Re: Disabling SCSI parity checking to dump disk on ACB4000 MFM-SCSI adapter?

[Camiel Vanderhoeven via cctalk]

> On Sep 24, 2020, at 5:23 PM, Grant Taylor via cctalk <cctalk@classiccmp.org> 
> wrote:
> 
> On 9/24/20 1:12 AM, Camiel Vanderhoeven via cctalk wrote:
>> I used an Ancot Ultra2080/Lite SCSI-bus Analyzer. This is a device that 
>> connects to a SCSI bus and has a serial port. Over the serial port, you can 
>> monitor the signals on the SCSI bus, and use it as a SCSI protocol analyzer. 
>> There’s also the possibility to construct and send a SCSI command. Rather 
>> than connect a serial terminal to the serial port, I connected a PC, then 
>> wrote a C program to control the Ancot. I had the Ancot send commands to the 
>> disk to read a sector at a time, and recorded the data sent in response to a 
>> file to create a disk image. Slow as hell (each byte on the disk requires 
>> sending two hex characters and a space over a slow serial line), but it 
>> works. I had to make several passes over the disk, because occasionally the 
>> data received from the disk turned out to be data from a different sector 
>> than the one I was trying to read. By reading the disk multiple times, I 
>> could get rid of these mis-read sectors.
> 
> Very NICE hack Camiel.  I like it!
> 
> I am a little surprised by getting different data for the same sectors. I 
> find that mildly concerning.  Did you do something like read each byte 
> multiple times to find a majority sample?  2/3, 3/5, 4/6, etc?  Do you think 
> the different data was an artifact of the old drive?  Or was it a tickle of a 
> bug elsewhere in the chain?


The drive was what seems to be a pre-production Fujitsu model (with a serial 
number of 37), 17MB in size; I’m pretty sure the disk itself had issues. On the 
first pass of reading the disk, about 8% of all sectors wouldn’t read, 
returning an “Unrecoverable Read Error”; when I did a second pass reading just 
these failed sectors, I found that I could read about 2/3rds of them. I 
repeated this 20 times or so, and then I had almost everything. There were 43 
sectors near the end of the disk that I never managed to read, but these were 
beyond the filesystems on the disk. That gave me a disk that would boot, but I 
has some corrupt files and a corrupt directory. Looking at these, I found that 
the corrupt bits were sector-sized, and were identical copies of sectors 
elsewhere on the disk.

In the end, what I ended up doing was read the entire disk 2 more times (so two 
more times the 20 iterations of reading the missing sectors until I had a 
complete image), and compared the resulting three disk images sector-by-sector. 
For some 99% of the sectors, all three copies were in agreement; where they 
were not, there were always two images in agreement, so I knew which one to 
pick.

I’ve documented some of this here:

https://www.vaxbarn.com/index.php/42-repair/577-convex-c1-xp-power-on 
<https://www.vaxbarn.com/index.php/42-repair/577-convex-c1-xp-power-on>

https://www.vaxbarn.com/index.php/42-repair/579-getting-a-convex-c1-to-pass-all-diagnostics
 
<https://www.vaxbarn.com/index.php/42-repair/579-getting-a-convex-c1-to-pass-all-diagnostics%5C>