[GreenKeys] Range finders and wrong speeds.

[Jeffrey D Angus]

On 7/4/2015 6:03 PM, Jim Haynes wrote:
> On Sat, 4 Jul 2015, Roy Morgan wrote:
>> The operating speed of the receiving machine is not being
>> changed.If you have a governed motor, it seems likely that
>> you can changethe actual machine speed, but I have not
>> heard of this.
> It was done during WW-II to adjust U.S. Model 15 machines to
> inter-operate with British 50 baud machines.  There is a
> special tuning fork for the purpose.
And by the same token, I "slowed down" the governed motor on my
Lorenz Lo15c to operate at 45 baud instead of 50. Not having the
correct tuning fork for that, I used a small rare-earth magnet
and aninductive pickup coil.

On the subject of how the range finder works.
A "normal" 45 baud teletype signal consists of an initial start bit, five
data bits, and a stop bit. The start bit, and the five data bits are 22 mS
each, and the stop bit is 33 mS. A total time of 165 mS.
The timing of these are physically fixed by the position of the rotating
shaft in the machine. Each operation, except the stop bit, is 48 degrees
of rotation. 22 mS for a 60 WPM machine running at the correct speed.

With the range control set at the mid point, on receipt of the initial
start pulse, it releases the clutch on the main shaft, at 72 degrees, or
33 mS after the initial start pulse, the first data bit is sampled and the
code bar for that bit is set to either mark or space. Then every 48
degrees or 22 mS afterwards, the 2nd, 3rd, 4th and 5th bit is sampled
and set on the code bars. At some point in time during the early part
of the stop pulse, the machine acts on the setting of the five code bars
and operates, reaches the end of the cycle and locks the shaft clutch
and waits for the next start pulse to come in.

Moving the range control ONLY adjusts the timing (or physical number
of degrees) from the initial start pulse to the time when the first data
bit is sampled. I.e., It can vary from 27 to 39 mS after the initial start
pulse. What does NOT change, is each subsequent sample time. The
2nd bit is sampled 22 mS after the first bit is sampled. The 3rd bit is
sampled 22 mS after the 2nd bit and so forth.

Nor does it change the rotational speed of the main shaft.

You can adjust the range control to skew the timing such that the
sample and set time is somewhere inside the 22 mS bit time of a
45 baud signal that will operate (i.e. print correctly) on a 66 WPM
machine. It has to have a nearly perfect signal, very little distortion,
to operate though. In the opposite direction, IF (a very BIG if) the
66 WPM machine is operating at less that 66 WPM, typically around
33 WPM, or "every other operation" you can copy 66 WPM data on
a 60 WPM machine. If the 66 WPM operator loads a tape and sends
at full speed, all bets are off.

Like the classic carpenter's dilemma, "I cut it three times, and it's
STILL too short."

At 50 baud, 66 WPM, the sample interval is 20 mS instead of 22 mS.







-- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com