sysop@sandv.UUCP (System Operator Paul Jerkatis) (06/27/91)
Two Meg CoCo Modification Document
This file describes how to make your 1-meg CoCo into a 2-meg
CoCo. ONLY EXPERIENCED HARDWARE hackers should even consider
attempting this modification.
DISCLAIMER: I, OR ANY OTHER PERSON/COMPANY MENTIONED IN THIS
FILE WILL NOT, AND CANNOT BE HELD RESPONSIBLE FOR ANY DAMAGE
THAT RESULTS TO YOUR EQUIPMENT IF YOU ATTEMPT THE MODIFICATION
DESCRIBED IN THIS FILE. YOU ACCEPT _ALL_ RESPONSIBILITY FOR
YOUR ACTIONS!
The following is a Delphi Forum message from Dr. Marty Goodman
describing some of the background on how the 2-meg modification
came about and some words of caution.
[ *** Begin Delphi Message *** ]
43967 24-JUN 03:10 General Information
2 meg upgrade caution!
From: MARTYGOODMAN To: ALL
Recently my friend Dennis Mc Millian (COCOKIWI) has been
flooding this SIG with news of his apparantly successful 2 meg
upgrade using the Disto 1 meg board. I'd just like to insert
some notes of caution here:
(and to add some background facts)
HISTORY:
This whole business started with Tony Di Stefano sold
me two bare circuit boards (at the "rabbi rate") to enable me
to solder up my own 1 meg Disto upgrade on the cheap. In his
hasty description to me of what to look out for on the boards,
Tony noted that "the second bit" of the two high order GIME MMU
bits WAS provided on the Disto 1 meg CPU board. Tony showed
where it was available. However, he seriously questioned whether
it would be a good idea to TRY 2 megs, citing the massively
increased load on the address line drivers that the additional
32 chips would put. It was his impression that such an upgrade
would likely be flakey, if it worked at all.
When I got back I told Dennis about this second bit,
and Dennis immediately wanted to try out the experiment. I
aided Dennis further by some suggestions on how to deal with
the memory decoding on the F139 chip on the memory board of
the Disto upgrade.
Dennis is an accomplished electronic technician, and
quite a clever and persistent fellow. He on his own added on
possibly CRITICAL wrinkle to the upgrade: He desoldered the
three LS244 and the one LS374 memory chip address and data line
buffers, and replaced them with F series chips. He did this
not so much because the F series chips were faster (tho that is
nice) but MAINLY because the F series chips provide 2.5 times
the amount of FAN OUT DRIVE on their outputs!!! Dennis then did
the tedious job of piggybacking THIRTY TWO DRAMs with pin 15
bent out, daisey chaining two sets of 16 pin 15's, and wiring
them appropriately to the F139 bank selector.
I was frankly suprised to hear Dennis was successful,
for I had tended before to agree with Tony that the 2 meg
upgrade could not be done and work reliably. It certainly is a
credit to Dennis' skills and abilities that it has worked as
well for him as it has.
HOWEVER:
Note that even a 1 meg upgrade can be VERY VERY touchy
about what DRAMs it uses. Several very reliable sources (Dave
Myers of CoCo PRO! who sells the bloody things and Bruce Isted,
one of THE most knowledgeable CoCo hardware types) have pointed
out to me that some DRAM chips (especially those rated for high
speed operation at 100 and 80 ns) which will work fine in a
straight 512K upgrade board will cause a Disto 1 meg upgrade to
FAIL. This implies that TIMING can be VERY VERY VERY critical
on Disto 1 meg upgrades... and is likely even MORE critical when
you push the thing to 2 megs.
I suspect Dennis was VERY lucky in his choice of DRAM
chips, for he could well have instead gotten ones whose timing
ideosyncrasies would have messed things up.
SUGGESTIONS:
DO NOT ATTEMPT the 2 meg modification unless you are a
VERY VERY experienced electronic hacker! The mere act of
piggybacking 32 chips requies significant soldeing skills! As
does the even more tedious DAISEY CHAINING of the CAS lines
(pin 15) of the DRAMs.
Use only 120 ns rated DRAMs. Use preferably NEC BRAND
Drams. DO NOT try using 100 or 80 ns rated DRAMs.
Be SURE to desolder from the CoCo mother board the 3
LS244 and the one LS374 memory chip buffers / latches and
replace them with socketted F244's and an F374 chip. If you are
in the SLIGHTEST bit hesitant about your ability to desolder a
chip from the CoCo 3 mother board and not damage the board, DO
NOT ATTEMPT this upgrade!
IF you have acess to them, you may want to try CMOS
41256 chips, to further reduce the loading on the address and
data line buffers.
Don't be suprised if the thing does not work... I
suspect that despite Dennis' success, the upgrade is VERY tricky,
and subject to a lot of vagaries in whether it succeeds or not.
