[GreenKeys] 7.00 vs 7.42 unit code

[Duncan Brown]

Correction - It was Western Electric, not WU that required 7.42

Here is Jim Haynes description (so he doesn't have to type it again):

===================================================
[23 Nov 2018]

Here's a little-known bit of trivia that I have posted before but
worth posting again I guess.

The question is, why 7.42 unit code (1.42 unit STOP pulse)?

Western Electric developed some teleprinters which used faceplate
receiving and transmitting distributors, and they had both distributors
running on the same shaft.  That obviously prevents stop-start from
working, since the receiver is ready to begin the start segment as soon
as the transmitter finishes the stop segment.  So they put a clutch on
the transmitting distributor, and used a relay to operate the clutch.
This added a little time after the transmitter had finished its rotation
before it could start off again.  Empirically they found that the relay
added 0.42 pulse duration to the process, so it effectively had a 1.42
unit stop pulse.

Now Morkrum (ancestor of Teletype) had solved the problem a different way,
by running the receiving distributor shaft faster than the transmitting
shaft.  So that when the transmitting shaft had completed a rotation the
receiving shaft had finished early and was held back by the clutch. This
continued through the whole life of the Teletype product line and meant 
that a Teletype printer could work satisfactorily on 7.00 unit code.

The Bell System early on wanted interoperability between the Morkrum and
the Western Electric printers, so they demanded that the Morkrum
transmitters should generate 7.42 unit code.  And this they continued
ever after, even though the Western Electric printers were long extinct.
And for some reason was applied to the machines sold to the military and
other non-Bell customers.  Only Western Union got 7.00 unit code keyboards.

So the 7.42 code is ultimately the result of a design short-sightedness
at Western Electric.

-Jim Haynes
==============================================

On 8/7/2019 14:49, Duncan Brown wrote:
> The extra 9ms on the stop pulse (to make 7.42) was a requirement for 
> an early Western Union machine that was obsoleted 80-90 years ago. 
> Most (if not all) "modern" machines (designed since about 1925) should 
> copy 7.00 with no problem.
>
> Computer TTY software is typically "tuned" to a specific data rate. I 
> know that MMTTY will not copy 66wpm when set for  60wpm. (I finally 
> found the hidden control on MMTTY that lets you change the speed). 
> Don't know how stop bit length would effect it.
>
> Have fun,
>
> Duncan
> K2OEQ
>
> On 8/7/2019 10:41, Nick England wrote:
>> Doing some machine shuffling this week made me think about this - 
>> please steer me back on course if my thinking is off.....
>> Truth-
>> 7.42 unit code has a start bit and 5 data bits of equal length (1.00 
>> unit each) and a stop bit 1.42 units long.
>> 7.00 unit code has a 1.00 unit stop bit.
>> 45.45 baud code can be either - each 1.00 unit data bit is 22ms.
>> So 7.42 unit at 45.45 baud is 60 wpm
>> and 7.00 unit code at 45.45 baud is 65 wpm
>> There's a chart at http://www.navy-radio.com/tty/speed-feed.gif
>>
>> Consequences-
>> These two do require different gear sets - you can't just put 60wpm 
>> gears (368 opm) into a 7.00 unit machine (390 opm) - that would make 
>> the baud rate wrong and your selector and keyboard transmitter timing 
>> would be off.
>> A 7.00 printer can copy a 7.42 keyboard or 7.42 TD no problem.
>> A 7.42 printer can copy a 7.00 keyboard if you don't type at max rate
>> A 7.42 printer probably cannot copy a 7.00 TD
>>
>> What about sending RTTY with a 7.00 keyboard or TD? - will the 
>> computer software RTTY folks copy it OK? I assume so, but don't know 
>> for sure.
>>
>> Comments welcome.....
>>
>> best regards,
>> Nick England K4NYW
>> www.navy-radio.com <http://www.navy-radio.com>
>>
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