mark@tove.UUCP (Mark Weiser) (01/17/85)
What can anyone tell me about RJ-42S and RJ-45S telephone connections?
I'll tell you what I know.
I just bought a couple of Anderson Jacobson 4800 baud modems for use
over regular 2-wire telephone lines. However they arrived with,
instead of the little 4 connector modular plug (RJ-11C), with an
8-wire modular plug into the phone system, which they call an
RJ-41S or RJ-45S (I don't know what the difference is). They claim
the phone company must come out and install the corresponding 4-wire jack.
Here is what they say about using the RJ-11S:
"Connecting the modem to the telephone line with an RJ-11C jack
is not recommended, although it may work under certain conditions.
An RJ-11C voice jack requires a 4-wire telephone cable, not supplied
with the AJ-4048 modem."
Actually not only do they not supply the 4-wire cable, but they don't supply
anyplace on their modem where the 4-wire cable could plug in.
I also think I am going to need to get myself a phoneset with an 8-wire
connector since the AJ doesn't autodial. Are these available?
My questions are:
A. Is it likely to be a big deal or a little deal for the phone
company to install that 8-wire jack in my house? Are the 8 wires
in the wall somewhere or does a major restringing occur? (I would
guess from the descriptions of wires 7 and 8 below that they are
just a volume control from the phone company built into the box.)
B. Are there adaptors available to go from 8-wire to 4-wire connectors
and back? The quote above implies it is somehow possible.
C. What is the real difference? What are the circumstances under
which RJ-11C will not work but RJ-41S will?
Thanks for any and all help.
By the way, the 8 wires are identified as follows (in the AJ manual):
1. no connection
2. no connection
3. Mode indicator. Selects voice or data mode depending on state of
telset exclusion switch (or other control).
4. Ring. One side of telephone line.
5. Tip. Other side of telephone line.
6. Mode Indicator Common. Ground.
7. Programming Resister. Resistor (on data block) used to
control modem transmit level.
8. Programming resistor common. Return side of PR signal (7. above).
7 and 8 are identifed as: "Passive or isolated signal", and none of the
rest are, whatever that means.
--
Spoken: Mark Weiser ARPA: mark@maryland Phone: +1-301-454-7817
CSNet: mark@umcp-cs UUCP: {seismo,allegra}!umcp-cs!mark
USPS: Computer Science Dept., University of Maryland, College Park, MD 20742ron@brl-tgr.ARPA (Ron Natalie <ron>) (01/17/85)
> "Connecting the modem to the telephone line with an RJ-11C jack > is not recommended, although it may work under certain conditions. > An RJ-11C voice jack requires a 4-wire telephone cable, not supplied > with the AJ-4048 modem." > This is exactly true. We hacked some eight wire->2 wire jobs on some other modems, but they were only running at 1200 baud. When AJ demonstrated the modems they did exactly the same thing. > I also think I am going to need to get myself a phoneset with an 8-wire > connector since the AJ doesn't autodial. Are these available? > No, you can mix a RJ-11 plug on the eight wire line just fine. > My questions are: > A. Is it likely to be a big deal or a little deal for the phone > company to install that 8-wire jack in my house? Are the 8 wires > in the wall somewhere or does a major restringing occur? (I would > guess from the descriptions of wires 7 and 8 below that they are > just a volume control from the phone company built into the box.) > B. Are there adaptors available to go from 8-wire to 4-wire connectors > and back? The quote above implies it is somehow possible. > C. What is the real difference? What are the circumstances under > which RJ-11C will not work but RJ-41S will? > There are still only two wires running to your house. The jack is entirely local to end of your two wires. > Thanks for any and all help. > By the way, the 8 wires are identified as follows (in the AJ manual): > > 1. no connection > 2. no connection > 3. Mode indicator. Selects voice or data mode depending on state of > telset exclusion switch (or other control). > 4. Ring. One side of telephone line. > 5. Tip. Other side of telephone line. > 6. Mode Indicator Common. Ground. > 7. Programming Resister. Resistor (on data block) used to > control modem transmit level. > 8. Programming resistor common. Return side of PR signal (7. above). > > 7 and 8 are identifed as: "Passive or isolated signal", and none of the > rest are, whatever that means. Ring and Tip are your two wires to the phone connector. The Mode indicator and it's common are a signal as to wheter the modem should connect itself to the line. Even the RJ-11 vadic modems use that feature. You can either put a little toggle switch on the extra two wires of the RJ-11C, or disable the feature entirely by using the little adapter that VADIC gives you which does nothing but disconnect these two wires. This is usually used by the use of an Exclusion key (that's that funny white plunger on the handset, or the little clear twist knob on the top of the dial). The programming resistor is a resistor. It's value is set up by the phone company to give you an idea what the loss of your loop to the central office is. I suppose you can just do this by trial and error. Anyway, AJ demonstrated the 4800 baud modems here just by connecting them to an RJ11 so their disclaimer is probably just CYA stuff. -Ron
