HP9133D disk replacement.

[Chuck Guzis]

On 31 Oct 2008 at 21:16, Tony Duell wrote:

> The obvious thing to do is to sample the data stream (either from the
> controller wehn writing/formatting, or from a good drive if you want
> to copy it) at about 10 times the data rate (that's where the 'high
> speed' requirement comes from) and record it in flash memory or
> something. Then replay it back to the cotnroller for reading. It's
> doable, but getting it all working at 50MHz is not going to be
> trivial. 

I suspect that 8x oversampling may do the trick; i.e. 40MHz for plain-
Jane MFM. Assume that this will work--will timings really be faster 
than is practicable?

Let's see if the math works.  Back-of-the-envelope, so you're advised 
to double-check my numbers.  Assume a brute-force, one-bit-per-sample 
implementation.

An ST412 spins at 3600 RPM, giving a rotational period of 16.667 
milliseconds.  The datarate is 5MHz, so a track can hold about 83335 
transitions (less than a 1.44MB floppy track, BTW).  We want to 
oversample by 8, so we need to sample 666680 times per revolution.

If we use a 16-bit-wide RAM to hold sample data, that's about 42K 16-
bit words, so a 16x65K memory will be more than sufficient and 
perhaps allow for a bit faster sampling rate (Tony's 50 MHz) if we 
need it.

So, while the bit shifting (or its equivalent) is performed at a 40 
MHz clock rate, access to the RAM occurs at 40/16 = 2.5 MHz or 400 
nsec--or perhaps 200 nsec while writing (a read is necessary to 
"splice" the bit stream).  Not bad at all--and a 40MHz shift register 
(or its equivalent) is certainly doable using some of the faster TTL-
compatible logic families (e.g. 74AS).  And note that you're only 
sampling a bit stream whose maximum frequency is 5MHz.

Who wants to build one?

Cheers,
Chuck