[GreenKeys] FRXD tape unit

[Jim Haynes]

The primary use of the FRXD machines was in automatic switching systems,
for message storage in the store-and-forward operation.  These systems
have somewhat varying architectures, so I'll describe a typical one.

An FRXD is on the incoming side of the office, copying the traffic that
is submitted by an out station.  When a complete message is received
the incoming position requests a connection to an automatic switching
unit.  When this is available, it will run the reader portion of the
FRXD to read the portion of the message header containing the routing
information - commonly called routing indicators, or selection characters.

The automatic switching unit decodes the routing indicator(s) and sets
up a potential connection to an FRXD on the outgoing side of the office,
associated with the destination.  It is then free to handle other 
messages.  When the outgoing FRXD is idle the potential connection is
converted to an actual connection; and the outgoing side tells the
incoming side FRXD to transmit the message all the way to the 
end-of-message code.  Thus the message is punched into tape at the 
outgoing side.  If the outgoing line is idle the outgoing FRXD can
have its reader start sending immediately.  If it is not idle the
outgoing FRXD reader waits for the line to become available.  So
a message is stored and forwarded twice, once on the incoming side and
again on the outgoing side.

A principle of this kind of switching is that you want to make the
cross-office transmission as fast as possible.  The reason is that you
want to get the messages out of the incoming FRXD as quickly as
possible so they don't have to wait behind the messages in front of
them.  To help that, a busy outgoing line may have two FRXDs feeding
it, each one sending alternately and the cross-office connections
sending into whichever one is available first.  With the FRXD you can
operate the outstations at 60 or 75 wpm and do cross-office at 75 wpm
always.  So the FRXD is also a speed converter.

Without the crawling read head on the FRXD the incoming side of the
office would have to punch out enough LTRS or blanks after a message
is completely received to let the end of the message get through
the reader head.  But this tape feedout operation has to quit
instantly if a new message starts coming in.  This feature is available
in some reperfs and is called non-interfering letters feedout.

Likewise on the outgoing side of the office it would be necessary to
have non-interfering feedout in the case of a fixed-head reader there.

In the Model 28 line there was more versatility in which punches could
be combined with which readers.  The usual configuration was the narrow
R-T "stand", which allowed two of them to go side-by-side in a rack
cabinet and allowed for large tape supply and takeup reels underneath.
There was an LFXD which was Model 28 equipment mounted on a base exactly
the same size and configuration as the FRXD, so it could replace the 
latter.

In Plan 55 for the Air Force, Western Union used LARP punches running
at 200 wpm for the outgoing side reperforators, giving cross-office
transmission at that speed (using their own tape readers).

That kind of machine could also be used in a torn-tape operation.
You usually think of torn tape as reperforators receiving directly from
incoming lines, and carrying the tapes to tape readers sending
directly to outgoing lines.  I've read that the Postal Telegraph
system, before the merger with W.U., was torn tape but had hidden
reperforator-transmitter sets on the outgoing side.  I don't know
if these were FRXDs or more common stuff with tape feedout.  The
reason again was to make cross office transmission as rapid as possible,
so that the tape readers being fed by the operators would empty
out faster and the hidden R-T set would buffer the traffic going
outward.

Another application of FRXDs that I am aware of was in the C.A.A.
weather distribution system.  The weather sequence messages were
collected from airports and other observing points on multistation
area circuits.  It was necessary to get some weather messages
from one area circuit to another, so that an airplane crossing from
one area to another could get the weather at the destination.
There was a system called MEDIS for Message Diversion that used
FRXDs at stations that sat on two different area circuits, copying
selected traffic from one to the other.  This is another example of
store-and-forward, but one where the storing and forwarding are not
centralized in a switching center.  The storing is necessary because
the receiving area circuit might be busy at a time when the message
was received from the originating area circuit.

Seems like someone on this group said that before MEDIS they had
FRXDs with manual plug and jack switching to do this forwarding
of weather traffic.  By the time of commercial jets MEDIS became
just too slow and the F.A.A. contracted with Teletype for a new
system, ADIS, but that is another story.