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

[Camiel Vanderhoeven]

> On Sep 24, 2020, at 6:21 PM, Camiel Vanderhoeven via cctalk <cctalk at classiccmp.org> wrote:
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>> On Sep 24, 2020, at 5:23 PM, Grant Taylor via cctalk <cctalk at classiccmp.org> wrote:
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>> 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.
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>> Very NICE hack Camiel.  I like it!
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>> 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?
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> 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.
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> 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.
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> I’ve documented some of this here:
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> 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/577-convex-c1-xp-power-on <https://www.vaxbarn.com/index.php/42-repair/577-convex-c1-xp-power-on>>
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> 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> <https://www.vaxbarn.com/index.php/42-repair/579-getting-a-convex-c1-to-pass-all-diagnostics%5C <https://www.vaxbarn.com/index.php/42-repair/579-getting-a-convex-c1-to-pass-all-diagnostics%5C>>

Oh, I was lucky when I spotted it; the first corrupt file I encountered was part of the microcode to be loaded onto the vector processor. Those files have checksums, and the microcode loader complained. I wrote a small utility to extract the files from the disk image (it uses a variant of the UFS file system), and had a look at the corrupt file; one of the blocks in the file looked like part of a directory, which was easy to spot, and that led me to the conclusion that it was valid data, but read from the wrong place.

Camiel