Make your own Ethernet Loopback connector

Thu, 2005-08-04 16:08 — Barry

Occasionally I have the need for a network card to operate as though it has a live network connection, without it actually being connected to a network. Typically for testing purposes, and occasionally for specific software configurations. To do this I need a quick and easy way to make a loopback connector. Something small enough to keep in my PC toolkit, yet large enough that it won't be lost. Mine end up about the size of a ballpoint pen. The following is a quick "how to" for making an Ethernet loopback connector.

Cut 4 or 5 inches of the end off of a network cable, keeping the connector intact.
Cut away two inches of the main sheath covering the eight wires. (Be careful not to cut the wires inside.)
Cut the sheath on Orange-White and Green-White and twist them together. Tape this pair. (Soldering is recommended)
Cut the sheath on Green and Orange and twist them together. Tape this pair as well. (Soldering is recommended)
Cut the sheath on Blue and Brown-White and twist them together. Tape this pair as well. (Soldering is recommended)
Cut the sheath on Blue-White and Brown and twist them together. Tape this pair as well. (Soldering is recommended)
Tape the end down to make it look presentable.
Plug your new RJ-45 loopback into your Network Card.

Comments

Wed, 2008-04-02 15:05 — Martin (not verified)

software loopback adapter

if you only need a loopback for test applications pourpous or something like a bridge with a virtual machine and you use winxp you can use the microsoft loopback adapter. That is a software tool look it http://www.windowsnetworking.com/articles_tutorials/Install-Microsoft-Lo.... if you need test the interface you must have this hard loopback. Good job

Mon, 2008-02-25 03:19 — tlol (not verified)

After completing this

After completing this cross-over connection, you have a cable that is capable of loopback at Ethernet speeds of 10, 100, or 1000 Mbps.

The more difficult task of GigE loopback is making sure the NEXT (Near End Cross Talk) cancellation is turned off in the transceiver. GigE must use all four (4) pairs of wires at a base baud rate of 125Mhz in order to function. This configuration creates a lot of crosstalk1, so all GigE PHYs must incorporate some sort of crosstalk cancellation. However, when you are performing loopback and have the wires directly connected to each other, the crosstalk will appear to be at 100% even though this configuration is intentional. Therefore, the NEXT canceller needs to be disabled for loopback testing.

Some GigE PHYs do not have an option to disable the NEXT canceller, but most do. You will need to consult the datasheet for your transceiver to determine the specific sequence. Some can be as simple as enabling a single bit in a control register -- others require a more complicated sequence such as:

1. Force the PHY to “Master” mode
2. Perform PHY reset
3. Force the PHY into GigE mode
4. Enable the Gigabit stub (i.e. - the loopback connector)
5. Disable NEXT cancellation

Once the NEXT canceller is disabled, you should be able to perform loopback testing using the GigE loopback cable.

Tue, 2007-05-22 11:46 — Plater (not verified)

10mbps

I have need of a way to simulate a 10mbps connection. I was hoping that a loopback device could be created to make the card think it has a 10mbps connection (and then I would create another one to be 100mpbs connection and depending on its support a 1000mbps connection)

Is that possible? Connecting just orange-green pairs gave me the 100mbps.
Any ideas on how to accomplish this?

Thu, 2007-12-20 13:38 — hunkydorry (not verified)

loopback

I'm sure you have gotten an answer on this by now, but to simulate a 10mb connection, only loopback pairs one and three (Orange and Green) while ommiting two and four (Blue and Brown).

The device will step down to 10mb.

Sun, 2007-12-23 01:20 — Barry

Good to know, I think we now

Good to know, I think we now have an answer.

Wed, 2007-05-23 01:15 — Barry

Wrong part of the problem.

I don't think the loopback cables are capable of this. To my knowledge this is dependent on the speeds supported by the port to which the loopback cable is connected. I would check the settings for the device in question. The same would apply to the 1000mbps (Gigabit) option, with the caveats stated in previous comments.

