HP9133D disk replacement.
cclist at sydex.com
Fri Oct 31 17:23:50 CDT 2008
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
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
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
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?
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