[GreenKeys] Torn tape switching

[Jim Haynes]

On Sun, 18 Nov 2018, Nick wrote:
> Thanks for sharing - very interesting to see the developing time line. I am
> still learning about teletype machines and I'm wondering what a ..... torn
> tape switching systems  ..... is.
> 
Torn tape is one category of store-and-forward switching.  It covers a lot 
of territory, from small to large systems.  The general idea is that 
messages are received on reperforators. Then operators tear off the tape 
at the end of a message, read the destination, and carry the tape to a 
transmitter sending toward the destination.  This beats receiving the 
message on a printer and then re-keyboarding it on a machine sending 
toward the destination.

At the small end is something like a reperforator and a tape transmitter
for one circuit in one set; then replicating that to as many sets as are
needed for the number of circuits involved.  Slightly more elaborate is
something like a Model 19 with reperforator attached.  There were parts
kits for the Model 15 printer to allow an incoming message to turn the
reperforator off or on.  Thus if the distant sending operator knew the
message would need to be relayed he would include characters to turn
on the reperforator and turn it off at the end of the message.  And if he
knew the Model 19 set was the final destination he would leave out the
reperforator characters to that the message was simply printed for
delivery.

The military AN/TGC-1 set housed two reperforators and a three-headed
tape transmitter-distributor (MXD)  The reperforators could be receiving
from one line, switching between them when one ran out of tape supply,
or receiving from two lines.  Two of the readers could be sending to two
different lines or could be operated "flip flop" meaning that when one
finished reading the other would start if it had a tape waiting to be
sent.  The third reader normally held a tape containing message sequence
numbers and would send one ahead of each outgoing message.

That kind of set is good for small offices and for situations such as
the military where the whole system is frequently reconfigured.  You just
keep adding sets to build an office as large as needed.  It gets difficult
to use as offices get large because the operators get in one another's 
way.  In any event torn tape allows for very flexible message routing
since operators read message destinations and can be instructed by
a routing desk or by chalkboard where to send messages for various
destinations.  And the equipment is fairly simple and doesn't involve any
wired-in routing that has to be changed when routing instructions change.
But a lot of operators may be needed to keep the traffic moving.

For larger offices it is considered better to separate the receiving and
transmitting equipment.  For this purpose a six-headed MXD was developed.
In some architectures the sending tape readers do not send directly to
outgoing lines, but send to out-of-sight reperforators feeding readers
going to the outgoing lines.  One reason for this is to keep traffic
moving out of the operator-fed readers; if traffic to a destination is
delayed for some reason the messages are stored in the second-level of
tape out of the operators' way.  However this is a drawback for systems
requiring different precedence levels of messages, since it's hard to
pre-empt a message that has already been stored in the outgoing 
reperforator transmitter set.

In some Western Union torn-tape switching offices the operators wore
roller skates to get between the machines more quickly.  W.U. later
developed plug-and-jack switching and pushbutton-operated stepping
switch systems so the operators didn't have to run around so much.