[GreenKeys] Model 15 without blank key

[Harold Hallikainen]

On Sat, November 20, 2021 8:34 pm, Jim Haynes wrote:
> On Sat, 20 Nov 2021, Harold Hallikainen wrote:
>> Also, all 5 bits are space. I suspect they made that not print anything
>> so
>> that an open loop (machine "running open") would not result in a
>> character
>
> I believe the real reason is that time division multiplex came before
> start-stop, and in TDM you need some character to send when you don't
> have anything to send.  In the 21-A printer there is a little mechanism
> such that if all bits are spacing the print solenoid is prevented from
> whomping for that character.
>
> In the post WW-II military time division multiplex - AN/FGC-5, AN/UGC-1
> there are six bits per character transmitted, the 6th pulse used only
> to show whether the other five are real characters.  Hence blank can
> be treated as a real character rather than an idle character.  And in
> the weather symbol type arrangement blank is actually a printing
> character.
>

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 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
are in FIGS, an open loop would not print a bunch of minus signs. It's
also tricky in that the carriage needs to not advance on LTRS-blank, while
it needs to advance on FIGS-blank. But, I guess that's no worse than
getting the carriage to not advance on FIGS-S (Bell).

These are truly amazing machines!

Harold
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