Date   : Fri, 29 Apr 1988 13:53:54 PDT
From   : rzh@freedom.llnl.gov (Roger H. Hanscom (415) 423-0441)
Subject: Xerox 820-1 and Double Density

In response to some Xerox 820-1 questions on the net recently
(young@ee.ecn.purdue.edu and leftlane!leadfoot@sun.com), as has
already been pointed out, the Xerox 820-1 uses a WD 1771 controller
chip that *only* does single-density disk I/O.

I am a Xerox user (sometimes), and until now I relied on SSSD 8"
drives for my Xerox.  But again, as has already been pointed out,
single-density is no fun!  I'm in the process of doing the
Emerald Microware upgrade on one of my Xerox's.  The 5 MHz mod
was no big deal.  I put an 'A' cpu in the board, and went to
faster EPROM's for the monitor (2716-1 is a must).  The monitor
software has to be patched because it uses the CTC to generate
regular interrupts, and they are used in turn to time disk
access, etc.

The Emerald add-on board uses a WD 1793 chip, and that (in
combination with their Xpro EPROM set) lets one write several
popular double density formats.  The 5 1/4" Kaypro seems to be
the most useful.  I've got the board built and stuck on the
Xerox.  It's up and running at 5 MHz in monitor mode.  Now I've
got to calibrate it, and make up some CP/M disks that are
boot-able on the new hardware.  The Emerald upgrade requires
semi-major mods. to the 820 board.  I avoided most of the
trace cuts by installing sockets under about 5 chips so that
I can "cut" runs by removing chip pins from the socket.  That
leaves the option of going back if I don't like this whole thing.

Emerald is selling the DD-mod. bare boards for $15, or 2 for
$25, right now as a close-out.  Pity I can't give more info. about
how it all works(yet).  Somebody told me that it is slow to boot from
8" drives because it tries several different formats to determine
what kind of hardware is out there -- 5" formats first.  Since
I'd like to run from Tandon 5" drives, I hope that won't be a
problem.  BTW, one doesn't have to do the 5MHz upgrade *except*
for 8" DD.  Apparently, the 2.5 MHz cpu just can't keep up with
8" DD transfer rates.

If anyone is interested in this upgrade, I can keep you posted
about how it works (if it works!).

Roger Hanscom
rzh%freedom.llnl.gov@lll-lcc.llnl.gov


End of INFO-CPM Digest
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