rpw3@fortune.UUCP (01/12/84)
#R:decwrl:-489100:fortune:22000001:000:2385
fortune!rpw3 Jan 11 23:56:00 1984
I don't want to dampen your enthusiasm for home computer peripheral
construction, but unless you are a senior analog electronics engineer,
a high density disk controller (you said 25 Mbyte Tandon) is not to
be attempted lightly. Besides the data rate (625 Kbyes/sec, peak) and
the hassles with formatting, retry algorithms, CRC, ECC, and the rest
(all of which CAN be handled by a bright digital electronics fan), the
real bugaboo is what is called the "data separator" -- that portion
of the controller which decodes the MFM code that comes off the disk
(with LOTS of distortion and jitter on it) and separates the pulses
into clock and data. This function requires a phase-locked loop with
fast acquisition time, low noise, low jitter, stable tracking, high
gain, and (last and worst) very low "static phase offset".
Even if you have somebody's schematics (ignoring legal questions),
there's no guarantee you can tune the PLL properly. It needs AT LEAST
a fast 'scope and a disk that is already properly recorded on, as
well as some good luck with the noise and cross-coupling on that
wire-wrap board you mentioned. (They don't make 'em with 4-layer P.C.
boards for grins.)
Unless you are really sure you want the hassles, may I suggest that you
simply buy one of the single-board SASI-to-ST506 controllers that are
offered at single-unit prices of around $300 (from Xebec, DTC, Shugart,
and many others). Thes cards typically include sector buffer memory,
ECC/auto-retry, commands to format the disk, and other fun stuff.
Interface it to your favorite parallel port, and write a SASI driver
for the parallel port (fairly easy). These boards are the same size as
the 5-1/4" winchester disk drives; in fact, many drives and controllers
have matching holes to bolt them together. The disk drive power supply
(which you will need) can be selected to power the controller as well.
Caution: these things (both) draw bongo watts, compared to your normal
controllers/devices.
This is what we did while our own controller was being developed,
and I know of home users who have done the same. Depending on your
experience and resources, you could have it going in anywhere from
a few days to a few weeks.
Good luck!
Rob Warnock
UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3
DDD: (415)595-8444
USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065