On Thu, 1 Oct 1998, Allison J Parent wrote:
> < PCs use a disk controller chip based on an NEC 765 design that can not
> < read ANYTHING other than MFM (some can also do FM), with IBM 3740 style
> Excuse me! That is patently incorrect! If you qualified that as *most
> PC controllers cannot read some formats* that would be true.
We appear to differ only on the definition of "PC".
The PC disk controller, AS IMPLEMENTED BY IBM, did NOT provide access to
all of the capabilities of the 765. In particular, FM can NOT be done
with that board unless you make a few modifications, or replace it with
any of the better designed after-market boards.
> The 765 and all of the chips based on the 765 core can read FM encoded
> disks. It's not always possible with PC based designs as some are
> hardware crippled (sloppy data sep circuits) and most of the PC bios code
> is marginally brain dead.
> However the 9266, 37c65 and 92c665 have been seen doing FM.
With the 37C65 and the like, the switch between MFM and FM can be done
entirely in software, not requiring the extra hardware gate needed for
the original 765 as used by IBM. Thus, any of the FDC boards using those
chips are generally a better choice than the IBM ones. And some, such as
the MicroSolutions Compaticard were specifically marketed for their
versatility.
Almost any of the controllers with 1.2M capability can handle 8" MFM (for
obvious reasons). Most of those with 37C65 or the like, or modified to
permit FM switching can also do 8" FM.
> In the mean time my S100 card design and the compupro DISK1A both happly
> read 8" SSSD (FM) disks and even single density 5.25 media.
An excellent choice for the task.
> Try the 765 read diagnostic command. Hex 02 (fm) or 42 for MFM.
Very useful. But I'd STILL rather have a real MFM (and FM) raw track read.
> <They were used in TRS-80s and a LOT of machines OTHER THAN PC.
> The TRS80 used the 1771 that was FM only.
The TRS-80 model I did indeed use the 1771 (FM). Data separation was poorly
done, and there were after market boards to patch it.
The Percom Doubler for the model 1 used either the 1791 or 1793 (I forget
which).
So did the Lobo expansion interface,
and so did the TRS-80 model 3 and 4. But Radio Shack made some WEIRD
design decisions regarding use of Data Address Marks.
> the WD 177x or WD179x still cannot read an apple disk.
Absolutely correct. But they CAN read some MFM formats that do NOT have
IBM style sector headers, unlike the 765 series.
There were several add-on boards for PCs for dealing with Apple ][ disks,
including the Apple Turnover and Matchpoint. They installed between the
existing FDC and the drives, similar to the Option Board. Reliability
was less than ideal, and some drives worked better than others.
--
Fred Cisin cisin_at_xenosoft.com
XenoSoft http://www.xenosoft.com
2210 Sixth St. (510) 644-9366
Berkeley, CA 94710-2219
Received on Fri Oct 02 1998 - 01:29:29 BST