Re: D9060 Help Needed

On 12/6/09, hwarin@neuf.fr <hwarin@neuf.fr> wrote:
> Hi ...
> Just reading this - Did anyone tried to directly use a SCSI 1 HDD in place
> of the SASI->MFM card ?

I have not tried it.  Because the firmware sends low-level format
commands to the SASI<->MFM card, I have always wanted to disassemble
the ROMs far enough to understand the sorts of SASI request packets
that the ROM builds and issues, but never got far enough along on the
disassembly to reach that level of understanding.

> PS : A good source for 4 plates MFM HDD are Atari megafiles .... that costs
> nearly nothing on eBay.

It is not enough to match merely the number of platters.  Back in the
days of 10MB and 20MB disks, you had to match the number of platters,
the number of cylinders (the disc can have more cylinders than the
controller expects, of course), and two now-mysterious-and-forgotten
parameters - reduced write current and write precompensation.  Without
going into the details of RWC and Precomp (as they were often
described at the time), they are numbers that relate to what cylinder
the drive needs the controller to change how it writes to the platters
(since that is all analog and entirely controlled by the controller,
unlike IDE and embedded SCSI drives).  Even though the Seagate ST506
and the Tandon TM602S have the same number of heads and about
(exactly?) the same number of cylinders, you can *not* replace one for
the other - their RWC and Precomp numbers are different and not
mutually compatible.  There are also different, shall we call it,
"versions" of the ST506 and ST412 ("MFM") interface implementation -
not vastly different, but different enough that it matters when trying
to put a newer drive on an older controller and vice versa.

Now... *if* the Atari Megafile you mentioned happens to have a drive
that will sufficiently imitate the TM602S (it will if, for example, it
has a Seagate ST225 inside), then that will work.  Without knowing the
drive model in the Megafile, it's impossible to determine in advance
if is a good substitute or not.  Other drives will probably work, but
I'd urge you to go find the specific numbers from a TM602S and check
them against what you'd like to try as a replacement drive (the TM603S
is identical except it has 6 heads instead of 4).  Of course, any of
these parameters could be changed with enough understanding about the
internals of the D90x0 drives, but I do not believe that there is that
depth of knowledge anywhere at present.  With enough research and
experimentation, many more adaptations and modern substitutions can be
tried.

I don't want to discourage you from trying new things, but I do want
to caution you that drives from those days are much more complicated
to set up to work reliably than drives are now.

-ethan

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