Introduction: Ethernet RJ45 Cable Tester
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
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
- 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)
- 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
- 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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Please be positive and constructive.