[GreenKeys] 1 or 1.5 stop bits?

[Paul Heller]

I think the entire idea of a stop and start pulse to synchronize communication is a fantastic invention. I always wondered, however, if the constant mechanical locking/unlocking of the clutches/shaft on every character was a good idea. It seems it would wear out parts faster, specially the stop/start pawl. But that does not seem to be the case.

Paul
W2TTY

> On Jan 2, 2022, at 7:54 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
> 
> I've given this lecture a few times before, so if you are tired of it
> just delete.
> 
> The principle of start-stop synchronization, which makes teletypewriter
> operation simple at the expense of a little slowness, is that the
> receiving mechanism must come to rest in between characters.  Western
> Electric built some machinery using electromechanical sending and
> receiving distributors, and put the sending and receiving distributors
> on the same shaft.  This doesn't allow any time for the receiving
> distributor to stop between characters, so they added a relay to slow
> down the transmitting distributor trip a little bit.
> 
> Morkrum, ancestor of Teletype, ran the receiving distributor a little
> faster than the transmitting, so the receiving distributor always got to
> tne end of its rotation before the transmitting distributor finished
> sending a single unit stop pulse.
> 
> Western Electric measured the time delay introduced by the relay and found
> it to be 0.42 as long as a normal pulse.  Thus to have their machines
> interoperable with Teletype they demanded that the Teletype transmit an
> elongated stop pulse 1.42 times as long as the normal pulses.  The
> source of this is a very obscure old document that someone made available
> to me.
> 
> Teletype equipment has always been capable of operating with 7.00 unit
> code because of the different sending and receiving shaft speeds, but
> the elongated stop pulse demanded by the Bell System became the U.S.
> standard.  Long after the Western Electric equipment was no longer in
> use so the need for the elongated stop pulse had ceased.
> 
> Western Union didn't like the delay added by the longer stop pulse, so
> they tended to operate machines set up for 7.00 unit code.  For Teletype
> equipment this means no change to the receiving selector shaft speed, but
> changing the gearing and camming on the keyboards and tape transmitters
> to eliminate the extra-long stop pulse.  I you happen to get old W.U.
> equipment you may find the keyboard gears and cams different from the
> ones in non-WU machines.
> 
> Somehow, and I don't know how, the standard speed for Teletype equipment
> in the U.S. was nominally 60 wpm, with a 22 millisecond signal pulse
> length, speed of 368 operations per minute.  22 x 7.42 gives 163.24
> milliseconds as the minimum length of a character, This gives 6.12595
> characters per second, or 367.55 characters per minute.  And at 6 characters per word (including the space between words) that's 61.26
> wpm.
> 
> Keeping 22 ms pulse length for the sake of interoperability, the 7.0
> code gives 154 milliseconds as the minimum length of a character.
> That gives 6.5 characters per second, or 65 wpm.
> 
> Now in Europe they use 50 baud, or 20 ms per signal pulse.  I have no
> idea if the designers of European teleprinters decided they needed an
> elongated stop pulse and if so settled on 1.5 unit length.  50 baud
> and 7.5 unit code means 150 milliseconds per character, or 66 wpm.
> 
> From this you can see why it's not a good idea to throw around words
> per minute casually, as 7.00 code 45.45 baud is almost the same wpm
> as 7.50 code 50 baud, yet the two are hardly interoperable.
> 
> When we get into electronics it's obvious that it's a lot easier to
> generate 7.50 code than 7.42.  There's no problem receiving 7.00
> code.
> 
> So if you are talking to Teletype equipment it should not matter whether
> you generate 7.00, 7.42 or 7.50 code.  If you are talking to other equipment I will not offer an opinion whether the elongated stop pulse]
> is needed.
> 
>    ---
> 
>    "Ya can argue all ya wanna, but it's dif'rent than it was."
>    "No it ain't! No it ain't!  But ya gotta know the territory."
>        Meredith Willson, The Music Man
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