[GreenKeys] Model 28 Operations Per Minute

[Keelan Lightfoot]

Hello greenkeyers. Long time no post.

I found myself curious about the main shaft speed of the Model 28 vs.
Baud Rate. I'll use 100 WPM/74.2 baud for this example. On the motor
there is a 20 tooth pinion and driving the 84 tooth driven gear on the
intermediate shaft, which is coupled to a 48 tooth gear that drives a
60 tooth gear on the main shaft:

3600 RPM * 20/84 * 48/60 = 685.71 RPM

685.71 RPM = 685.71 operations per minute, divided by 60 = 11.43
operations (characters) per second, or 87.5 ms mer character

So, stepping back for a second: 74.2 baud = 13.48 ms bit period * 7.42
bits per character = 100 milliseconds per character.

At first this confused me, it seemed like the main shaft was spinning
fast enough that there wasn't time to receive all 7.42 bits, but then
I realized that the start bit trips the selector clutch, and then the
5 datas bits follow, 6 * 13.48 ms = 80.88 ms. The stop bit doesn't
need to be "received". The main shaft is rotating fast enough for the
selector to receive all of the bits, the stop bit period gives the
clutch time to disengage ready for the next character. Makes sense to
me. (Am I understanding this properly?)

I noticed that the shaft rotational period (87.5 ms per character) is
7/8ths of the actual character period (100ms). This is 7/8 ratio
consistent across baud rates (e.g. 22 ms vs 19.25 ms at 45.45 baud).

Why is 7/8 the magic number?

- Keelan