[GreenKeys] Why and When 7.00 vs. 7.42 unit code?

[Jim Haynes]

On Mon, 10 Mar 2014, Duncan Brown wrote:
>> 
> Because the receive shaft is turning faster, the selector mechanism gets a 
> longer stop period?
>
Yes, the receiving shaft reaches the point to lock up the clutch around
11ms into the STOP pulse.

When you get a radio fade (resulting in lost of STOP pulse) the receiving
clutch fails to lock up when it should, so the shaft continues to run
until there happens to be a marking signal at the time it should latch up.
That might or might not be the real stop pulse of a character, but
eventually after printing several garbage characters it will get back
where it belongs.

Western Union fitted some of their machines with an open-line stop, which
was a contact operated by a cam on the selector that could detect when
the signal was spacing at stop pulse time.  This could supply an 
artificial stop pulse in time to lock up the selector clutch.  They used
this in some full-duplex switching applications where they wanted to send
out break signals to control the transmitting side.  They could send
timed breaks in the middle of outgoing messages without printing any
garbage characters because of the open-line-stop.

I just received a W.U. Model 105 printer, and it appears to have some kind
of mechanical open-line-stop.  I haven't investigated yet, but it doesn't
run open when the signal line is open.

Don't know why Teletype never developed such a feature, as it would seem
to be useful.