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From: tanner@ki4pv.uucp (Dr. T. Andrews)
Newsgroups: comp.dcom.telecom
Subject: Re: The 'Public Telegraph Office'
Message-ID: 
Date: 23 Sep 89 12:01:45 GMT
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X-TELECOM-Digest: volume 9, issue 402, message 2 of 11

) ... you mentioned that the [baudot tty] speed
) was around 60 baud.  Well, it was actually something called "75-speed"
) which indicated a maximum speed of 75 WPM, transmitted in 5-level
) "baudot" encoding.

Some of these  5-level  devices  could  actually  be  geared  for
different  speeds.   The  most common speed for the units, if you
find them today, is 45.45 baud (with 1.5 stop bits!), which works
out to ~6 chars/sec (commonly called 60 WPM).

) Actually, they used a "switch" character to flip-flop between
) meanings of particular bit patterns.
They  used  two  characters.   There  was  a  "shift"   character
(sometimes  called  "figs", bit pattern 0b11111) which forced the
second character set.  To drop back  to  letters,  the  "unshift"
character  (sometimes  called  "ltrs",  bit  pattern 0b11011) was
sent.

As a user-settable option, the  machine  could  also  be  set  to
"unshift-on-space",  which  meant  that  if a space (0b00100) was
received when the machine wsa in shift mode, it would  drop  back
into  unshift mode.  The user selection of the "unshift-on-space"
option is made by moving a metal bar under the vanes.

I do mean "drop" - shift mode was handled by raising the carriage
so  that  the  other  row on the type slugs would hit the platen.
The "unshift" got a nice gravity assist, and made a nice sound.

Those old model 15 and model 19  teletypes  were  fascinating  to
watch.

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