From: William Levak (wlevak_at_cyberspace.org)
Date: 2003-11-07 07:00:23
On Fri, 7 Nov 2003, MAY Peter wrote:
> Last night I swapped the 6520s over, it made no difference, likewise
> swapping the video ram 6550s with two other 6550s made no difference.
> Keyboard is still dead as well.
> Interested to note that when I unplugged the 6540 Character ROM, it gave a
> white screen and the two flashing cursors were still prominent, but in
> negative/inverted.
The white screen is normal for no character ROM. The two cursors is not.
The output below suggests a bad address decoder for the video RAM. This
would not affect the keyboard.
> > *** COMMODORE BASIC ***YTES FREE
> > SIC ***YTES FREE
> > []cursor
> > READY.
> > []cursor READY
Another possibility is bad low RAM. Try switching the RAM at I1 and Ji
with other RAM.
The output above is enough to see that you have the original ROM's. There
is a built in diagnostic routine. Later versions of ROM had the machine
language monitor instead. The diagnostic uses two connectors wired with
jumpers, one on the user port, the other on the keyboard connector. They
are easy to make. I will try to work out the wiring over the weekend.
The bad news is that the output goes to the screen.
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