Troublesome T-Tap Turmoil

Discussion in 'General Electronics Chat' started by thingmaker3, Feb 1, 2007.

  1. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
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    My cohorts and I have run across an odd problem.

    We're installing (among other things) phone lines in hotel rooms. One suite has a length of CAT5e coming in from the 110 block in the building communications room, which is T-tapped via scotch-locks to three RJ45 outlets.

    cat5ttap.jpg

    When we test each of the two cables in the room without scotch-locks in place, our DSP4800 indicates no problems - headroom on the order of 8 or 10dB. Similar good results testing with DSP4800 between the "close" jack and the 110 block.

    When everything gets "beaned" together with scotchlocks, and double checked by TWO journeyman technicians for correct wiring, the DSP4800 comes up with "wiremap" error. Says we've got an open orange pair at two-hundred-some-odd feet. Now, we don't expect to pass any CAT5e test with the T-tap in place, but we should at least pass wiremap, should we not? Also, we get random readings with a simpler tester - an Ideal LinkMaster. The Linkmaster alternates between opens (sometimes green, sometimes orange), shorts, voltage present, and pass. The DSP4800 gives only the open - no voltage present.

    I know that we increase stray capacitance by T-tapping, but would that alone be enough to cause such odd test results?

    Also, there are three such rooms wired identically - one on top of another. It is the lowest floor - closest to the 110 block - that is giving us trouble. The other rooms test just fine.

    Are we going to get reliable dial-tone in this room?:confused:
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Am I clear on this - you are using the Cat5 4 pair for POTS via RJ45 ports?

    Considering the crud that passes for telco wiring, it's hard to imagine how it would be possible for it not to work - gigabit Ethernet aside.

    What happens if you use plain old wire nuts for the splices? Again, for POTS, capacitance is kind of hard to worry about.

    Think about some white roosters to have on hand to appease the demons, though.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    And don't for get the little yellow screwdriver and St. Vidicon of Cathode

    Have you hooked up the phones and tried to use them? I know you'll have figure it out but it might be a phantom error cause by the meter.

    Do you have enough of another kind of interconnect to test that? About all I can recall scotch locks doing is to cause high potential or an open. Well unless it's carrying current and scotch blocks suck at that.
     
  4. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    I wonder if you're not possibly picking up induced AC hum, possibly combined with questionable contact from the T taps that's showing up on the analyzer as a fault. From my experience with security systems, I agree with beenthere, try using small [blue or orange] wire nuts instead of the T taps & see if this makes a difference.
     
  5. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
    5,072
    6
    I can scrounge up a doulbe-handful of the blue wirenuts. I'll try them next time I'm at that jobsite. We can't put dialtone on the line yet, as the switch won't go in until a day or two before the hotel opens. At that time I'll be hip deep with an IDF that is still being designed. (Too many chefs...)

    Induced voltage is something we were wondering about at the time. We've seen problems when fire alarm cable runs too close to ethernet. Its also the only believable explanation for only one room being afflicted. Too much hard-pan in the ceilings to re-pull on a different path now, though. If the wirenuts don't clear the phantom test results and the problem does show up on the phones, those white roosters may be our only hope.:(
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Things could always be wierder. At the U., we needed a line to a second room some distance away. We just ran some surplus 5 wire serial cabling over the fluorescents and through the walls. Worked ok, buy the cross-talk on the two lines was pretty bad (no pairs).
     
  7. CATV

    Member

    Apr 17, 2007
    11
    0
    This might work for analog POTS but if the switch is digital you are probably asking for trouble.
     
  8. thingmaker3

    Thread Starter Retired Moderator

    May 16, 2005
    5,072
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    If the switch were digital the RCDD would never have specified t-taps!;)
     
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