21 Apr
1998
21 Apr
'98
9:45 a.m.
Tony Duell wrote:
Punched cards have one major design bug IMHO
that's not shared by paper
tape. There's no 'column reference' on a card. On a tape, you can strobe
off the sprocket track, but alas on a card a totally unpunched column is
valid (=a space character).
My Documation M200 uses the leading edge of the card as a reference. It
then counts pulses from a toothed wheel/pickup head on the card roller
shaft to deteremine where the columns should be.
It's therefore almost impossible to make a hand-pulled card reader.
I rather liked Allison's long array of photodetectors (of whatever
flavour) to tell you where the leading edge is.
But why not have the same wheel to determine the position, but the card
turns the wheel rather than vice versa? You lose the non-contact
element of hand pulling, but you don't get motors chewing delicate
cards.
Philip.