How to make a LAN cable tester

Written by dave wilson
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How to make a LAN cable tester
Build a network cable test jig to verify network patch cable integrity. (computer cable image by Christopher Hall from

Ethernet network cables transmit data at very high speeds in both directions (full-duplex), so the integrity of Ethernet network patch cables is important to ensure that network connections perform reliably without fail. If a network connection is erratic for a particular host, the network patch cable that connects the host to the network could be the cause. Build a simple CAT5 LAN cable tester that can be used to determine if network patch cables need to be replaced.

Skill level:

Things you need

  • Four inches of CAT5 or CAT5E cable
  • One RJ45 connector
  • RJ45 crimper tool
  • Wire stripper tool
  • RJ45 eight-pin Ethernet coupler
  • CAT5 patch cable
  • Microsoft Windows computer

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  1. 1

    Strip the plastic sheath off the CAT5 cable and remove the four twisted pair sets of wires.

  2. 2

    Separate and straighten the wires in the twisted pair sets.

  3. 3

    Position the RJ45 connector with the socket toward you and the tab facing down. Insert the wires into the RJ45 connector according to the pinout below, counting the pins from left to right:

    Pin 1 - Green wire

    Pin 2 - Blue wire

    Pin 3 - Green wire (other end)

    Pin 4 - Orange wire

    Pin 5 - Brown wire

    Pin 6 - Blue wire

    Pin 7 - Orange wire

    Pin 8 - Brown wire

    Ensure that each wire touches the inside end of the RJ45 connector before proceeding to the next step.

  4. 4

    Insert the RJ45 connector into an RJ45 crimping tool and squeeze the handle to push the RJ45 pins through the wires.

  5. 5

    Plug the new loopback adaptor into an eight-pin RJ45 Ethernet coupler. Plug a CAT5 patch cable into the other end of the RJ45 coupler, then plug the CAT5 cable into a Windows computer. View the network icon in the lower right corner of the Windows PC desktop. If the network icon displays that the network is connected, the CAT5 patch cable is working. If the Windows computer displays that the network is offline, the CAT5 patch cable should be replaced.

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