31 Mar
2010
31 Mar
'10
2:34 p.m.
Yes. The main problem is that there is no 'clock
track' on a punched card.
On paper tape you can use the smaller sprocket holes as a timing
reference. on cards you have nothing like that, there will be blank
columns with no holes punched at all. The way my reader (A Documation
M200) does it is to move the card at constant speed using known-diamter
rollers, then to detect the leading edge of the card.
IBM used to use a known diameter BRASS roller. It was based on a
combination of which wire brush made contact with the roller and the
rotational position of the roller.
An unreliable source (my father), IBM's patents on that were a major
impetus for the development of photo-detecting readers, and that IBM's
attempt to shut out competitors actually caused them (CDC?) to leapfrog
ahead.
--
Grumpy Ol' Fred cisin at xenosoft.com