Re: CBM900 hard disk timeout

From: Michal Pleban <>
Date: Thu, 21 Aug 2014 21:25:26 +0200
Message-ID: <>

I did some more tests and I discovered more interesting stuff. I changed 
the drive select jumpers back so that only drive #0 was selected. Then I 
selected drive #1 to see if it makes any difference in the messages I 
see, and it did!

* Hard disk jumpered as drive #1, the machine says:

Read error
Inode read error
Cannot boot

* Hard disk jumpered as drive #0, the machine says:

Read error
Controller/drive initialization failed
Read error
Inode read error
Cannot boot

Note that there are more error messages when the drive is jumpered as 
drive #0. That means to me that there must be some communication between 
the drive and the controller, which is absent when the drive is jumpered 
as #1. So both the controller and the drive must be working enough to 
communicate in some way.

But what is really interesting, and what I have completely overlooked 
the first time, is that when I select drive #0, the first error message 
I receive is actually not a Timeout, but a Read error (on block 
0x00000002, whatever that means).

Which leads me to this possible theory: maybe the drive is OK, and the 
controller is OK, but the drive is simply not formatted? Which explains 
the first read error. Then the computer tries do "something" with the 
drive, which results in timeouts - I have no idea what that could be, I 
will try looking at the BIOS disassembly to find out. But maybe the root 
cause of all problems is the first read error, which indicates that the 
computer cannot read the sector it needs. And the cause of this may be 
simply the drive is not formatted?

So, now the big question is: how to format the drive. From what I read 
about MFM drive controllers, each of them can have a different low-level 
format, not interchangeable with other controllers. The controller here 
is WD1003-CMD, with some custom DMA channel interface. Would it use the 
same format as a PC-compatible ISA WD1003-something controller? There 
seem to be some on Ebay so maybe that is the way to go?


W dniu 2014-08-09 20:15, Michał Pleban napisał(a):
> Hello!
> I am back home, and I tried to fix the broken C900. Three RAM chips
> needed replacement, I exchaged them for new ones and the computer again
> boots normally.
> I disassembled the hard drive and there were no fuses inside, but after
> I rotated the drive by hand a few times and reseated all the connectors
> it started to spin again. After I assaembled it back it stopped
> spinning, so I repeated the process and it now spins properly. It looks
> like some kind of mechanical problem.
> Anyway, the computer is still not able to communicate with the drive.
> All operations result in timeout. I tried these solution:
> * Select all four "drive select" DIP switches.
> * Use another data connector on the controller.
> * Play with the undocumented first DIP switch.
> * Play with the "Set drive type" option in BIOS; apparently the option 
> 2
> is for 20MB disk.
> * Use "Park drive 0" and "Park drive 1" commands from BIOS.
> All I get is only a bunch of timeouts on every oepration (seek, reset
> drive etc.)
> Here are the theories I came up with:
> * The controller is dead. That doesn't seem to be the case, as removing
> the controller gives additional error message "Controller/drive
> initialization failure" which is not present when the controller is
> inserted. So there must be at least some communication between the
> computer and the controller.
> * The controller is partially dead. Maybe exchanging the ASIC chips 
> with
> another WD1003 would help in such case, but all WD1003 PC controllers I
> found on eBay have the chips soldered. I wasn;t able to find a
> WD1003-CMD anywhere for sale, only WD1003-WA? ISA cards.
> * The drive is partially dead. I guess timeout issues would rather
> indicate errors on the PCB and not the drive mechanics. On the other
> hand, the drive is able to perform self-test and flash the LED, so at
> least it must be partially working. Maybe some chips related to the MFM
> interface are dead. Exchaning the PCB with another Miniscribe 3425 
> mught
> help here. If MikeS' offer to do that still stands, I would be glad to
> take it.
> * The computer is able only to communicate with select types of MFM
> disks and the 3425 is not one of them. This seems unlikely, as the
> WD1003 controller is pretty standard ans there should be no reason for
> some special kinds of disks to generate timeouts with it.
> Any other ideas would be welcome :-)
> Regards,
> Michau.
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Received on 2014-08-21 20:00:02

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