The "cent key"

Jerry Feldman gaf at blu.org
Mon Jul 9 07:36:01 EDT 2007


Thanks Rick,
It's been a few years since I worked with Baudot code (1971), but the
ASR 33 was used at Burger King Corp in the early to mid 1970s with their
point of sale test in the office.  The BK POS device was a 4K core
memory PDP-8M with no storage device, no punched paper tape. The
printer code actually had to issue the code to strike the hammers on
the drum. No UART, so the bits had to be timed by code loops. The
advantage of this, is that it was possible to adjust the baudrate on
the fly. The BK system had a powerfail board that would save all the
registers (1 12 bit accumulator, 1 link bit, possibly the MQ, and the
Program Counter).  

 On Sun,  8 Jul 2007
23:01:15 -0400 (EDT) Ric Werme <ewerme at comcast.net> wrote:

> Jerry Feldman wrote:
> On Sun, 08 Jul 2007 11:36:02 -0400
> 
> > The ASCII character set was originally a 7 bit character set defined in
> > the late 1950s when the input device was an ASR 33 teletype. ASCII
> > started to replace the old baudot code which was (if I remember a 4 bit
> > code).
> 
> Five bit code, see
> http://www.baudot.net/
> http://www.science.uva.nl/faculteit/museum/papertape.html
> 
> OBtW:  I worked as a contractor at a place from time to time and one of the
> young 'uns tried to impress his coworkers with the following:
> 
>   Core memory always used odd parity because reading core memory caused the
>   data to be zeroed.  That way odd parity tests would catch the read and
>   forgot-to-write cycle.
> 
>   Why does RS232 data generally use even parity?
> 
> I looked up (I was the only one working at the time), looked straight at
> him, and said "Paper tape."
> 
> He was crestfallen, the others were merely confused.
> 
> The reason, obvious to anyone who has had to prepare a paper tape offline,
> is that if you made a mistake, you backed up the tape, and pressed the
> rubout key.  On ASCII teletypes, that punched all 8 holes, and programs
> that would read the tape would discard rubouts.  Even parity allowed
> rubouts to pass the parity test.
> 
>     -Ric Werme
> 
> P.S. "The only thing worse than fan-fold paper tape is non-fan-fold paper
> tape.  - Bob Clements
> 
> -- 
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-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix user group
http://www.blu.org PGP key id:C5061EA9
PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9
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