4 Jul
2000
4 Jul
'00
5:11 a.m.
For quite some time I've tried to persuade Eric
Smith, who's quite
knowledgable about programming the SCENIX SX processor, to write some
firmware that would create, functionally, a FDC chip out of one of these
ultra-fast single-chippers.
"Better" is the enemy of "good enough". If I can throw something
together
in an afternoon out of TTL chips from Radio Shack, why go to the effort of
creating what amounts to a custom chip?
Maybe my priorities are too much on the "just do it" side, and not enough
on the "do it Dick's way" side :-).
Interpreting it in light of the modulation, data
format, data rate, etc, is
quite involved, but certainly achievable, though someone has to undertake to
write the code with which to accomplish this.
The code obviously varies depending on the encoding method, but my method -
a completely public circuit design with easy interface to a wide variety
of computers - allows the user to write the decryption code in whatever he/she
might be familiar with on whatever platform he/she wants.
Having the entire diskette
sampled as has been suggested, a track at a time means that one's computer
can, at its own pace, reduce, interpret, reformat, etc, the data prior to
writing it to a duplicate. The reformatting of the data into its original
format offers the advantage of phase coherency between sectors so the PLL on
the controller doesn't have to shift phase between sectors. That will make
the job easy in a case where the PLL has, over time, drifted off its nominal
data rate.
??? There is no need for a hardware PLL if you oversample by a factor of a
few.
There are, IIRC, 10416 byte-times, nominally in an
8" FD track at MFM. at
16x ovrsampling, that's a fair amount of data.
That's absolutely true. But RAM chips are fairly cheap these days, and I took
advantage of that in my design.
While there are a number of
256Kx8 SRAMS out there, they're not likely to be lying in the corner unused.
I used an ISSI 62C1024-7, a 128K*8 70ns 32-pin DIP, and it's good enough
for me. Cost was $12.00. Looking in my Digi-Key catalog, it seems you can
get 512K*8 parts for about $15.00 today, but they're in TSSOP's which aren't
so quickly breadboarded for me. (Though they are the obvious solution
if you transfer my design to a PCB layout.)
I'm sure a fair number of people on this list have access to unused 486
motherboards with socketed cache RAM parts in the 32K*8 to 128K*8 range.
These oughta do fine too.
I recommend, therefore that such a circuit be devised
with DRAMS.
Again, "better" is the enemy of "good enough". I made my circuit
with parts that were easily available to me, and I decided that it
wasn't worth the effort to build a DRAM controller when SRAM is so
cheap. (And SRAM kept the parts count down, too...) You obviously
have different priorities.
--
Tim Shoppa Email: shoppa(a)trailing-edge.com
Trailing Edge Technology WWW: http://www.trailing-edge.com/
7328 Bradley Blvd Voice: 301-767-5917
Bethesda, MD, USA 20817 Fax: 301-767-5927