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Which is a pair?

Discussion in 'Electronic Basics' started by Terry, Oct 17, 2003.

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  1. Terry

    Terry Guest

    Am using a four conductor low voltage cable between two buildings
    on same property. Total cable run is about 120 feet.
    My understanding is that it is 'direct burial' telecom cable; it
    has a very heavy/thick light grey plastic sheath.
    It has four conductors surrounded by a transparent plastic tape
    or tube and three nylon reinforcing strands all within the
    sheath. Conductors are approximately 22 or 24 AWG.
    The four conductors are; blue, orange, white and red.
    Problem is that even though I've stripped back the sheath for
    about a foot I can't determine which of the conductors are a
    'pair'. There doesn't seem to be any lay or twist of the
    conductors or maybe I've destroyed it in the process of stripping
    the substantial covering!
    So questions are;
    Are there 'pairs' in this cable? I had understood that in a
    normal residential telephone use of this cable one pair was used
    with other conductors as either spares or for a second telephone
    line?
    If that was so they are not likely, IMHO, to be all twisted
    together with the same lay? That could lead to cross talk between
    pair 1 an pair 2?
    Using telecom practice (the Blue, orange, green, brown, slate
    code etc.)?
    a) Is it. Blue+white = pair 1 ? Or b) Orange+white = pair 2
    ....... Or?????
    In present application it isn't too critical (only two wires
    required) but I'd like to know the 'proper' way.
    Can anybody help out. Assistance welcomed.
    Thanks. Terry.
     
  2. Dbowey

    Dbowey Guest

    << Are there 'pairs' in this cable? >>

    I think not, based on the colors, and your not seeing paired wires in the
    length you have exposed.

    << a) Is it. Blue+white = pair 1 ? Or b) Orange+white = pair 2 >><BR><BR>

    The code runs Blue, Orange, Green, Brown, Slate, along with their White "other
    side."

    Any time the wires are not paired, you will pick up noise, perhaps mostle hum.
    It is somewhat critical.

    Don
     
  3. John Fortier

    John Fortier Guest

    Terry,

    I've only just started working with North American Telephone infrastructure,
    so, despite my many years experience with British and German standard
    practices, I may be wrong in the following. However - -

    It sounds like what you have is the type of cable which is normally used to
    connect a PABX (Private Automatic Branch Exchange) to the feature phones
    with which a particular PABX will be designed to work. Of the four wires,
    two are used for speech as a balanced pair, 1 is used for the recall signal
    and 1 is the earth used for such a signal. You will probably notice that
    they are twisted together as a quad, rather than a pair, although, as they
    are intended strictly for speech frequency use, the twist may be quite long.

    Blue and red are normally used for the speech siganal, however, there is
    nothing about the physics of the cable which demands that this be so. Any
    pair can be used as your signal conductor. Twists are normally used to
    prevent the ingress of crosstalk from adjacent pairs, but since there are
    normally no adjacent pairs as this cable is normally used, crosstalk is not
    normally a problem. You may find, however, that the cable is sensetive to
    external noise, and, depending on the frequency and spectral bandwidth of
    the signal to be sent, you may find strange noises or breaks in
    communication caused by external noise.

    I doubt that this is direct buried cable. It doesn't sound nearly robust
    enough. So, if you are laying it between two buildings as buried cable,
    make sure it is deep enough to avoid being damaged by any casual digging and
    you may want to follow the European practice of laying a yellow tape 10 cm
    above the level of the cable to warn people of its presence.

    I hope this helps.

    John
     
  4. You should strip off at least 2 m or 6 ft of the cable sheath. You
    might see the twist if you do. Normally, as you said, the white and
    blue are one pair. If the cable has markings on it, try to look up
    the specifications in the manufacturer's catalog. You could also use
    a cable tester to tell more about the pairs, if there are any. More
    help should be available on the comp.dcom.telecom.tech and
    comp.dcom.cabling newsgroups.

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  5. SHAUN

    SHAUN Guest

    First of all Terry, it kinda of goes to what John was implying, depending in
    what geographical region you live in, i.e., U.S., Canada, Europe, Australia,
    Japan, and so on your standards will be different.

    From the sound of things white/blue would be pair 1, with white being your
    tip wire and blue being your ring and pair two would be red/orange, with red
    being your tipwire and orange being your ring wire and by the way John it is
    (PBX) private branch exchange. Oh by the way, it seems as if coming out of
    your 110 block someone decided to only use old cat 3 pbx cable! This is not
    good. Please make sure that your twist are there, no matter what type of
    media you use other than fiber optics, you need those twists to prevent
    (N.E.X.T.) near end cross-talk. However, this is a guesstimation of the
    info you gave.
     
  6. Terry

    Terry Guest

    Many thanks to posters for the replies and ideas. In particular
    that it may be a quad (for audio frequencies plus signalling
    leads) with the four conductors having a common lay.
    For information; from the first building the cable is run inside
    a plastic pipe for about 50 feet (16 metres) then enters a small
    shed. From the small shed it continues, directly buried about
    eight inches under grass that has only occasional foot traffic,
    and is also power mowed, for about 30 feet (10 metres) to the
    second building. The total length of wire including within
    buildings at each end is about 100-120 feet. Nowhere is it
    subject to, or ever likely to be, auto traffic. The wire has been
    in place unused for some 15 years and appears to be in excellent
    condition.
    The construction of the cable is very much more substantial than
    the common two pair, Red/Green/Yellow/Black, four conductor
    commonly used for residential phone wiring in Canada which lent
    credence to the idea that it was for 'direct burial'.
    One 'pair' will be used for a voice intercom and/or to extend a
    telephone line. Another possibility is that one pair will be used
    experimentally to transmit a security camera video signal (with
    conversion back to coax immediately it reaches the main building!
    Or we'll wireless the video!
    Thanks again. Terry.
     
  7. Dbowey

    Dbowey Guest

    Shaun posted:
    << Oh by the way, it seems as if coming out of
    your 110 block someone decided to only use old cat 3 pbx cable! This is not
    good. >>

    That is not correct. Cat 3 cable is twisted, paired cable, and is very good
    for voice grade to 6 MHz or so, service. It just does not have the same spec
    as Cat 5 in the higher frequencys.

    Don
     
  8. SHAUN

    SHAUN Guest

    Don Posted:
    Sorry about that done what I somehow failed to write was, that if the pairs
    were not twisted and also if they were Cat 3 then that was not good, because
    cat 3 has to be twisted, that is what I meant, Thanks for the correction
    Don!

    Shaun
     
  9. Standards should be pretty much the same in any country if they're
    current. Most countries base their standards on ISO 11801. See the
    first few paragraphs of http://www.kwhw.co.uk/spec_11801.htm


    You can't prevent NEXT, the cabling just reduces it to a level that
    doesn't interfere with the signal.

    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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