[Coco] Broken CC3

[Darren A]

On 9/15/08, Robert Gault wrote:
>
> George,
> You need to take a step back, calm down, and approach this problem with
> some logic.
>
> 1) Remove the disk controller from the Coco and try all the Basic
> commands. Try the RESET button. If everything works normally, then the
> problem is not in the Coco but in the disk controller.

That's not necessarily true. You can't rule out a problem with the
CoCo based on that information alone. The presence of a disk
controller can reveal a problem with the CoCo that otherwise does not
show up. A blown HALT line is a prime example of this (George has
already tested for that one). The NMI is another hardware aspect that
only comes into play when a disk controller is used. If the controller
and drives can be tested with a different CoCo, that would be an easy
way to determine if the CoCo is at fault.


> 2) With the disk controller installed, don't issue any disk commands but
> just test the RESET button. If it works correctly, the problem is not
> the DOS ROM but probably the Western Digital chip of the actual drive.

How would pressing the RESET button reveal bad bytes in the ROM that
are only executed during disk I/O?  Nevertheless, I would consider a
bad ROM to be very unlikely.  Also, I believe George has indicated
that he has replaced the FDC and still sees the same behavior.


> 3) What happens with the disk controller installed but the drive off
> when you issue drive commands? How does the RESET button work in this case?
>
> In short, you need to isolate the problem to as narrow a range of
> hardware or software as possible. Then you will know what needs to be
> tested or replaced. Heck, the problem could even be oxidized contact
> strips on the controller or CART socket.
> Much of this can be done without test equipment. However, for best
> results at least some test equipment will be needed.
>

By all means, clean the controller's contacts if that hasn't been done already.

Based on the information supplied, we know that the FDC is performing
SEEK and RESTORE commands successfully. It also seems to be correctly
handling the VERIFY during SEEK because he hears the usual RESTORE and
RESEEK that occurs when the physical head location does not match
where the CoCo thinks it is.  The problem reportedly happens once the
head has been located over the correct track. The next step in the
process would be the Read Sector command. The following test may
reveal if any data is actually being read:

WIDTH 32
POKE 234,2 : POKE 235, 0
POKE 236,17 : POKE 237, 3
POKE 238,4 : POKE 239, 0
?@256,; :EXEC &HD75F   (&HD66C for DECB 1.0)

This will attempt to read the first sector of an RSDOS directory into
the first 256 bytes of video RAM. Whatever data is being read from the
disk (if any) should appear in the top half of the screen.

Darren