PET startup sequence?? (was LF: Commodore PET schematics, troubleshooting info)

[Dwight K. Elvey]

Hi Dave
 A common failure item in these old machines is the RAM.
Any failure of the RAM makes things like subroutines fail
quickly.
Dwight


>From: "Dave Dunfield" <dave04a at dunfield.com>
>
>>> Deos ftp://ftp.funet.fi/ still exist? That used to be a good place for 
>>> Commodore schematics and technical info,
>>
>>Yup, FUNET's still alive. <ftp://ftp.funet.fi:/pub/cbm> for the CBM stuff.
>>There's also a web-based version at <http://nic.funet.fi/pub/cbm/index.html>.
>
>Thanks guys - that helps a lot.
>
>+++
>
>Does anyone have "inside" information on exactly what the PET firmware does 
when
>it starts up?
>
>I am still working on the same pair of SuperPET 9000's (one works, one does 
not).
>Both machines have been stripped to just the base 6502 board.
>
>The "bad" machine plays it's tune and clears the screen - then appears to hang.
>(If you reset it with the monitor warmed up, you can see the screen fill with
>"garbage" and then clear - just like the working one).
>
>Code appears to be running:
>
> - If I remove the KERNAL rom, the startup beep does not occur, and the screen
>   never initializes, so it looks like this ROM is executing.
>
> - After it appears to "hang", the code still seems to be running. Looking at
>   the 74LS154 decoder which shows accesses within 4k boundaries, I see lots of
>   accesses to 0xxx (probably page0 RAM), no accesses to intervening locations
>   and lots of access to the ROM's, which follow a clearly identifiable 
pattern.
>   Removing any ROM prevents this from happening, with the machine just 
"flailing"
>   until it gets a HLT - so it looks like it may actually be running ROM code.
>
> - I can see accesses to the keyboard scanner.
>
>On the working machine, after accessing all ROM's as BASIC starts (and prints 
it
>welcome message), it settles into a keyboard scan which appears to exist within
>the KERNAL and EDIT rom's - the BASIC ROM's are not accessed unless some 
function
>is activated.
>
>The non-working machine appears to hang during the phase where it is accessing 
all
>ROM's - it continues to do this indefinately.
>
>It looks like the system is hanging during the initialization of BASIC - it 
never
>prints the welcome message (hangs with blank screen).
>
>So far I have been unable to determine what it is doing exactly - any info on 
the
>startup sequence would be MOST appreciated!
>
>I have verified that the ROM's contain exactly the same code as the other 
machine
>(I've seen several PET's fail with bad ROM's, however this does not appear to 
be
>the case here).
>
>Regards,
>Dave
>-- 
>dave04a (at)    Dave Dunfield
>dunfield (dot)  Firmware development services & tools: www.dunfield.com
>com             Vintage computing equipment collector.
>                http://www.parse.com/~ddunfield/museum/index.html
>
>
>