Re: Fw: Re: Fw: Re: Modern myths

From: Andre Fachat (
Date: 1999-04-23 09:48:43

William Levak wrote:
> I can make no sense out of this message.
> 59460 refferred to in the original message is the 6522 timer.
> 59406 referred to in  this message is not connected to anything on any
> PET.
> 59424 is the IEEE data input.

He probably meant 59456, i.e. $e840 VIA, Port B.
(I know why I normally use symbolic names for pin names... :-)

VIA PB5 has the mentioned Sync input, that is also connected to 
PIA#1, CB1 (should be $e813, 59411), to generate the system interrupt 
once a frame (see the RESET routine, it enables the CB1 interrupt).

The other stuff is PIA#1, CA2 (should be $e811, 59409) that drives 
the "BLANK TV / -EOI" line
and that, in early PETs, give that blinking when saving to disk,
or the flash effects Jim mentioned.
Later the "blank screen" feature for this output pin has been removed.


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Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG
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