smb@ulysses.UUCP (Steven Bellovin) (01/19/85)
The purpose of RJ41 and RJ45 jacks is to provide a measurement of the loss to the central office. This permits a modem to depend on amplitude of the received signal. What happens when the jack is installed is that the actual loss is measured, and a resistor of an appropriate value is made available (via pins 7 and 8, as noted) to the modem to permit it to adjust its transmit level. If you want to try faking things, buy an ordinary RJ11 cord (at Radio Shack or wherever) and plug it in to the modem and your wall jack. The RJ11 cord (which has provision for 6 pins, though normally only the central 4 are used) is designed so that the important signals (RING and TIP) are properly connected when that cord is plugged into an 8-pin jack. Similarly, your voice phone that you'll use to make the connection can be connected via an RJ11 plug to the modem. (As for how you make the connection: you dial the number normally, then press the CONN "button" on the AJ modem and hang up your phone. No special phone is needed.) One thing to watch out for: if you have two phone lines in your house, the second pair of wires running through the wall (black and yellow) may be the other line; this signal will show up on AJ's pins 3&6 (mode indicator and mode indicator common), which will confuse matters... If this is the case, disconnect the wires from your jack to the junction box, leaving only the red and green connected. But let the black and yellow feed through the box, of course. (Btw, the arrangement of having two lines hooked up to one sack is an increasingly common one, and there are phones and gadgets thawayuse this configuration. Radio Shack even has something that looks like a 3-way T-splitter that will break out line 1, line 2, and line 1+2. I believe this arrangement is called an RJ13 jack.) It's problematic whether or not this arrangement will work with the AJ modem, though. It really does need an accurate level setting, we've found. When I brought one home, it worked well most of the time; from my supervisor's house, though, it wouldn't work at all till he played games with the resistor setting. (We were using a RJ45 jack attached to a short RJ-11 plug; that let us have some value in place but still use an ordinary phone line. We didn't try using the RJ11 option setting inside the modem.) --Steve Bellovin AT&T Bell Laboratories P.S. These are my opinions, etc., only, not the company's.
darrelj@sdcrdcf.UUCP (Darrel VanBuer) (01/21/85)
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.ARPAdsi@unccvax.UUCP (Dataspan Inc) (01/21/85)
(now, to dig out the FCC Rules and Regulations)
Dreaded jack RJ41S possibly specifies a tariff concerning the SLIC card
and the use of certain compressing devices at the C.O. , but I can't confirm
this. We installed (and install) 9600 bps dial-through UDS modems in people's
homes all the time (doctors) and it seems that specification of this jack
makes one whale of a difference in the modem's performance.
Jack RJ41S is the 8-pin kind with the little switch marked on the side
FLL (fixed level loss) and P (programmed.) The pinout of such a jack is:
1 Ring (FLL)
2 Tip (FLL) (pad switched in to Telco in FLL, out from Telco in P)
3 M1 (has something to do with the Exclusion Key
4 Ring (P) (Always connected to Ring of the subscriber loop)
5 Tip (P) (same, but Tip of s.l.)
6) M1C (Exclusion key)
7 PR Programming resistor
8 PC Programming resistor
There is a little table on page 99 of part 68 which describes the pad
to be used. These resistor values hold for either the FLL pad (for modems
with no level programming position inside the modem) or the selection of PR-PC
(for modems with this provision). The UDS modem uses the PR-PC resistor when
strapped for this operation. PR-PC is selected by the telephone company
installer when the line is conditioned for dial-through data and voice
service, for (I believe) -9 dBm at the C.O. Around here, they test it at
1001 Hz (?) and the whole thing takes about 1 minute.
Jack RJ45S is identical to jack RJ41S except that it brings out fixed
(oops, it does NOT bring out) fixed loop loss paddage to pins 1 and 2.
We use the UDS modem on dial-through all the time. Internally, it has
a jumper for "Permissive" (-9 dBm) for connection to the plain ole RJ11 jack,
and "Program" (using the program resistor, it delivers X dB to the C.O.).
We have found that it is almost mandatory to specify RJ45S or RJ41S to the
phone company whenever compressors are installed. This is because the
attack time of a noise-limiting device is slow enough to mangle the training
burst from the modem.
I have personally installed these modems in three doctors' offices
and homes, and many more hospitals. There should be no problem although the
tariff that comes with the jack may be more than you want to pay.
-dya-
.