[GreenKeys] Model 15 without blank key

[Jim Haynes]

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.