31 Mar
2010
31 Mar
'10
9:39 p.m.
On Mar 31, 2010, at 4:08 PM, Tony Duell wrote:
I wonder if this could be handled with a simple PLL-based clock-recovery circuit. I
suppose the ones density isn't exactly guaranteed though.
every so
often). Making a punchcard reader from scratch is a very
different level of effort, so back then I figured that it'd be easier
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 (basically when the
optical sensors g form all on to all off, then use that to produce an
internal clock signal that should align with the centres of the columns
(if you see what I mean) and us that to strobe the read logic.
What if the card slips?
-Dave
--
Dave McGuire
Port Charlotte, FL