Re: Apple II Minus Question

[mjmahon@aol.com (Michael J. Mahon)]

Bill Garber wrote:

>"Wayne Stewart" <waynes@telus.dotnet> wrote in message
>news:BkX%b.24007$A12.19763@edtnps84...
>: Bill Garber wrote:
>:
>: > Well, I can't really check what the non-autostart
>: > ROM does, I only have autostart machines, and a ROM
>: > card. Would it do that with the ROM card and no CPU?
>:
>: With the ROMcard the Apple II will use the ROMs on the card
>instead
>: of the ROMs on the motherboard. Provided of course that the
>switch
>: is flipped. So if the ROMcard has the nonautostart ROM
>(3410004-00)
>: then it'll go to the monitor instead of into basic. If you want
>to
>: use the F8 on the ROMcard then the solder pads marked F8 should
>be
>: bridged.
>: If you use a ROMcard containing a non autostart F8 then the
>screen
>: will look pretty much the same with or without a CPU on powerup
>:
>: Wayne
>
>So, therefore, if Bryan is starting up and
>no monitor prompt comes up on RESET, then
>the CPU is probably at fault, but since it
>works in another board, then the fault is
>a trace on the 5v line somewhere. This is
>provided the RESET key is working at all.
>I wonder now if he has tested the keyboard
>on another motherboard.

All it means is that the processor is, for some reason, not running
the reset code.

This could be caused by many things, since a normal reset requires
that page zero RAM and the text page RAM (plus its decoders and
buffers) must be working, the F8 ROM (plus its decoders) must be
working, the processor must be working, and reset must be working.

In short, if an Apple II resets properly, then all but the video refresh
and the I/O decoding are pretty well shown to be working.  (And if you
can see the screen, the video refresh is working.  ;-)

Since the processor chip is known to be working, a static check
of its power and ground connections is in order, followed by a 
check of its receipt of the reset signal, followed by checks of
the memory buffers/transceivers and the decode logic.  The failure
of any address or data wire or buffer or a chip select line will
effectively prevent the processor from running the reset code.

A simple check for a fault in the reset line is momentarily grounding
the /RESET pin of the 6502.  This cannot harm the system, and if
it causes it to reset successfully, then the fault is isolated to the
reset signal.

Since the chips have been replaced without alteration of the symptoms,
the sockets, solder joints, and traces of the main board are called
into question, as well as any chips which have not been swapped
(like the F8 ROM, for example).

-michael

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