At 10:36 AM 16/06/2002 -0400, Kev wrote:
...
*Weird problems are sometimes traceable by momentarily shorting circuits to ground on the video board that use resistor pull-ups, the control ICs do not have strong pull-ups (otherwise they would not need the resistors) and you can learn a lot of how the images are built this way. Interesting watching lines appear, characters vanish, stars scroll sideways....
Is there some method to this madness?
Yup, by shorting various pull-up lines to Vss momentarily you can often identify the area of the board where non-rebootable video errors occur. In other words, things that don't trip the watchdog reset. Fun, and shows how different Video RAM are used to build the images...not quite what I expected while poking around...
*The horizontal delay timing is CRITICAL and if you are getting RAM errors but the RAM is good, then check the 220pf cap on the delay circuit...
That's all for today. But I did get in my Z80QT pod and boy that one is FAST on the RAM tests! A couple of seconds to verify a 1k block instead of a minute or two (short test). What a difference to troubleshooting time!
Sure rub it in! ;-)
Don't be jealous, I'm thinking of building an upgrade for standard Z80 pods...I have a couple of guys who like designing circuit boards on tap at my shop now, so things might actually start happening!
Thanks for the tips!
Kev
You guys are welcome! John :-#)#