Looking for AT&T 3B1/UNIXPC/PC7300 software

Dave Dunfield dave06a at dunfield.com
Sat Oct 11 18:18:16 CDT 2008

> > It appears ImageDisk doesn't work with 8 sector ones.
> >
> > The 3.51 utilities disks were copied OK with ImageDisk, except for the
> > VDI disk, which apparently is 8 sector.
> >
> Hrm... any ideas why it won't image 8 sector ?  I'd think that
> smaller # of sectors would be easier....

I doub't it's because if the number of sectors. ImageDisk has no trouble
with 8 sectors - format an 8-sector disk on an old DOS system and try it.

Exactly what do you get when you try and use ImageDisk on these disks..?
Do you get some sectors, but not others? Does it fail to see any sectors
at all?

More likely it's because the 8-sector format has other charactistics
which make it incompatible with the PC controller.

For example, if you are missing one sector of every track, then a likely
candidate is that it may squeeze things a little too tightly, moving data
into the dead area following the index pulse of the PC controller. If it's
only slightly too close, then slowing the drive may help - I've used this
technique very successfully to both save and recreate Cromemco disks,
which most of my systems can't read on a full speed drive.

If you don't see any sectors, or get errors trying to read all sectors,
then it may be another factor - perhaps the 8-sector format uses non-
standard marks, or a different encoding altogether.

Does anyone know the details of the AT&T 3B1 diskette format?  What FDC
is used in this machine?


dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:

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