[GreenKeys] More Teletypes with Mainframes

[Jim Haynes]

On Tue, 16 Jul 2013, George B. Hutchison wrote:

> Hmmmmmm.
>
> No one seems to remember the Model 29.
>
> It looked like a 28, smelled like a 28, had parts that were
> interchangeable with a model 28,
>
> But was in fact a machine that responded to some variant of EBCDIC.
>
Seems like I keep getting moved into telling Long Stories, without
knowing if anybody is all that interested.

Model 29 started out in life, I'm told, as a 28-line replacement for
the Model 20 up/low case six-level machine used on Teletypesetter
circuits.  Trouble was, nobody wanted an up/low 28; they apparently had
all the Model 20s they needed.

Now in the computer biz at the time there were a bunch of six-level
codes, generally called something BCD meaning binary-coded decimal
meaning the decimal digits were represented by their binary equivalents.
Each computer manufacturer had its own variant of BCD, and some like IBM
had several different codes based on BCD.  These codes were more or less
easy to translate into Hollerith punched card code.  A funny bug or 
feature of Hollerith is that the alphabetical characters have breaks in
the alphabet.  This will come up later.

Western Electric and probably some of the Bell operating companies
saw it would be useful to have Teletype equipment operating with BCD
code for their computerized applications, so the Model 29 was recoded
for BCD.  The most often seen variant was an ASR set called the IDP
ASR set for Integrated Data Processing, one of the buzzwords of the
period.  There would have been a large market for this kind of equipment
in the business world, but AT&T didn't want to appear to be favoring
IBM over the other computer companies, and didn't want to have to make
a dozen variations of the set for all the different computer companies'
BCD codes.  Univac had an odd one called excess-three.  The decimal digits
instead of being represented by binary codes were represented by binary
codes with three added.  This is convenient for doing decimal arithmetic.

So the Model 29s, officially called the Model 28 IDP ASR sets, were 
supposed to be for Bell System internal use only.  However it happened
I know some escaped.  When I worked for G.E. in 1966 there was a man
whose job was converting Model 29 typing units to Model 35.  Later
there was a surplus store near the Oakland airport that had the remains
of an RCA computer system and that included a bunch of Model 29 RO
machines.  One of my friends bought a number of those, converted them
to Model 35, and sold them to a dealer in that kind of stuff.

The military was unhappy about all the varieties of BCD, and proposed
a code called Fieldata that was to be the military standard.  Fieldata
had its own set of problems, so there was the big effort that led to what
we know as ASCII.  Much has been written about this so I won't go into
it.  AT&T and the military backed ASCII, and most of the computer 
companies decided they could live with it, but not IBM.  The sticking
point was that ASCII had the whole alphabet in a contiguous set of
codes, while IBM wanted to hold out for the code with breaks in the
alphabet but easy to translate to Hollerith.  The end result was that
IBM quit objecting to ASCII and just went off in its own direction with
a new code called EBCDIC - Extended Binary Coded Decimal Interchange
Code.  Which had breaks in the alphabet.  Presumably IBM thought their
market power was such that they could bury ASCII; but they were up against
AT&T and the federal government.

I know Teletype built a limited number of ASR keyboards for Fieldata;
presumably there were printers as well, and presumably they were also
considered Model 29.  It doesn't really matter because the numbered
models are a Bell System thing and Bell never offered the Fieldata
machines to the public.

I believe EBCDIC was announced in 1964 concurrently with the System/360
announcement.  Some computer manufacturers felt they had to support
EBCDIC or they couldn't compete against IBM.  IBM made some CRT terminals
that used EBCDIC.  Their hard copy terminals were based on the Selectric
typewriter and they had quite a zoo of codes for those.