Re: [PLUG] RJ45 Cables - crimping

[Keith via plug <plug@lists.phillylinux.org>]

On 5/1/22 11:41, Joe Rosato via plug wrote:

I own this rj45 tester, and am having an odd problem. Long story short running some cable and both 3 and 4 came up blank. Figured I messed up one side, did the dance to replace them and still the same. Grabbed a known good cable and it is doing the same thing. Uhg. Have no idea how the internal electronics of this works - but is that a possible failure state? The tester has had very light usage...

    That looks like one a more modern tester.  I did a job some years
    back and invested in one of the Fluke
    oh-my-god-I-have-always-wanted-this-meter units and it included a
    similar type of prob you have that can also act as a terminator for
    testing.  The type of tests you are doing are pretty basic- just
    pairing (568-A/B, custom, etc.) or continuity.  You have ethernet
    link detection on yours too which is nice to have.  In my
    experience, if it has a low battery indicator then it is pretty
    accurate and you should change the batteries when it says to.  If
    they don't, then you are playing a bit of guessing game.  I forgot
    what a "dead" 9v battery voltage is but if it the same as other
    cells then being down more than 20% (so 7.2v) would mean you
    probably need to get a new battery.

Since the cable run has PoE I thought I did something stupid, even though I made sure the cables were unplugged. But if that was true, PoE for T568-B is on 4,5 and 7,8. Not 3.

I was about to go get some new 9v batteries in the hopes this was some odd "weak battery" problem.

Thoughts?

Plugged in or not active PoE systems (48v or higher) don't have power always on the pins like "passive" PoE which is 24v.  Always smart to disconnect though.  You could still have a bad wire because of crappy heads or crappy wire where you have high crosstalk or broken strands for some reason always the way.  If you had an old wire on spool sitting around for more than 15 years depending on storage condition it **might** not be viable to use.  Another thing to note that complicates matters is that testers will often indicated things in a mirrored or reflected way.  So a problem on pair 2 (the green pair), pin 3 could really mean an issue on pin 6.  The orange (1,2), blue (4,5) and brown (7,8) pairs are more likely to see that sort of thing.  If you have a broken wire (usually close to the crimp), your meter can miss this with regular pin detection.  Meters that give you distance reading will only indicate that kind of thing to you if test each pair from BOTH directions and get measures that are significantly different.

If your tester is saying a cable is "bad" on a known to be good cable it could be tester but known-to-be-good I would say ONLY applies to manufactured cables.  I can doing wiring in my sleep and in my inventory of "good" cables- things I had been using for years, will all of a sudden be problematic.  My tester has made me eat some humble pie in that regard especially when I tried to do large data dumps at nominal 1Gbs speeds or PoE work.  Depending on what you're doing and what devices you have, good crimps are not be as good as you think because you're running up against electrical tolerances.  For example, how much you are stripping and untwisting wire when you are putting heads on punching to a keystone jack matter more today that it did even 10 years ago.

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Keith C. Perry, MS E.E.
Managing Member, DAO Technologies LLC
(O) +1.215.525.4165 x2033
(M) +1.215.432.5167
www.daotechnologies.com
  

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Philadelphia Linux Users Group         --        http://www.phillylinux.org
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