[GreenKeys] Model 15 without blank key

[Jim Haynes]

On Sat, 20 Nov 2021, Harold Hallikainen wrote:
>
> Doesn't the Baudot code predate time division multiplex? Or, instead of
> time division multiplex, are you referring to synchronous transmission
> (which, I guess is time division multiplex of the 5 bits onto one line). I
> agree that without start-stop, you need something to put in the spaces
> between characters if there is space, and the "blank" key is a good choice
> for that.

The original Baudot system was four channel synchronous time division 
multiplex, which was high technology for its day.  And they needed a
synchronous or multiplex blank character when there was no traffic
character available to send at the time one was required.  The code we
currently use is actually Murray's, but we tend to call it Baudot in
honor of the original inventor.  Murray just made some improvements and
used a different code but kept the concept of 5 bits per character.
(and a ltrs/figs shift to get more characters)

And Western Union used time-division multiplex into the 1950s or so.

It was Howard Krum's invention of start-stop synchronization that made
a non-synchronous teleprinter operation possible.  This got around the
high technology of keeping mechanical distributors running in sync,
which was only practical between offices where there were maintenance
people on hand at both ends.

>
> The use of the blank as a printing character on the weather keyboard
> seemed strange to me. Looking at
> https://en.wikipedia.org/wiki/Baudot_code, I see that FIGS-blank is the
> minus sign, while LTRS blank is blank. That's pretty clever! Unless you

I guess they just needed more characters than they could get with just
two cases and so chose blank in figs case as a way to get one more.  But
that precludes use of blank as the synchronous idle character.  Note that
ASCII has a character devoted to that purpose.  In fact it has a whole 
bunch of characters reserved for control purposes.  Which was more or less
unnecessary.

IBM confronted the problem of wanting to send pure binary data over a 
link, and pure binary would of course include all those control 
characters.  Their solution, embodied in what was called Binary 
Synchronous Communication, was to designate a single character as
DLE - data link escape.  When sending pure binary data if a DLE occurred
in the data it would be transmitted as DLE DLE.  The receiver would
recognize this and discard one of the DLE characters.  For other control
characters they were sent as DLE followed by the control character,
and the receiver would recognize a single DLE as a prefix for a control
character.  Of course you just about have to have a computer to do all
this recognition and substitution.