[GreenKeys] Model 15 without blank key

[Harold Hallikainen]

On Sun, November 21, 2021 10:28 am, Jim Haynes wrote:
> 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.
>

Thanks for all the info! Pretty amazing stuff. The DLE concept is
interesting. Hayes modems did something similar with +++ sent very quickly
dropping you out of transparent data mode to command mode. Also, years ago
I did a run length encoding system for sending images over SPI to a
display. A code of 0x01 would be followed by a count of how many times to
repeat the previous pixel value. A sequence of 0x01 0x01 would actually
set the pixel to 0x01. This worked well since it was rare to set a pixel
to 0x01 (we put an RGB value into the 8 bit byte).

I really appreciate you knowledge of the history of all this.

Harold

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