[GreenKeys] Police teletype

[Jim Haynes]

On Wed, 18 Dec 2019, Jim Cooper wrote:

> were detected mechanically in the local stunt box
> and a switch closed to activate something like the
> solenoid, which would then disable the print
> suppression (double negative!) ...  local reperfs
k
But that's not quite how it worked.  Remember the engineers
at Teletype were (some of them anyway) prima donnas at mechanical
design.  Selective calling was done entirely mechanically, with
stuntbox parts moving a code bar to enable or disable the printing
clutch.  And I'm sure they could have made some entirely mechanical
way of enabling or disabling print suppression with a manual
control.  But for some reason they put in the solenoid to prevent
print suppression from happening when it was not desired.

As I recall the code bar that made selective calling work was the
same one used for automatic carriage return and line feed, so you
couldn't have both of those features on the same typing unit.

Selective calling is pretty complicated.  You have to have a state in
which, with the machine non-printing, it responds to selective calling
turn-on codes.  Then you have to put the machine into a state where it
is not sensing the selective calling codes, so it doesn't respond to
them during traffic with the machine printing or non-printing.  And
then an end-of-message-text sequence puts it back into the state of
sensing turn-on codes and not printing.

Model 28 had a big impact on FAA stations.  Previous to Model 28 they
had a little box with a 6 by 6 array of pushbuttons that could connect
any of up to six machines with up to six circuits.  All it switched
were the signal lines.  With Model 28 they had the stunt box controlling
all kinds of things, so the little pushbutton box turned into a seven
foot rack with panels that could switch I don't remember how many wires
in addition to the signal circuit.

Previous to Model 28 there was a machine called SOTUS which could do the
kinds of things that the stuntbox does, detecting characters or sequences
of characters mechanically and closing switch contacts.  That's all it
did - nothing connected with printing.  Model 28 made that machine
unnecessary, provided the stunt box was big enough for everything you
wanted to do.  Later on there was a machine called the Model 28 Sequential
Selector.  This was basically a typing unit with the front plate and other
printing parts omitted, so it just operated the stunt box.  This was a
replacement for the SOTUS when some existing systems were upgraded to
100 wpm operation.  There was also an LRXD, a 28-line replacement for the
FRXD typing reperforator transmitter distributor.  The preferred sort
of machine for new projects was the R-T stand which accomodated 3000 foot
paper tape reels.  But the LRXD was exactly the same size and form factor
as the FRXD for direct replacement in existing systems.