Fair price and ways to find a teletype

Brent Hilpert hilpert at cs.ubc.ca
Fri Oct 16 00:06:44 CDT 2015

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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