Re: CBM900 hard disk timeout

From: Michał Pleban <>
Date: Wed, 08 Oct 2014 14:10:14 +0200
Message-ID: <>

The controller issues are now resolved! Here's what happened:

I exchanged the WD ASIC chips on the controller board, which did not
make any difference. Then I exchanged the controller CPU (thanks Ruud
for programming it!) and it also did not help. So I started examining
the bard carefully and trying to find the crucial signals: where they
are routed and what happens to them. That's how I found that the /ENABLE
line for the bus buffer was being shorted to the ground. After further
examination I found that a broken 74LS14 driver was responsible for
that. I replaced it with a 74LS04.

The controller operation improved significantly. The 0x74 error was
gone. The controller stopped hanging on the first disk access when the
disk was attached. It still, however, hung on the second initialization
command (after first disk read fails).

This led me to thinking that the controller hardware may be OK, and
maybe it's the firmware that is acting badly. Specifically, maybe the
controller does not accept the initialization command sent to it for the
second time.

The BIOS disassembly shows that the controller is being re-initialized
after the first disk read fails. This would mean that the BIOS will make
controller stop working after any read error, be it "disk not present"
or "disk not formatted". So that would imply that most probably my
MiniScribe disk is not formatted or otherwise faulty, so the controller
cannot read from it.

Therefore I bought another MFM disk on an auction and attached it to the
controller. I didn't know anything about it, but it turned out that this
disk is working, and formatted in a way that the controller can read it!
After attaching it, all the errors are gone. The BIOS now displays the
message "Bad file type" which makes perfect sense since there is
obviously no Coherent filesystem on the disk.

Moreover, all the disk commands from the BIOS menu (such as "park
heads") are now working.

So I have now a working controller and a working disk (too bad I cannot
put it in the C900 case since it is full-height, not half-height as the
MiniScribe). The only thing missing now are the Coherent installation
disks :-)


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Received on 2014-10-08 13:00:03

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