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June 22, 2009

Ethernet Cable - Color Coding Diagram

The information listed here is to assist Network Administrators in the color coding of Ethernet cables. Please be aware that modifying Ethernet cables improperly may cause loss of network connectivity. Use this information at your own risk, and insure all connectors and cables are modified in accordance with standards. The Internet Centre and its affiliates cannot be held liable for the use of this information in whole or in part

T-568B Straight-Through Ethernet Cable

The TIA/EIA 568-A standard which was ratified in 1995, was replaced by the TIA/EIA 568-B standard in 2002 and has been updated since. Both standards define the T-568A and T-568B pin-outs for using Unshielded Twisted Pair cable and RJ-45 connectors for Ethernet connectivity. The standards and pin-out specification appear to be related and interchangeable, but are not the same and should not be used interchangeably
T-568B Straight-Through Ethernet Cable

Both the T-568A and the T-568B standard Straight-Through cables are used most often as patch cords for your Ethernet connections. If you require a cable to connect two Ethernet devices directly together without a hub or when you connect two hubs together, you will need to use a Crossover cable instead.

RJ-45 Crossover Ethernet Cable

A good way of remembering how to wire a Crossover Ethernet cable is to wire one end using the T-568A standard and the other end using the T-568B standard. Another way of remembering the color coding is to simply switch the Green set of wires in place with the Orange set of wires. Specifically, switch the solid Green (G) with the solid Orange, and switch the green/white with the orange/white.

Ethernet Cable Instructions:

Pull the cable off the reel to the desired length and cut. If you are pulling cables through holes, its easier to attach the RJ-45 plugs after the cable is pulled. The total length of wire segments between a PC and a hub or between two PC's cannot exceed 100 Meters (328 feet) for 100BASE-TX and 300 Meters for 10BASE-T. Start on one end and strip the cable jacket off (about 1") using a stripper or a knife. Be extra careful not to nick the wires, otherwise you will need to start over. Spread, untwist the pairs, and arrange the wires in the order of the desired cable end. Flatten the end between your thumb and forefinger. Trim the ends of the wires so they are even with one another, leaving only 1/2" in wire length. If it is longer than 1/2" it will be out-of-spec and susceptible to crosstalk. Flatten and insure there are no spaces between wires. Hold the RJ-45 plug with the clip facing down or away from you. Push the wires firmly into the plug. Inspect each wire is flat even at the front of the plug. Check the order of the wires. Double check again. Check that the jacket is fitted right against the stop of the plug. Carefully hold the wire and firmly crimp the RJ-45 with the crimper. Check the color orientation, check that the crimped connection is not about to come apart, and check to see if the wires are flat against the front of the plug. If even one of these are incorrect, you will have to start over. Test the Ethernet cable.
Ethernet Cable Tips:
*A straight-thru cable has identical ends.
*A crossover cable has different ends.
*A straight-thru is used as a patch cord in Ethernet connections.
*A crossover is used to connect two Ethernet devices without a hub or for connecting two hubs.
*A crossover has one end with the Orange set of wires switched with the Green set.
Odd numbered pins are always striped, even numbered pins are always solid colored.
Looking at the RJ-45 with the clip facing away from you, Brown is always on the right, and pin 1 is on the left.
*No more than 1/2" of the Ethernet cable should be untwisted otherwise it will be susceptible to crosstalk.
*Do not deform, do not bend, do not stretch, do not staple, do not run parallel with power cables, and do not run Ethernet cables near noise inducing components

BASIC THEOREY

By looking at a T-568A UTP Ethernet straight-thru cable and an Ethernet crossover cable with a T-568B end, we see that the TX (transmitter) pins are connected to the corresponding RX (receiver) pins, plus to plus and minus to minus. You can also see that both the blue and brown wire pairs on pins 4, 5, 7, and 8 are not used in either standard. What you may not realize is that, these same pins 4, 5, 7, and 8 are not used or required in 100BASE-TX as well. So why bother using these wires, well for one thing its simply easier to make a connection with all the wires grouped together. Otherwise you'll be spending time trying to fit those tiny little wires into each of the corresponding holes in the RJ-45 connector.

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June 17, 2009

Secrets Of SMPS Repair And Troubleshooting (Switch Mode Power Supply)

SMPS repair sometimes can be easy and sometimes are quite difficult to solve the problem. The power supply fault could be an open start up resistor only or could be even more than ten components burnt due to heavy lightning strike. In this article, I’m going to explain to you the power supply problems that caused by defective components at secondary side. Sometimes no power, power blink and low output power symptoms are due to problems in the main circuit board. It is not necessary the fault must be in the primary power section and shorted secondary output diodes.

The fault can be further down the secondary output lines which are in the main board. If you had measured all the components in the power supply section and could not find the defective component, then try troubleshoot components beyond the power supply section. A shorted small ceramic capacitor in the main board can cause the whole power supply to malfunction.

True case study- A Samtron Monitor 4Bn sent for repair with the complaint of no power. After troubleshooting the smps section for sometime, I’ve concluded that the power supply is working and suspect something shorted after the secondary output diodes and this made me to trace beyond the power supply unit. By using the explanation above, I managed to traced and found a shorted small resin ceramic capacitor located at the 80 volt line in the Monitor color board. The capacitor shorted to ground and made the power supply to stop operating.

The value of the shorted capacitor was .22 microfarad 100 volt. Replacing only the small capacitor brought the power supply back to life again. In smps repair, we as an electronic
repairer must know how to distinguish as which section is giving problem. Spending and concentrating your whole time to check on a working section is just a waste of time, you should only concentrate on section that have problems so that you could locate the fault and make the equipment work again.