🚀 go-pugleaf

The response to my question about 4 MB SIMMs was so timely and useful that
I decided to post what I got in case this turns out to be a question of
general interest.  My thanks to Joe and Jeff for their help.

SUMMARY: "You have an old 386/25 motherboard.  Populate with 4 1MB SIMMs
first, then add all the 4 MB SIMMs you want."
-----------------
DETAILS:

From jwhite@oasys.dt.navy.mil Mon Aug 31 15:08:13 1992
Organization: Annapolis Detachment, CARDEROCKDIV, NSWC  Code 2723 
	      (formerly David Taylor Research Center, Annapolis MD)

Well I just upgraded a 386/25 that resides in a Z-248 box, via a motherboard
replacement kit from Zenith, from the stock 4 1Mb SIMMS to 20Mb via
the addition of 4 ZA3800MK compatible SIMMS.  The problem you might
be having is that the upgrade manual does not match the motherboard
that came with it.  Zenith upgraded the motherboard at some point, and
the two different motherboards take different maximum amounts of RAM.
The "Old" motherboard had a max capacity of 20Mb (4) 1Mb Simms, and
(4) 4Mb simms.  The new motherboard has a max capacity of 32Mb (8) 4Mb
Simms.

The "Old" motherboard has some jumpers on the board, and has one socket
for either the Intel or Weitek coprocessor.  The "New" motherboard has
no jumpers on the board, and two coprocessor sockets, one for the Intel
and one for the Weitek.  How you add memory to each is different.

The Old motherboard MUST have the 4 1Mb SIMMs in the first 4 banks (U103
-U106), the 4 Mb SIMMs go in the other sockets.

The New motherboard MUST have a 4 Mb SIMM in bank U103, if there are
any 4Mb SIMMS at all.

To go from 4Mb to 20Mb, my memory banks are filled as follows:

BANK     SIMM
U103     4Mb SIMM
U104     1Mb SIMM
U105     1Mb SIMM
U106     1Mb SIMM
U107     1Mb SIMM
U108     4Mb SIMM
U109     4Mb SIMM
U110     4Mb SIMM

With no problems
Hope this helps, I got the above info from a hardware service rep at
Zenith Data Systems (1-800-843-4128)
Joe White
jwhite@oasys.dt.navy.mil
--------------
From: easton@peter.cs.andrews.edu (Jeff Easton)

  I suspect that you are suffering from the 'memory controller asic' bug.
After the Z386/25 had been shipping for a while, a bug was found that 
prevented 4 Mb SIMM's from working correctly in the first four SIMM
sockets.  The 'hole' between 640K and 1M would show up in the first
meg, the second meg, third, etc.  Not good for extended memory.
The explanation at the time was that we designed the asic before there
even was such a thing as a 4 Mb SIMM and we had no way of testing it
until somebody actually made one.

  Anyway, a fixed ASIC eventually made it into the product around mid
way thru its life.  The same ASIC is used on the Z386/33E btw.

  I was under the impression that a insert was added to the manuals
explaining that the system had to be expanded up to 4 Mb with 1 Mb
SIMM's before adding any 4Mb SIMM's.  Of course later systems could
put the 4Mb SIMM anywhere.

  Does that answer your question?

[start salesman mode]

  Check out my latest creation, the Z420/SX -> Z433/DX, a line of
low cost 486 ISA machines, at your local ZDS sales outlet.  They
come bundled with a SVGA monitor and cost from $1599 to $2399.
They were announced a week or so ago.

[end salesman mode]

  ___  ___    Jeff Easton             easton@andrews.edu
 (__  (__     Zenith Data Systems     easton@zds-oem.zds.com
 ___) ___)    Saint Joseph, Mich.     j.easton@mi04.zds.com
Monte Carlo   Z433/DX    Team Rigel
------------------
-- 
"Moby" Dick O'Connor                         djo7613@u.washington.edu 
Washington Department of Fisheries           *I brake for salmonids*