Re: Commodore 9060/9090 repair

On Tue, Jan 28, 2014 at 8:29 AM, didier derny <didier@aida.org> wrote:
> Apparently a solution almost exist to repair the broken 9060/9090
>
> a board:  SCSI2SD permit to connect a board converting SCSI  to a sdcard
> http://www.codesrc.com
>
> the commodore is not SCSI but SASI  some differences exists:
>
> - no termpower on SASI   (it is possible to build a custom cable with the
> termpower wire removed)
> - on the 9060/9090  the ATN and Parity Lines are not connected
> - the 9060/9090 is using vendor specific commands.

Some of those vendor specific commands are related to low-level
formatting the HDA through the SASI bridge, so they wouldn't apply to
a more modern SCSI target anyway.  Parity is something that could be
enabled or disabled in early SCSI targets.  Perhaps it could be
disabled in the SCSI2SD (especially if you have to load custom
firmware in the SCSI2SD anyway.  See 'sector sizes' below).

> this board has a support for a replacement of SCSI drives in apples.

A very middle-of-the-road platform that doesn't require any exotic
options on the SCSI2SD.

> he could eventually add the support for SASI   commodore 9060/9090 if
> someone is interested.

Start with the fact that the D90x0 firmware uses and expects 256 byte
sectors.  The same physical drive when used with contemporary PC MFM
controllers is formatted to 17 512-byte sectors per track, but in the
D90x0, it's formatted to 32 sectors of 256 bytes.

Start with how the SCSI2SD would handle that and you will have gone a
long way to getting it to work.

The alternative is to completely rewrite the D90x0 firmware to work
with 512 byte sectors but ignore half the data space (not a problem
with modern hardware because the max filesystem with DOS 3.0 is around
16MB, even if you extend the geometry past the real size of a TM603S).

We've devolved to the point where all drives act like big wads of
sectors that are perfect (no bad blocks to work around) and are always
512 bytes per block.  Things were not always so, and a lot of the
options and flexibility and extra complexity of 30 years ago has just
vanished.  It comes roaring back when you want to hook a modern device
to a vintage one.  It's not an unsolvable problem, but there are
specific technical issues to be overcome.  Unlike a 1541, of which
millions were made, the D90x0 drives only number in the thousands, so
it's less appealing to custom-craft hardware when you might only sell
a couple dozen boards.  It also comes down to "how much would you
pay?"  If the answer is "$200", then there are a lot more options than
if the answer was "$50".

Please don't take this as discouragement, but it is meant to point out
a specific technical problem which must be solved by modifying the
thing at one end of the SASI cable or the other for it to work.

-ethan

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