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

From: Dwight K. Elvey <>
Date: Tue Aug 10 19:43:37 2004

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

>From: "Dave Dunfield" <>
>>> Deos still exist? That used to be a good place for
>>> Commodore schematics and technical info,
>>Yup, FUNET's still alive. <> for the CBM stuff.
>>There's also a web-based version at <>.
>Thanks guys - that helps a lot.
>Does anyone have "inside" information on exactly what the PET firmware does
>it starts up?
>I am still working on the same pair of SuperPET 9000's (one works, one does
>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
> Removing any ROM prevents this from happening, with the machine just
> 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
>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
>is activated.
>The non-working machine appears to hang during the phase where it is accessing
>ROM's - it continues to do this indefinately.
>It looks like the system is hanging during the initialization of BASIC - it
>prints the welcome message (hangs with blank screen).
>So far I have been unable to determine what it is doing exactly - any info on
>startup sequence would be MOST appreciated!
>I have verified that the ROM's contain exactly the same code as the other
>(I've seen several PET's fail with bad ROM's, however this does not appear to
>the case here).
>dave04a (at) Dave Dunfield
>dunfield (dot) Firmware development services & tools:
>com Vintage computing equipment collector.
Received on Tue Aug 10 2004 - 19:43:37 BST

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