[GreenKeys] Early printing telegraphs

[John Nagle]

> From: Duncan Brown <duncanancy at earthlink.net>
> Subject: Re: [GreenKeys] Navy Drops All Caps Format
> 
> "Printing telegraphs" were built & patented in the mid 1800s, soon after 
> the development of the telegraph, but did not become practical until the 
> early 1900s.  The earliest indication of use of a teleprinter by the US  
> Navy is in a 10 Aug 1922 article in the New York Times:
> http://query.nytimes.com/mem/archive-free/pdf?res=F40715FA385D1A7A93C2A81783D85F468285F9

  There were quite a few printing telegraph systems developed between
1850 and 1900.   Some of them worked.  The House and Phelps systems
were used widely by the 1870s.  Stock "tickers", which were slower,
were first invented by Callahan (1867).

  The big problem was keeping both ends in sync.  Printing telegraphs
of the period had rotating typewheels at both ends, and a pulse on
the line printed a letter at the current typewheel position.
The House sync mechanism was simply to drive a pawl into a gear
whenever a letter came in, so that any errors of less than half
a character time were corrected.  But there was no automatic
way to resynch.  At startup, the sender would send AAAAA for
a while to allow the receiver to adjust their printer.  Once
in sync, the machines would usually stay in sync until
something abnormal happened.  If the receive end got out
of sync, the receiving operator would open the line
(sending a long BREAK) so the sending operator would
notice.  Then they'd go back to the AAAAA drill.

  Stock tickers worked by sending "step" and "print"
pulses, like that toy TeleType thing that was on eBay
recently.  Each "step" pulse advanced the typewheel
one notch.  The Callahan machine had to be set
manually to get started and could get out of sync.
Edison's first profitable invention was a stock ticker
that could be resynched remotely.  At last, there was
something that could print without an operator constantly
watching.

   All these machines had a much lower parts count than
a Teletype machine.  It wasn't until manufacturing
technology improved, and a typewriter industry had
developed, that Krum came up with the first Teletype
machines.  The first few generations of those (the
"Blue Code" and "Green Code" machines, and the Model
12 Teletype) had electrical selectors with commutators
and relays.  The problem was that the relays all had
specific time constant requirements (some had to be
slow, some had to be fast) and adjusting the timing
was a huge headache.  This was long before oscilloscopes,
remember.  Some of the pre-model 12 machines used a
three-bit-time code with four voltage levels (two
polarities, two voltages) for each bit time.
For the Model 12, that was discarded, and the straight
5-bit code we know today was used.  Now the transmission
scheme was right, but the selector was still hard to
adjust.

   Finally Krum developed the fully mechanical
selector in the Model 14.  Now, as long as the motor
was turning at approximately the right speed, the
camshaft kept everything timed properly.  The Model
15 uses exactly the same selector mechanism.
Teletype continued to use mechanical selectors
well into the 1960s, longer than they should have.
You all know the later history, of course.

			John Nagle