This small file is a composite difference description between the
820-I Etch 1 and 820-I Etch 2. An assumption is made that you
have one or the other (hopefully, both) of the 820 schematics.
In addition, I would like to thank Dan Costello for helping me
out with this document by providing some nicely drawn difference
schematics to work from.
Briefly, the first board Xerox released was the Etch 1 (which had
a serial number ending in 629A). About a year later, Etch 2 was
phased into production (which had a serial number ending in
644A). There were a number of minor technical differences
between them; it is these items that this article addresses.
Note that with the sole exception of being able to detect double-
sided 460 drives (special input line), there are NO programming
differences between the two versions. All software which works
on Etch 1 will work on Etch 2. However, the only item to observe
is that there was a slightly different ROM release which
coincided with the Etch 2 (v1.0 ROMs replaced by v2.0 ROMs).
Although all jump vectors perform the same operation, there were
many more added in v2.0. In addition, location of monitor
variables (real-time clock, track, sector, etc.) appear elsewhere
in the later release.
There are four topic areas which are different in the two etch
versions: processor clock, CRT display generator, system port,
and floppy controller circuitry. Each will be addressed.
PA
PROCESSOR CLOCK
---------------
The clock and clock driver are slightly different between the two
versions.
The clock is again different here. Also, the Not Vertical Sync
(NVSync) is processed through a one-shot 74LS123 in Etch 1, but
directly taken to the output gate (U117) in Etch 2. The earlier
version used the 'LS123 to extend the NVSync signal to work with
the Ball monitors which required it for clocking. Most present
monitors could care less; hence, Etch 2 removed it.
ETCH 1: U11 = 74LS04 R4, R5 = 470
C26 = marked as 33pF BUT IS a jumper wire
XTAL = 10.69425MHz
C27 = marked as .01uF BUT IS 100 ohm
Etch 1 and 2 differed in the data seperation circuitry. Etch 2
saw the addition of a one-shot to improve clock/data seperation
as well as extensive re-routing of the RAW DATA signal as part of
the improved circuitry.
