[GreenKeys] Tape punch question

[Jim Haynes]

That reminded me of a funny story.

At first Teletype didn't have a parity-generating keyboard, so the 8th bit
on the tape was always punched marking.  After parity keyboards came into
use there was a problem with one of the tape readers making errors in the
8th bit.  Investigation revealed that the designers of that tape reader
had trouble getting the tape reading mechanism to settle in time for the
8th bit to be generated; it would sometimes make errors in that bit.
So, knowing that at the time the 8th bit was always punched marking, they
added a little bias to the mechanism so that if the 8th bit was unsettled
it would always be transmitted marking.

There was also a controversy in the area of standards.  Teletype wanted
8-level signals to be transmitted even parity.  The obvious reason is that
the rubout character for erasing punching mistakes is the all-marking
one, and that is even parity in 8-level.  IBM wanted signals to be 
transmitted in odd parity.  I guess the reason was so that a false start
pulse or a break signal would produce characters with even parity,
and thus they could be distinguished from valid characters.  I suppose
what should have been done was to punch tape with even parity, but to
invert the 8th bit in the signal generator so that odd parity would be
transmitted.