Date: Sat, 12 Dec 92 13:33:44 CST
From: varney@ihlpl.att.com (Alan L Varney)
Newsgroups: comp.dcom.telecom
Subject: Re: What is a 1A ESS Master Scanner?
Organization: AT&T Network Systems, Lisle, IL
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 901, Message 3 of 5

In article <telecom12.898.6@eecs.nwu.edu> bote@access.digex.com (John
Boteler) writes:

> When our ring plant decided not to ring the phones every so often, the
> switch guru for our ESS#1A said "We rebuilt the master scanner and we
> haven't found a lick of trouble since."

> Would someone who actually has working knowledge please describe a
> master scanner? Is it software or hardware? If it's hardware, how do
> you effectively "rebuild" it without disrupting service?

> I am most curious.

   If you are MOST curious (i.e., willing to spend money), you should
know that almost all the hardware (and some software, tools, testing
details, etc.) associated with 1/1A ESS(tm) switches is described at a
high level in two special issues of the Bell Labs Technical Journal
(BSTJ), one on the 1 ESS switch, and a later one that describes the 1A
Processor (used in both 1A ESS and 4ESS(tm) switches).

   "No. 1 Electronic Switching System", BSTJ, Vol. 43 No. 5,
          September 1964, Parts 1 & 2.
   "The 1A Processor", BSTJ, Vol. 43 No. 5, February 1977.

   I'm told the AT&T Customer Information Center maintains copies of
the BSTJ "special" issues, so even if your library doesn't have it --
it's still available.  For even more money, you could order AT&T
Practice 231-030-010, "Scanners - Description and Theory, No. 1 and
No. 1A ESS", which describes a "middle" level of detail on scanners in
these systems (51 pages, including fold-outs!).

   For those who are somewhat less curious, ...

   The master scanner was one of the original frames of equipment
designed for No. 1 ESS, and is one of the few to have never been
upgraded in later years.  Essentially, these frames (each switch must
have one, but many have more) serve as input devices for arbitrary DC
signals in the system.  Lines and trunks have scanners dedicated to
those functions, so master scanners are used for detecting other
things.  For example, detection of blown fuses, power failures,
diagnostic results, open doors (if they are alarmed), low power in the
battery plant, low paper in a printer, someone pressing a key on a
control panel, etc.

   Each master scanner contains some duplicated control circuits and a
64-row by 16-point matrix of current detectors.  Each detector is
called a "ferrod" (a ferrite rod with some wire threaded through and
around it).  These operate essentially like old "core" memory units,
in that read-out is controlled by pulsing X and Y leads, with the
selected row responding because the coincident-current exceeds some
minimum.  But unlike core memory, the 0 or 1 response is determined by
the amount of current flow in the "control" winding of the ferrod.

   For master scanners, a ferrod has about 35 ohms resistance and will
respond with a 0 if more than 3.9 ma in flowing.  Less than 1.8 ma
will yield a 1.  In between those values, the readout is not
predictable.

   One of the units monitored by a master scanner is the ringing and
tone plant.  Among other things, the beginning of various ringing
current phases is detected.  If this is not reliable, ringing will be
unreliable.

   Failure of a ferrod and it's windings is rare, but it can happen.
Since it's just a transformer + some wire, it's pretty durable.
Because the ferrod matrix is not duplicated, a failure requires some
non-obvious steps to remove the ferrod with minimum interference.
Some folks might call this "rebuilding", since it's not the simple
matter of circuit board removal used with electronic parts.


Al Varney - just MY opinion