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From: DEFRANCO@RADC-TOPS20.ARPA.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Reliability of Modular Jacks
Message-ID: <12282132923.11.DEFRANCO@RADC-TOPS20.ARPA>
Date: Thu, 26-Feb-87 09:51:11 EST
Article-I.D.: RADC-TOP.12282132923.11.DEFRANCO
Posted: Thu Feb 26 09:51:11 1987
Date-Received: Sat, 28-Feb-87 03:34:41 EST
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Concerning the discussion of modular jacks and related electrical data:

1.  Most telephone switches operate at -48VDC nominal, actually closer to
-52V with battery systems.  This provides operating power for your telephone.
Any voltage drop is due to line losses.
2.  Ringing systems vary from switch to switch.  Old standards used 105VAC
20 Hz nominal ringing placed across the line.  Frequencies used can range
from 16 2/3 to 33 Hz, most often on party lines.  Other schemes include
divided ringing, i.e. ring signal is placed only on one of the two wires
leading to the phone superimposed on the DC power supply, with steering
diodes used in detector circuits.  In most cases, private line telephones
use line-to-line or "bridged" ringing, which most modems are designed to
recognize.
3.  The DC polarity of your telephone line does not change when you answer
a call.  Older electromechanical switching systems used the "reverse
battery" technique within the switchtrain to recognize answer.  There should
not be any effect on a modem even with these systems.
4.  Newer electronic switching systems usually do a "foreign battery" and
"AC" test before a connection is made, but only to protect the switching
equipment.  The phone company really doesn't care about your equipment
except be sure it doesn't hurt theirs.
5.  Modular jacks are designed to withstand all the normal electrical
stresses imposed by regular telehpone systems, including the AC ringing
voltages, and are specified to some extent within the FCC Regulations
governing interconnect between private and common carrier systems. While
the mechanical capabilities depend upon the manufacturer's materials
and quality control, variations are expected to be minor.
6.  If you wish to positively avoid misconnecting computer equipment with
telephone lines, do as suggested - use radically different types of
connectors.  Otherwise, simply insure that the equipment can withstand the
maximum electrical stress presented by the telephone system.
Remember that modems built for telephone connections are designed to
handle telephone line stresses - or should be.

     Most building communications distribution systems I have had a
hand in designing use common access boxes for both voice and data
connections, using either dual 4 pin modular jacks or single 8 pin
ones.  Obviously, some attention must be paid to the jacks when a
line is connected, but decent labelling and minimal user instructions
are all that are normally required to avoid problems.

     Hope this helps someone.

       Carl DeFranco                      "Calendars help you keep
       DeFranco@radc-tops20.arpa           track of appointments
                                           you missed."
       Disclaimer: "My boss doesn't even know I'm writing this, let
       alone agree with it."
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