Crossover patch cords provide proper pin-outs

When daisy-chaining Ethernet hubs (concentrators) with modular patch cords, you may sometimes find that your hubs cannot communicate with each other.

Mar 1st, 1997
Th Cim62957 14

Robert Walls

Wiltel

Problem

When daisy-chaining Ethernet hubs (concentrators) with modular patch cords, you may sometimes find that your hubs cannot communicate with each other.

Solution

Connect the hubs with a "crossover" patch cord; that is, terminate one end of the patch cord using the T568A pin/pair assignment, and the other end with the T568B pin/pair assignment. This provides the correct installation of the transmit wire pairs (pins 1 and 2) and the receive wire pairs (pins 3 and 6). Having a quick and easy way to recognize the pinouts of a modular plug can make it easier to eliminate pin/pair assignment confusion and ensure that the hubs can communicate properly.

Procedure

1) Examine the modular patch cord plug and determine whether it follows the T568A or T568B pin/pair assignment. The T568A pin/pair is the sequence recommended by the Telecommunications Industry Association/Electronic Industries Association tia/eia-568a standard for terminating unshielded twisted-pair (UTP) cabling on 8-pin modular plugs and jacks. However, the T568B sequence, specified by the ieee 802.3 10Base-T Ethernet standard, is widely used.

2) The sequence of the pin/pair assignment is the order in which the incoming pairs are terminated into the modular pins. Each pair is designated as a tip (T) and ring (R) conductor; for example, pair 1 is designated as T1 and R1. To identify the pairs, 4-pair copper cable is color-coded. The primary color in color-code option 1 is always white and the secondary colors are blue, orange, green, and brown, in that order. Pair 1 is white/blue, for example, where one conductor is white with a blue stripe and the other conductor in this pair is blue with a white stripe. Similarly, pair 2 is white/orange, pair 3 white/green, and pair 4 white/brown.

3) Once you have identified the pin-out sequence for one end of the patch cord, go to the other end. Cut off the plug at that end and reterminate it using the other standard pin/pair sequence; that is, if the pin-out on one end of the modular patch cord is T568A, then you should reterminate the other end of the cord with the T568B pin/pair assignment. This ensures that pins 1 and 2 connect to pins 3 and 6.

4) When terminating the modular plug, always follow the tia/eia-568a standard guidelines; that is, allow no more than 1/2 inch untwist for Category 5 and no more than 1 inch for Category 4 UTP patch cords.

5) Test the patch cord when completed, and your tester will indicate a reversal on pairs 2 and 3.

Note: This type of crossover patch cord is typically used on lower-end hubs. Higher-end hubs generally have a built-in polarity-reversing capability.

Click here to enlarge image

In this crossover modular patch cord, the plug on the left has a T568A pin-out sequence and the other end has been reterminated to the T568B pin/pair assignment.

Robert Walls, installation manager at Wiltel (Marlborough, MA), submitted this article in the Installer Tips Contest at Cabling Installation Expo `96, held last October in Charlotte, NC.

This tip was compiled by Dennis Mazaris, registered communications distribution designer (rcdd), a design consultant at PerfectSite, 100 Glenn Drive, Suite A-11, Sterling, VA 20164, tel: (703) 450-8986, fax: (703) 450-9343, e-mail: dmazaris@perfectsite.com.

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