6502 CPU schematics

[Philip Belben]

Pete Turnbull, quoting me:

>> When I was 13 (1980.  Ouch!) I was given a PET for my birthday.  A few 
>> years later I blew up an output buffer in one of the 6520s trying to 
>> use the Cassette 2 motor output as a controlled power supply.  We 
>> connected one of those two-tone doobells that uses the back emf from 
>> the "ding" solenoid when you open the switch to sound the "dong".  
>> Said back emf killed the output buffer on that pin of the 6520.  It 
>> managed to do so without any noticeable damage to the intervening 
>> circuitry, too!
> 
> Common problem -- 6520s are fragile.  For that reason, it was common, at 
> least here, to replace them with Motorola MC6821s, which was a later 
> version of the MC6820, which is just the same as the MCS6520.  Lots of 
> the PETs at the college where I worked in the early 80s had Motorola 
> replacements because the 6821 seems to be more robust than the 6520.
> 
> So replacing the missing or damaged 6520 with a Motorola MC6821, which 
> should be much easier to find, even has a degree of authenticity :-)

Thanks for the tip.  6821 or 6820 goes on my mental wish list.

Philip.