[HCARC] TTY

[Virgil Bierschwale]

I hear you

I'm wanting the model 28 for the reasons you described on the 33's and
because of the offshoring stuff we have discussed.

Consider it a statement that us ole ????s still have lots of miles left in
us, that I hope to see in the news one of these days

Virgil
N5IVV


-----Original Message-----
From: hcarc-bounces at mailman.qth.net [mailto:hcarc-bounces at mailman.qth.net]
On Behalf Of John Canfield
Sent: Tuesday, January 28, 2014 12:00 PM
To: hcarc at mailman.qth.net
Subject: Re: [HCARC] TTY

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 selectr
 ic 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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