[GreenKeys] Teletype monopulse printer

[John Nagle]

>> [Original Message]
>> From: John Nagle <nagle at animats.com>
>> To: <greenkeys at mailman.qth.net>
>> Date: 20-Nov-09 12:45:52
>> Subject: Re: [GreenKeys] Teletype Monopulse printer
>>
>>
>> The early printing telegraphs had trouble staying in sync, but for the
>> Teletype 36, they seem to have assumed that both ends were running from
>> the same synchronous AC line, so the sync problem wasn't too hard.
>>
> 
> I don't think synchronization was a big problem with the M36, since it was
> such a simple system.  If TX & RX were always in the same "rest" position
> at the start of a letter, then no other synchronization would be needed.  
> 
> Contrary to what I said before, the signalling pulse was a NO current
> condition. Here is a description I found from the transcript of a slide
> show of the original Teletype Corp. Museum.  (Someone on Greenkeys
> supposedly has the orignal slides and is going to put them onto a DVD????)
> 
> -----------------------
> [from "History of Telegraphy from the Teletype Museum"; Ransom D. Slayton,
> Consultant;1983]
> 
> 
> 
> "203. There was always the push to develop lower cost units for simpler
> systems. One such system was the Model 36 Monopulse.
> 
> "204. This operated much like some early stock tickers, with the line
> signal being an OPEN interval during which sending and receiving devices
> rotated until the proper point was reached for printing the desired
> character. the Keyboard, at the left here, had its Keys mounted in a
> circle, which was non-standard and took a bit of training by the operator.
> The sending contact was opened by depressing a key, and remained open until
> the selecting arm reached that key after leaving its rest of STOP position.
> 
> "205. Meanwhile, the selecting arm at the receiving Printer was similarly
> seeking the printing point. When the line closed, it was there, so the
> printing hammer struck the rotating typewheel, printing the character. The
> Printer typewheel was inked, much as the old BLUE and GREEN code printers
> had been. The inking roller can be seen at the top of the printer, with the
> typewheel under it. then comes the printing tape, the supply roll of which
> is at the bottom. The printing hammer is just under the tape and print
> wheel, and moves upward to cause printing. The system was very simple and
> therefore low cost, but only a limited number were made. "
> ------------------------------------------
> 
> I will send Brooke some pictures of the TX unit and he can put them on his
> site.
> 
> The only problem in synch on the M36 was how to get the TX & RX back to the
> home position.  Perhaps this was done through the SPACE function??

     Probably, the typewheel holds at the start point at the end of the
cycle, so it resynchronizes once per character time.

     Both ends have to rotate at the same speed, to within half a character
time.  The allowed speed error for that design is 0.5/(number of wheel 
positions), which is about 1%.  Start-stop Teletypes will tolerate much
more than that.  A 3% speed difference isn't even noticed, and 6% can
be tolerated.  The governed Teletype motors were able to stay within
those limits, but 1% would have been tough. A spring-loaded on/off
governor switch speed control can only do so much.

     Stock tickers are a different approach.  Those step the wheel to
the desired position, driven by pulses from the line.  They don't
require precise synchronization, or at least they didn't once
Edison came up with something that would stay in sync.  Read the
patent history of "unison devices" for how inventors struggled
to get something that would stay in sync.

					John Nagle