ST-506 hard drive emulation

Jules Richardson julesrichardsonuk at
Tue Jul 26 09:42:12 CDT 2005

On Mon, 2005-07-25 at 22:03 -0400, Kelly Leavitt wrote:
> Anyone know about anything similar to this?
> Mostly interested in something I could build. Probably couldn't even afford
> the shipping on a unit like this.

I'm not sure how complex such a box would be. I *think* an ST506-
interface drive looks pretty much like a big floppy, just with more
heads. Far as I know, it doesn't do anything intelligent on the drive
itself, just responds to head select / step commands from the controller
and reads or writes data.

The bit I don't know is the nature of the data signal at the interface -
it uses +/- signal lines for both read and write data. I'm not sure if
that's an analogue signal or a digital one.

I could imagine a device containing a pile of non-volatile memory in a
row/column arrangement. There'd be a free-running counter to address the
memory row and simulate the rotation of the drive (this would also drive
the index line when at address 0). Then there'd be a second counter
linked to the memory column and controlled by the step line on the
interface, in order to simulate a drive track; this would also drive the
track 0 line when at address 0.  

OK, that's only simulating a single head; but it just scales up with
more memory planes to simulate more platter surfaces.

I *think* that would be it; the controller writes the necessary bit
patterns when doing a LLF of the drive to mark out sector boundaries, so
I don't think there's a need to handle anything complex in the virtual
drive. I believe an ST506 drive's purely a data store/replay device and
it knows nothing of the actual data stored on it - most of the circuirty
on board is presumably just motor control and head amplifiers / filters.

I'd have no idea how to properly design something on a PCB that'd work
though at the sort of necessary speeds without noise problems! But I
think *in theory* it's probably not a difficult thing to design, and
just needs a good electronic engineer to do the PCB work. Arguments
welcome though :-)

Yes we've been here before on this list - I think that was working out
how to replace a ST506 drive with IDE though (which of course does know
about sectors and stuff) - hopefully driving a pile of NVRAM as a linear
sequence of bits is a lot easier.

>  Just dreading the day I finally run out of ST-506 hard drives.

I'm got a few stockpiled at the museum, both for ourselves and for if
anyone else needs one in an emergency - people locally seem to dispose
of them quite often, so I just save what I can. Of course the majority
of them are probably in a non-working condition anyway!



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