[GreenKeys] Information Requested

[Jim Haynes]

On Wed, 19 Sep 2012, Nick England wrote:

> So now I'm curious - what does an LFXD actually do? What does "tape
> pull-back" mean?
> Nick
>
I've told this so many times I should save it in a file for use when
someone asks.

LFXD is part of an ASR set for the "Developmental Line-Switched 
Teletypewriter System" which was first done for Delta Airlines.
Delta needed a new message switching system in a hurry because
the building containing theirs was to be torn down as part of
expansion of the Atlanta airport.  Instead of a store-and-forward
system like they had in mind, AT&T proposed a system that would use
the DDD phone network for switching.  This required a complicated
ASR set attached to a modem and auto-dialer.  The ASR set used
five-level code punched into eight-level tape, the other levels being
used for control purposes.  Messages typically have multiple addressees.

A message in the Delta system is preceded on tape by the phone numbers
of all the recipient stations.  LFXD reads the first phone number,
dials it, and if a connection is made skips to the beginning of the
message and transmits it.  Then it backs up the tape and stabs it to
mark that phone number as delivered and advances to the next phone
number and repeats the process until all copies of the message have
been delivered.  In case a number is busy or does not answer there
is provision for some retries and then delivery to an intercept position
for manual handling.  After all copies of the message have been delivered
the LFXD feeds out the tape to a point where it is no longer retrievable
and proceeds to handle the next message.

Since the airlines want mnemonic codes for message addresses, such at
ATLRVDL for Atlanta, reservations, Delta, there was a machine called
the Codomat containing cards, like a Rolodex file, with tabs for the
mnemonic codes and punched with the phone number corresponding to the
code.  The operator would locate the desired mnemonic, pull the card
sideways into a slot where a crawling-head reader would read the
perforations and copy them to the message tape being prepared.  This
would punch both the phone number and the mnemonic code into the tape.

There was also a large collection of equipment used to interchange
messages with the store-and-forward systems used by other airlines.

The system was used successfully by Delta for several years.  One was
also made for United Airlines.  All this depended on the WADS - Wide
Area Data Service proposed tariff, which the FCC rejected and then the
systems had to be replaced by something else.  Something else by this
time was a computer technology store-and-forward switch such as the
Collins C-8400.