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From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.dcom
Subject: Re: RJ-41S and RJ-45S
Message-ID: <1692@sdcrdcf.UUCP>
Date: Sun, 20-Jan-85 23:27:16 EST
Article-I.D.: sdcrdcf.1692
Posted: Sun Jan 20 23:27:16 1985
Date-Received: Thu, 24-Jan-85 05:33:12 EST
References: <90@tove.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Distribution: net
Organization: System Development Corp. R+D, Santa Monica
Lines: 46
Summary: 

An RJ-41 jack is a "universal" data jack.  It has the features of an RJ-45,
plus another circuit on pins 1 and 2 for fixed-loss loop modems.  It's quite
a bit more expensive, and seldom needed.

The middle two pins on either jack go directly to the phone line (in fact,
you can plug a RJ-11 plug into an RJ4x and it works).
The pins just outside this are part of local wiring to your telephone.
There are two possible uses: for phones on a keyset system (the kind where
the phone has 4 of 5 lines selected by a row of buttons), then called A-A1.
In this use, the modem plays the right games to make the off-hook light work
on all the telephones.
The other use is with a phone designed for dataset use, where the phone
opens and closes the contact to signal the modem that it should make a data
connection.  It's sufficient to use a toggle switch for this (closed usually
means the the modem should connect).

The last two pins (7 and 8) are what's unusual about data jacks.  Between
these terminals is a resistor from a standard set of 13 values which sets
the output level of the modem.  The way it's supposed to be installed is
that the Telco measures the actual loss in your phone line loop, then
chooses a modem output level which gives -12dBm level at the central office.
A permissive connection (the kind usually supplied on cheaper modems for
RJ-11 use) assumes an "average" loop loss of 3 to 4 dB, so output level is
set to -9dB.  This corresponds to a 5490 ohm resistor across the terminals.
A RJ-11 cable for a modem which can be either kind just contains this
resistor in the cable.
If you are (typically) between .5 and 1.5 miles from your switching office,
the permissive value is within 1 dB.  If you are closer, you would use a
larger resistor for lower output (9200=10dB, 19800=11dB, infinite=12dB).
If you are farther, you want a lower value for higher output.  The only
legal way to have output over -9dBm is thru a Telco calibrated RJ4x jack.
(Be sure to call the data people; the ordinary installers haven't a clue
about any of this, nor the equipment to measure loss).
You can buy the jacks (an RJ-45 is about 20 to $25 including a set of
resistors) from some of the phone supply places.  I got mine from EDS
Communications, made by Armerdinger.

-- 
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA