Date   : Wed, 24 Nov 2010 16:16:06 +1100
From   : station240@... (paul aslin)
Subject: Harddisc fakery using a microcontroller

> From: philb@...
> To: robert@...
> Date: Tue, 23 Nov 2010 11:08:29 +0000
> CC: bbc-micro@...
> Subject: Re: [BBC-Micro] Harddisc fakery using a microcontroller
> 
> On Tue, 2010-11-23 at 10:49 +0000, Rob wrote:
> > On 23 November 2010 10:39, Phil Blundell <philb@...> wrote:
> > > If you wanted to emulate an actual ST506 hard disk, i.e. something that
> > > you could plug directly into an MFM/RLL controller card, then this
> > > becomes a slightly lower level problem and it's possible you would need
> > > something a bit faster.  But I think there's a fair chance that an AVR
> > > would be OK even for that, actually.
> > 
> > ISTR this one being discussed on the cctech list some time back.  It's
> > harder because it's an /analogue/ interface!  Not sure if anything
> > came of that discussion.
> 
> It isn't exactly analogue, it's (iirc) just a digital signal transmitted
> over an RS422-style differential pair.  You'd need to provide a suitable
> line driver and receiver but there isn't anything particularly
> complicated about that.
> 
> The bits on the ST506 data interface correspond directly to flux
> reversals on the disk surface, though, so it is about as low-level a
> digital interface as it's possible to get.  If you wanted to just do a
> direct emulation of a hard disk (i.e. a black box which you can write
> data to and then later retrieve it) then, conceptually, it's quite a
> straightforward problem although I think getting all the timings right
> would be fairly hard in practice.  The data rate is fairly low, though,
> something like 5Mbps, so you wouldn't need a massively powerful
> processor to keep up with it.

Couldn't this be implemented with a shift register/UART chip and some AM26LS
series chips. The flux reversal part created using riseing/falling edge triggered
gates.
That would reduce the amount of work the AVR etc would have to do, and solve
some of the timing issues.

I did some some commercially available ST506 to CF/IDE adapters, 1500 euros.
Eeek!

If we could make one for less than that we could make a sizable sum selling
them. I'd like one myself if its $100 or less.