M200 interfacing continues...

From: John Lawson <jpl15_at_panix.com>
Date: Thu Jan 2 14:11:52 2003

On Thu, 2 Jan 2003, Sellam Ismail wrote:

>
> I'll certainly try that. But I'm wondering why that should be necessary
> when the 6522 data sheet says that particular pin is a TTL level output.

  I imagine that the relative low resitance of the 1K resistor is
diverting too much of the signal to ground, ie. it's loading that pin too
much, and the actual voltage level of the pulses generated are of
borderline value to positively switch the chip.

  In TTL (as in all other families) there are 'windows' of operation...
there is a window of voltage ranges that constitute a guaranteed '0', then
above that a window of ranges where it is uncertain (or not guaranteed)
that the pin will be at the state wanted, and a window above that, in
which it is guaranteed that the pin will be at a '1'. Above that, you
risk killing the chip.

  I think the 1K pull-down is loading that particular pin down to where it
is inside the uncertainty window, thus allowing that Prankster Chaos to
slip in the back door... the other pins are working because they are
probably just over the borderline... 10K Rs will clean you up, I betcha.


  Cheers

John
Received on Thu Jan 02 2003 - 14:11:52 GMT