Thu, 2007-05-10 19:25 — gokul (not verified)

Gigabit loopback in smartbits?

Hello all,
I was wondering if this connection works fine with SmartBits 600B on Gigabit ports.
I have crimped the loopback correctly, but I do not see the link port glowing after connecting it to the SmartBits port. I have also tested with Sunrise telecom tester, but same problem persisted.
Could anyone tell me the reason why I cannot get that running?

Thanks,
Gokul.

Fri, 2007-05-11 15:00 — Barry

...covered previously.

See Gigabit "sort of" supported below.

Tue, 2007-03-27 08:01 — Zero_Signal (not verified)

It worked on mine perfectly.

It worked on mine perfectly. Thanx a bunch!

Wed, 2007-03-07 10:05 — Barney Garrett (not verified)

Gigabit interfaces ?

Doesn't appear to work on any of the Gigabit interfaces I've tried.

Fri, 2007-09-07 03:48 — Mik (not verified)

RE: Gigabit interfaces ?

I've tried it on a Lenovo IBM T60 (Intel Pro/1000 PL) and it won't work at Gigabit speeds but if you manually adjust the link type to a non-gigabit setting then it will.

Fri, 2008-04-11 18:32 — Anonymous (not verified)

Gig E Nic Works - Sort of

I tried this loopback wiring on a Broadcom 1 Gb NIC in an HP laptop. It does not work at 1 Gb, but setting the adapter to 100 it works. Good enough form my needs.

Thanks mucho.

Wed, 2007-03-07 20:30 — Barry

Gigabit "sort of" supported

Consult the documentation for the equipment with regard to Gigabit loopback support. Not all Gigabit Ethernet hardware supports loopback operation. This loopback should work for hardware where Near End Crosstalk (NEXT) can be disabled and where the equipment can be looped back to itself.

Fri, 2007-11-30 15:05 — Anonymous (not verified)

Hmm

Maybe a 2,2 ohm resistor as loop will fix the NEXT issue??
Just an idea... (not tested)

Sun, 2007-12-23 01:21 — Barry

Well folks, have at it.

Well folks, have at it. Unfortunately, I do not currently have a gigabit connection to test this idea.

Sat, 2006-09-30 18:48 — Jose Campos (not verified)

Make your own Ethernet Loopback connector

PC network cards only use two pairs of cables to transmit data, why connect 4&7 and 5&8? Great diagram!

Tue, 2008-01-29 13:35 — IceMage (not verified)

Connections to 4,7 & 5,8

One a 100BaseTX this will work fine, as they are both ground cables.
On 1000BaseTX, you'll need to connect 4 to 5, and 7 to 8, instead of the other way around.
4 is Data -, and 5 is Data +, same for 7 and 8. Theoritically, connecting 4 to 8 may work, but I doubt this is the intention.
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=pincabnet_ethe...
This page shows the pinout.

Fri, 2008-02-01 14:42 — Barry

4 connects to 7 and 5 to 8

According to Sun Microsystems the pinouts for for proper for building a gigabit and 10/100 loopback are are: 1 to 3, 2 to 6, 4 to 7, and 5 to 8. The linked page at Sun also has a lot of loopback pin-outs for other cable types as well. (multiple types of: 8-pin, 9-pin, 25-pin, 37-pin, and 96-pin)

Sun, 2006-10-01 13:49 — Barry

Why the extra wires? (Answer)

10/100 Mbps ethernet only uses two pair of wires, however Gigabit ethernet uses the extra wiring.

Thu, 2006-11-16 14:34 — Yadin (not verified)

Ethernet Loopback

Hi,
1. What is the software used (like hyper terminal for rs232)?
Best Regards,
Yadin

Thu, 2006-11-16 15:54 — Barry

Loopback Software

This is useful with many types software, such as many network testing packages. Hyper terminal will require telnet on the PC, or if you have SSH installed/enabled, you can use Putty on Windows or ssh from the Linux/Mac shell. Also any software that requires an active network connection to start, there are several software packages that require network dongles for licensing purposes when a network connection is not available.