Ethernet RJ45 Cable Tester

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Introduction: Ethernet RJ45 Cable Tester

Hi all

this is my first instructable, so forgive my less-than-optimal description (and some missing photos)-

The idea (well, the need, actually) was to check the proper cabling of a long (40m or so) ethernet cable from my flat to the basement; the routing itself was tricky, with a lot of narrow passages, so the probability of damaging the cable was high. Obviously I had no professional ethernet tester available!

I got inspiration from xklathos' DIY-UltraCheap-RJ-45-UTP-Cable-Tester project, but it had a big limitation, in my case: it is not applicable when the two ends of the cable under test are far away, i.e. with the cable already in place.

Furthermore, I wanted something that was able to detect short-circuits, errors in wiring at the connectors, and, as an addition, suitable both for straight-through and cross-over cables.

There are plenty of "smart" projects out there, all relying on integrated circuits and leds, to perform a cyclic test of each channel, but I had no such hw available.

Summarising, the required characteristics are:

  • capable of testing cables "in-place"
  • detection of
    • open channels,
    • short circuits,
    • wrong wiring
  • applicable to Cat 5, 5e, 6 cables, shielded and unshielded
  • minimum hw required

The project ended in a "passive-only" couple of terminals, to be used in conjunction with a multimeter capable of reading resistances-

So... let's go!

Step 1: Collecting What Is Needed

hardware:

  • 3x female RJ45 shielded connectors ("jacks") (for example from a broken/old router/switch); you can use also unshielded jacks, but of course you cannot test STP cables for shield continuity
  • 2x small breadboards
  • 8x 1kOhm resistors "RA" (or similar value, the important is that they are equal to each other, and at least 2 orders of magnitude higher than the cable resistance...anything in the range of 470-4700 Ohm should be OK)
  • 1x 10kOhm resistor "RB" (or similar value, proportional to the 8 above)
  • about 20 cm of ethernet cable
  • some shrink tube (small diameter)

tools:

  • plier
  • knife/cutter/scissors
  • solder and soldering iron
  • multimeter, measuring resistances
  • hot glue gun (optional, even silicon sealant, vinyl glue, foam, anything to avoid shorting wires..)

Step 2: Preparing the Sockets

if you have 3 new female jacks of the wall-mounted type, for each of the jacks:

  • prepare a 6 cm piece of the ethernet cable, removing external insulation cover
  • separate each wire
  • insert the individual wires in the slots of the jack, press them with the tool or its cover
  • use another wire to connect the shield
  • remove individual insulations at the other end of the wires

if you have an old router/switch/NIC:

  • cut the PCB around the jacks, until you have 3 single connectors, already soldered in their small piece of PCB
  • for each of the jacks:
    • with a file or sand paper, smooth any edge of the PCB
    • prepare a 4 cm piece of the ethernet cable,
    • remove external insulation cover
    • separate each wire
    • completely remove individual insulation
    • solder each of them on the protruding ends of the leads
    • use another piece of naked wire to connect the shield

Step 3: Remote Terminal

This unit will be passive only, with only one female RJ45 connector on it, and all the resistors:

  • cut a piece of breadboard a bit larger than the jack (let's say 10 holes), and twice longer (let's say 15 holes)
  • take one of the already prepared sockets
  • insert the wires from the jack into a strip of 8+1 holes, and solder them (if you used salvage connectors, insert the naked wires in as much as possible, in order to avoid short-circuit between them)
  • trim the exceeding length of the wires
  • use hot glue to fix jack and breadboard to each other, thus avoiding short circuits
  • insert and solder the resistors according to the schematic

Step 4: Local Terminal

This unit will be the measuring one, with two RJ45 connectors (for testing both straight-through and cross-over cables, otherwise you can use only the straight-through connector):

  • cut a piece of breadboard a bit larger than two jacks width (let's say 13-14 holes), and 14-15 holes long
  • take the two already prepared jacks
  • insert the wires from the jacks into a matrix of 4x2 holes (plus 1 for shield), and solder them (if you used salvage connectors, insert the naked wires in as much as possible, in order to avoid short-circuit between them)
  • trim the exceeding lengtt of the wires
  • use hot glue to fix jacks and breadboard to each other, thus avoiding short circuits
  • use short pieces of the remaining wires to make point-to-point connections of the connectors terminals, according to the schematic above (pay attention at the swap between pairs 1-2 and 3-6!!); if needed, use shrink tubes to help with insulation
  • with the multimeter, verify for absence of short-circuits
  • again, use hot glue to fix all the wiring to avoid damage/shorts etc..
  • optionally, solder some rods at the test points, to facilitate use

Step 5: Using the Ethernet Tester

Ok.. everything is ready

Now we need a prepared ethernet cable (hopefully working!!!) as a test unit.. let's start with a straight cable.

  • plug the connector at the "remote-end" of the cable into the "remote terminal"
  • plug the "local-end" connector in the "local terminal" ("straight" receptacle)
  • set the multimeter in "Ohm" mode, with the appropriate range (greater than 8xRA, or RB)
  • connect the "black" multimeter probe to the Test Point 1 ("TP1" in the schematic), used as common reference
  • connect the red probe step by step to the TestPoints TPn:
    • if the cable is OK, the multimeter will display a value close to RA*n for each single point (for example, with the 1kOhm resistors, you should find 2 kOhm on TP2, 3 kOhm on TP3, and so on)
    • if you see (almost ) 0 Ohm, there is a short circuit between wire "1" and the wire under test
    • if more than one TP show the same resistance value, this means there is a short somewhere along the cable
    • if you see infinite resistance on TP"n", this means that wire "n" is interrupted somewhere
    • if you see infinite resistance on all channels, this means that wire "1" is interrupted somewhere
    • if the above formula is not matched with the right sequence, this means that there is some improper wiring
  • connect the red probe to TestPoints TPsh:
    • it the shield is OK, you should see the value of RA+RB (11 kOhm, for example)
    • if you see inifinite resistancet, shield is interrupted somewhere (unlikely) or is not present at all in the cable (probable)
    • if you see a resistance lower than RA+RB, it is short-circuited with another channel

If you have a crossed cable, simply use the "cross-over" receptacle, and the process is the same

NOTE 1: in the pictures you will see different values on the multimeter display, because I had no 1kOhm resistors available for the prototype

NOTE 2: To Be Done: find a small enclosure for the two terminals, to give them a more "solid" appearance

NOTE 3: by the way, the flat-2-basement cabling, tested with this tester, was OK!!

NOTE 4: all the post production was done with Free/Libre Software:

  • photo editing: GIMP 2.8 (GNU General Public License v.3)
  • schematic drawing: QUCS 0.0.18 (GNU General Public License version 2.0)
  • publishing: Firefox 57.0.3 (Mozilla Public License 2.0)

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    2 Discussions

    I've got a box full of old parts and cables, I could really use one of these!

    1 reply