NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/8522

Internet Message Format | 1996-08-05 | 3.7 KB

Message-ID: <[e-mail]>
Date: Sun, 17 Mar 1996 14:50:19 +0000
From: Stephen Satchell <[e-mail]>
Organization: Satchell Evaluations
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Subject: Re: Making a RJ11 "busy" plug
References: <4i4n3j$[e-mail]> <[e-mail]> <4iabts$[e-mail]> <4iag3c$[e-mail]>
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DON'T DO IT!

Zero-ohm plugs make the telephone company very angry.  Very angry indeed.
Angry enough to disconnect your line as a "network hazard."

The reason this is true has to do with the mechanics of a telephone
switch and how telcos deal with faults detection.  It also has to
do with 47 CFR 68, better known as Part 68 of the FCC Rules and
Regulations, the stuff that regulates what can and cannot be
connected to telephone lines of various sorts.

When a telephone goes off-hook, it exhibits an AC impedance
of 600 ohms and some DC resistance.  The DC resistance should be
selected to draw no less than 20 ma of current in the off-hook state
and no more than 60 ma, but shall not exhibit an impedance of more
than 1000 ohms at ringing frequencies.

The line cards in many switches now incorporate current limiters
which will limit the "loop current" to some value, such
as 35 ma.  That's not universally true, so the short-circuit
current (if you short tip and ring) may be as high as 500 ma.

47 CFR 68.312(k) is specific, though, that tying together tip
and ring, even with a resistor, is a no-no:  "Register terminal
equipment and registered protective circuitry shall not by
design leave the on-hook state by operations performed on
tip and ring leads for any purpose than to request service
or answer an incoming call, except that terminal equipment
which the user places on the off-hook state for the purpose
of manually placing telephone numbers in internal memory for
subsequenct automatic or repertory dialling shall be
registrable.  Make-busy indications shall be transmitted by
the use of make-busy leads [MB and MB1] only as defined in
68.3 and 68.200(j)."

47 CFR 68.502 further defines that the following jacks have
make-busy capabilities:  RJ18 (6-position plug), RJ2MB (50-
pin connector with up to 12 lines appearing).

*

Now that I've said all of that, use the 600 ohm 1/2 watt
resister instead of a shorting plug.  Either that, or call
your telephone company and ask them what they prefer to see
for make-busy.  Some might want a shorting plug, some might
want 600 ohms, some might 1000 ohms.  It varies with the
operating company.
---
Stephen Satchell



Floyd L. Davidson wrote:
>
> In article <4iabts$[e-mail]>,
> Robin Bassett <[e-mail]> wrote:
> >Don Horne <[e-mail]> wrote:
> >
> >>>I want to create / wire / solder a RJ11 plug than I can stick into the
> >>>socket so that the exchange thinks the line is busy. I imagine that that
> >>>should be fairly simple, but I don't know how exactly to do it. Should
> >>>I put a resistor between the leads or so? A diagram might be useful.
> >>
> >>Simply use a 600 ohm (or something close such as a 560 ohm) 1/2 watt
> >>resistor between the 3 & 4 pins (the two in the center) of the RJ11 plug.
> >>These are the green and red wires that can be seen through the clear
> >>plastic plugs.
> >>
> >>Don
> >This is the right way to do it, however, 600 ohms may not be low
> >enough.  Try anything more than 50 ohms (anything in the range of 50
> >to 300 should probably work).  Otherwise, use a cheap telephone and
> >leave it off hook. :)
>
> Zero ohm resistors are best (22-26 guage frame jumper wire).
>
>   Floyd
>   AT&T Alascom