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From: BRUCE@ccavax.camb.com (Barton F. Bruce)
Newsgroups: comp.dcom.telecom
Subject: Re: A/A1 Control For Key Telephone Systems
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Date: 27 Oct 90 04:59:11 GMT
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drears@pica.army.mil (Dennis G. Rears (FSAC)) asks about A/A1 and 1A2:

A 1A2 key system is the electronic replacement for the 1a1 all relay
ones that preceeded them. This is a relatively simple system that
typically has a KSU supporting 4, 6, or 13 cards. Each card is wired
in series with T+R going to the phones. The card does several things.
It can detect ringing and start a local interrupter (to have ringing
and lamp flashing for many lines all in step) and connect the flashing
lamp service to the lamp lead for that line. It will light that lamp
lead steady if a phone is picked up by noticing that the A lead for
that line and the A1 lead (local signal ground, and the pair-mate to
the phone's A lead for line 1) have been connected.

If the current through the T+R path breaks and then the A A1
connection breaks, (normal hangup sequence) the card knows you are
hanging up and the lamp goes out. If the A-A1 connection is broken BUT
the current is still flowing from Tip to Ring, the line card knows you
want to go on hold (things are in this state when you have the hold
button bottomed). The card immediately switches a 135 ohm 5 watt
(typical) resistor across T+R to simulate a phone off hook to hold the
line up. The card also starts the interrupter and switches the wink
supply to that lines lamp ckt.

The card monitors loop current on hold and if it drops (the far end
probably hung up) drops the hold. The timing of this can be extended
to bridge short duration opens such as those caused by call waiting by
a strapping option usually involving adding a cap.

Answering machines and modems sometimes have the A + A1 leads so they
can light the light and trip local ringing, etc. if used behind such a
1A2 key system. A non A A1 equipped device can be supplemented with an
adapter that watches for loop current and gives the A A1 signal.
Answering machine manufactuorers that no longer include A A1 support
often suggest a source for such a device if they don't offer it
themselves.

Does this give you enough, or do you have some specific other
questions?

There are also probably many similar answers someone could point you
to in the archives.