Fair price and ways to find a teletype

Brad unclefalter at yahoo.ca
Fri Oct 16 01:20:22 CDT 2015

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:

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brent
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

	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
			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.

No virus found in this message.
Checked by AVG - www.avg.com
Version: 2015.0.6172 / Virus Database: 4447/10805 - Release Date: 10/12/15

More information about the cctalk mailing list