Fair price and ways to find a teletype

[COURYHOUSE at aol.com]

the red apple  ref  book  had  a 5 level running off  game port or??? 
something as I remember....Ed#
 
 
In a message dated 10/15/2015 11:20:28 P.M. US Mountain Standard Tim,  
unclefalter at yahoo.ca writes:

Thanks  for this great explanation.  So would anyone doing computing back  
in
the early 70s have used a 5 level machine?

I saw this one on ebay  (or is it two?  Not sure what the deal is here)
It's probably  sacked:
http://www.ebay.com/itm/Teletype-equipment-1-model-28-writer-1-reperforater-
1-50vdc-supply-etc-/121784463105?hash=item1c5aeb6f01:g:UR8AAOSwnDZUJHWs

-----Original  Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf  Of Brent
Hilpert
Sent: Thursday, October 15, 2015 10:07 PM
To:  General Discussion: On-Topic and Off-Topic Posts  
<cctalk at classiccmp.org>
Subject: Re: Fair price and ways to find a  teletype

On 2015-Oct-15, at 6:25 PM, Brad wrote:
> 
> Also  separate question to others:  I want to stay away from the  Baudot
machines, right?  (ie. Model 28, etc)

I'd say it largely  depends on what your interests or purposes are.
A brief overview of the  technology:

In the main, there were 3 generations of  teletypes:

Era             Common Model    Code
Speed       Common Interface        Mechanism
======            ============     ==========
=======    ===============         ============

1)    1930s-40s:         Model 15,19        5-level
~30-50 bps    60mA  current loop        Modified Typewriter  Cage

2)    1950s-60s:        Model  28        5-level
<=75 bps    60mA current  loop        Typebox

3)     1960s-70s:        Model 33         7/8-level/ASCII    110
bps        20mA  current loop        Type Cylinder

These are the  "page printers" that would type across and down sheet paper
fed from a  roll.
There are other models, variations on the above. 
Not included  here are the tape printers, simpler mechanisms that printed in
one  dimension on a narrow paper tape, ala stock tickers.

The mechanism was  the overriding distinction between these generations as
speed and code  capability followed from the mechanism:

1) Modified  Typewriter Cage:    
Decoding bars select 1-of-30-odd symbol/type arms
arrayed in an arc, to  swing and hit the paper, just like a common
typewriter.

2) Typebox:
An ~ 1" by 2"  metal box holds typeface symbol pins
in two 4*8 matrices.
The box is shifted up/down and left/right to bring  a
selected
symbol pin between  a hammer and the paper.

3) Type Cylinder:
A cylinder embossed with the typeface is  moved
up/down and rotated CW/CCW to select a symbol.

The 5-level  devices are commonly referred to as Baudot devices but this is
not strictly  correct as they generally use the ITA2/USTTY codes
(International  Telegraphy Alphabet No.2).

The speed of Model 28s (at least) was  determined by a selected gear-set.

5-level machines need code  conversion of course.

They all need current loop interfaces.

A  lot of old computer equipment will do 110 bps as the 33s were so
associated  with computers.

For working form modern equipment, the bit rates for  all of them are
potentially awkward.
When working on the 28s, which were  geared for 75 bps, I lucked out as I
found the USB-serial interface I was  using could do 75 bps
- not entirely surprising as 75 is a factor of 2 down  in the common
9600,1200,300 bps series. How many USB-serial interfaces are  capable of 
this
I have no idea. 
Regardless, the baud rates are slow  enough that bit-banging from a program
is not difficult, or an adjustable  RC oscillator to a UART should do.

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