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From: <>
Date: Sun Feb 27 13:22:22 2005

used the open collector driver but you can check that easily.

>Jim Willing also warned of the glitches. He said it was so noisy that it
>was a challenge to build a proper receiver.
>At any rate, the way I understand it, I wait for the IM (Index Mark)
>signal from the reader to go high which indicates a column has been read.
>Shouldn't the character data be stable at that point? What am I missing?
The data should be stable. If the cable is long enough to get reflections or
cause slow rise times or the the type of reader uses mechanical contacts
(not an issue with this one) you can get glitches on the strobe. That
could cause you to read the same character twice. With the M200 and a 10
foot cable you shouldn't have any problems.

From: (Tony Duell)

>> or schematics for it? Polling from an Apple ][ should be slow enough that
>> it won't see the glitches that are likely to happen on transisions of the
>> index mark signal. I frequently will read strobe signals like that until
>Why should there be glitches on the index mark signal? The M200 has a
>fairly elegant design for this IMHO.
Your right it probably won't have any. If he had built a long cable
without good grounds that could cause some even if the M200 was clean.
I was more thinking of the >30ft 25 pin ribbon cable I used off the PC
parallel port to control my online PDP-8. It had glitch problems when I
tried to use the smart modes on the parallel port. Double sampling in
software fixed it.

David Gesswein -- Run an old computer with blinkenlights.
                         Have any PDP-8 stuff you're willing to part with?
Received on Sun Feb 27 2005 - 13:22:22 GMT

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