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Author Topic: RJ11, RJ14, RJ25 connections  (Read 3947 times)
KM3K
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Posts: 299




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« on: June 04, 2010, 07:49:04 AM »

I've finally gotten around to sorting out and marking my array of cables, then entering details about them into a spreadsheet matrix, so I know what I've got.
Eight of the forty cables are phone-cables and I've since found out from physical inspection that, although outwardly they all use the same plastic-piece, they have different number of contacts.
Then, from Internet searches I learned they have different designations; for example, a RJ11 is 6p2c for one phone-line, RJ14 is 6p4c for two phone-lines, and RJ25 is 6p6c for 3 phone-lines.
What I noticed in the physical inspection for each of my eight cables is the wiring is crossed-over; as an example, for RJ11, one plug will have red at contact#3 and green at contact#4, then at the other plug, they'll be reversed. Similar things happen for RJ14 and RJ25; every cable is crossed-over.
I'm puzzled and so my question is, "Why are they crossed-over?"
73 Jerry KM3K
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AA4PB
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« Reply #1 on: June 04, 2010, 09:35:48 AM »

I don't know why, but for some reason the phone company standard was to use opposite tip and ring pins for the phone and the wall jack. Most cables will be wired cross-over in order to maintain the proper polarity. On earlier "touch tone" phones having the polarity reversed would cause the keypad to not work. On later phones they used steering diodes so that it didn't matter if you got the wiring polarity reversed.
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KE5OFO
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« Reply #2 on: June 10, 2010, 11:59:12 PM »

As a phone co comm tech I can answer this one.  They have a reversal in them so that the tx from your equip goes to the rx of the phone co equip and vice versa.  Tip goes to ring and ring goes to tip.  Those terms come from the old bayonet type plugs from the manual switch boards.    Hope this helps.
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KM3K
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« Reply #3 on: June 11, 2010, 05:50:04 AM »

OK, got it.
73 Jerry KM3K
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AA4PB
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Posts: 12847




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« Reply #4 on: June 11, 2010, 08:28:50 AM »

As a phone co comm tech I can answer this one.  They have a reversal in them so that the tx from your equip goes to the rx of the phone co equip and vice versa.  Tip goes to ring and ring goes to tip.  Those terms come from the old bayonet type plugs from the manual switch boards.    Hope this helps.
In the POTS system the tip and ring are in the same circuit. One is not transmit and the other receive - it's a current loop. With older touch-tone phones it was necessary to maintain proper tip/ring polarity in order for the keypad to work. What I don't know is why they choose that standard rather than reversing at the phone and then using straight through cables (i.e. the same technique as CAT5 connections).
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