[Coco] Broken CC3

[Darren A]

This sounds a lot like a blown HALT in the CPU. From what you say, the
crash doesn't occur until the head is over the correct track, and the
sector is ready to be read. That's when the HALT line is activated.
What's strange is that you say you've replaced the CPU. It seems
unlikely that both CPU's would have a blown HALT line, but who knows.

You can test it by starting up the CoCo in Disk Basic using a standard
CoCo disk controller. Then enter the folllowing program:

10 POKE &HFF48,3
20 POKE &HFF40,128

When you run this program, the CoCo should freeze for about 7 1/2
seconds before the OK prompt appears.  If it crashes, or if the OK
prompt never appears, or if the OK prompt appears immediately then the
HALT line is the likely culprit.

Darren


On 9/15/08, George Ramsower <georgeramsower at gmail.com> wrote:
>  Well, I'm stumped.
>
>  I found the power supply was clean after all, I think.
>
>  The computer seems to work fine as long as I don't access the disk drive.
>
>  To bring you up to date I've changed:
>
>  The CPU, Gime, RAM Board, disk controller and disk drive and C14. That's
> the one just after the .1 ohm resistor from the voltage regulator
> transistor. The .1 ohm resistor is not in the circuit as, I'm using an
> external power supply.
>
>  AC noise on the 5V line is < 20mv. However, I'm seeing that noise on the
> ground also. So I suppose this is just noise on the power line to the scope.
>
>  Right now, from power up, when I DIR drive 0, it steps over to( I think)
> the directory track and then crashes. After a crash, I reset twice because
> the first reset doen't reset to RSBASIC. The crash gets better and more
> colorful. Upon the second reset, I get the initial startup screen. This
> time, when I do a dir, the disk doesn't find the track, tries again and then
> crashes when it( I think) gets to the dir track.
>  My 3.5 drives, upon power up, moves the head to track 0(I think). So after
> the crash, the drive isn't where the coco is expecting the head to be.
> Therefore, it goes to the wrong track, doesn't find the directory and tries
> again after moving it back to zero, tries again and crashes. I'm used to
> hearing this and I'm pretty sure it is stepping the correct number of tracks
> before it crashes.
>
>  I hope my info is not to vague or stupid sounding.
>
>  Suggestions are welcome!
>
>  Is it possible the ROM in the coco could have failed? Has anyone ever had a
> ROM failure in a CC3?
>
> George
>
>
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