between rolled and crossover.

0:06:12.400000 --> 0:06:15.040000
 Null modem is a crossover.

0:06:15.040000 --> 0:06:17.940000
 That is a rolled cable.

0:06:17.940000 --> 0:06:19.660000
 And that concludes this video.