[HCARC] TTY

[John Canfield]

Good explanation from Kerry.

When I worked for SW Bell in Austin, I was a Teletype tech for several 
years and worked on Models 28 to the Model 40.  Fortunately the Models 
15/19 were just phased out when I became a TTY tech.  The Model 28 
selector magnets were 60 ma, the 33/35 were 20 ma.  Model 37 & 40 were 
RS-232 and the 40/4 emulated IBM bisync terminals.

Models up to and including the 28 were Baudot, the rest ASCII (except 
for the 40/4 which was EBCDIC.)

The Model 33 was an inexpensive machine that was a nightmare to keep 
adjusted and running due to the design.  To make adjustments, parts were 
designed to be bent (really!) instead of loosening a screw or nut/bolt 
on the 28/35/37 and then making the adjustment.  The 33 keyboards were 
particularly horrible.

I operated RTTY for many years with a 35ASR and I'm back on the air in 
digital modes with a couputer and couldn't be happier.  The Teletypes 
were noisy, slow, and not particularly user friendly or easy to keep 
maintained.

73 de WB5THT <John>
Harper

On 1/28/2014 10:56 AM, SARA SANDSTROM wrote:
> Teletype is a fascinating system.  There were/are 2 basic interface standards:  60 ma current loop and 20 ma current loop.  The problem with the 60 ma current loop is not the current.  Most loop supplies for the 60 ma standard provided 200 - 300 Volts.  This was because of the large inducta nce of the selector magnets in parallel.  If you didn't have enough voltage you couldn't drive the magnets.  The 20 ma standard was almost compatible with RS-232.  You could use th e approximately 12 Volts provided by a lot of RS-232 drivers to operate the machines.  As I understand it, the basic difference between a 60 ma machine and a 20 ma machine is the 60 ma machine had the selector magnets in parallel while the 20 ma machine had them in series .  By the 70's most if not all military TTY gear for comm had been converted to 20 ma low level operation.  The TTY machines used as computer terminals were probably never converted to low level but just eventually replaced often by a selectric typewriter type terminal or a Texas Instrum ents termi nal called a "Silent 700 " which used heat sensitve paper, if I remember correctly .
>
> Even solid state TU's (terminal units) had no problem with the high level (60 ma) machines.  At l east through the HAL ST-5000/ST- 6000 TU's , high level interfaces as well as low level, RS-232 and Mil Std 188c interfaces wereprovided .   O ptical isolators were not required/used.
>
> Incidently, I believe one of our members, John Canfield, used to be an AT&T TTY guy .
>
> If you can still find one, most of the old terminal programs included Baudot as a choice and had no problem converting TTY signals to ASCII for internal computer use.  The computer became a "Glass TTY".  Works fine, very quiet, and doesn't smell of oil and dust!  Many year s ago I got an Atari 800 computer for  my son.  I had a short program that converted a TTY signal to the ASCII.  I used the Atari as my TTY terminal.  A slow 8-bit processor had no problem keeping up with Baudot.
>    
> Kerry
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