Again, it is a TRIBUTE to Dennis' skill (and perhaps to
some extent to his LUCK as well!) that he got the bloody thing
running. But there is NO WAY that this modification will become
very common among Coco 3 users, I predict. It is just too
complicated and too tedious and, I suspect, to hard to get
working in most cases, despite the success Dennis had.
The above are my immediate reactions to all this chit
chat about the 2 meg upgrade.
Oh... one MORE thing... Tony Di Stefano urged me in
strong terms NOT to spread word about this upgrade, fearing
that he or CRC might be deluged with requests for help in this
UNAUTHORIZED and UNSUPPORTED modification. If you have
problems with your upgrade, you WILL NOT be able to get ANY
help from Tony, CRC, or your dealer!!! This modification is NOT
supported by ANY of those!
---marty
[ *** End of Delphi Message *** ]
On to the actual description of the work involved. The first
section describes the actual hardware work that is required.
The second describes the software patches you will need to make
under OS-9 Level 2.
PLEASE read through the rest of this document THOROUGHLY before deciding if
you want to attempt this "operation".
Requirements:
1) A already FULLY functional and RELIABLE 1-Meg CoCo with the
Disto 1-Meg Upgrade.
2) Thirty-two (32) 41246 DRAM chips rated at 120ns (preferably
low power draw chips).
3) Three (3) 74F244 chips.
4) One (1) 74F374 chip.
5) Four (4) 20 pin sockets.
7) 26 gauge hook-up wire (about 30 inches).
8) Lots of guts!!! 8-)
Tools:
1) Soldering iron (15 watt with fine point works good).
2) Desoldering tool. ("solder wick" just doesn't cut it in my
book)
3) Small diagonal wire cutters.
HARDWARE:
0) Turn off all power to CoCo, and unplug! Open CoCo (voiding
your warrenty 8-) Make sure you are properly grounded to
avoind "zapping" any chips with static!
1) Remove RAM boards and CPU satilite board from CoCo.
2) Remove all RAM chips from memory boards. Place them into
non-static foam.
3) Cut small trace between pin #2 and pin #3 on the 74F139
chip on the Disto 1-Meg RAM board. It is on bottem side of
PCB.
4) Cut trace that runs from pin #7 of 74F139 chip on Disto 1-Meg
Board to the small "through hole" near jumper wire "J5".
It is on the tope side of the PCB.
5) Solder a small jumper wire from pin #6 of 74F139 to the
"through-hole" near jumper J5.
6) Solder a 4-5 inch jumper wire to pin #2 of 74F139 on Disto
1-Meg board.
7) Bend out pin #15 on all the new RAM chips. Bend them so they
are at a 90 deg angle from their "normal" pisition. Be
carefull not to break them off!
8) Position a new RAM chip over each of the old chips, making
sure that all the pins one the top chip line up with the
bottem chip. Also, make sure pin #1 on both chips corespond
with each other.
9) Carefully solder each of the 15 pins on each of the new chips
to its coresponding old chip. Be carefull not to make any
solder bridges and not to get the chips too hot! I recomend
doing 1 pin per chip at a time and moving to the next so they
can cool before doing the next pin.
10)After all the RAM chips are piggybacked, reinsert them into the
two RAM boards, makeing sure you get them aligned properly.
11)Cut two peices of 26 gauge wire about 15 inches long to be
be used to "daisy chain" all the pin #15s together on each
of the RAM boards. (ie, seperate wires for each RAM board)
You'll want the "free" end of the wire to end up close to the
front-right corner of each RAM board. I started in the
back-right corner chip, and "zig-zaged" from side to side to
end up at the single RAM chip in the front-left, leaving about
3-4 inches of extra wire.
12)Connect the wire from the pin #15's on Disto 1-Meg board to
pin #5 of 74F139.
13)Connect the wire from the pin #15's on the other RAM board
to pin #7 of 74F139.
14)Connect jumper wire J5 to pin #4 of the small header on the
Disto CPU Satilite board.
15)Connect the wire from pin #2 on the 74F139 to pin #2 of the
small header on the Disto CPU Satilite board.
You are now done working on the RAM boards, set them aside for
a bit.
16)You now must remove the three 74LS244 chips and the one
74LS374 chip on the CoCo mother board. They are located
right in the middle of the board. Chips IC10, IC11, IC12
and IC13. You may "destructively" remove these chips by
cutting them out with a small wire cutters (I like to use
a small fingernail cutter myself) and then remove the pin
stubs and clean out the holes. Be carefull not to overheat
the PCB and to not distroy any of the traces/holes.
17)Solder in a 20-pin socket in the places where you removed
the above chips.
18)Now, insert the new "F" series chips into their proper
sockets. The 74F374 goes in IC13 socket in case you forgot
to note that before 8-) Make sure the chips are alinged
properly.
19)Plug the two RAM boards together and reinsert into your CoCo.
Plug the CPU Satilite board back into its header.
20)You will also want to add a FAN to your CoCo if you have
already added one!
That completes the hardware work. You now want to power up and
check to see if your CoCo still works. If you don't see the
normal Tandy copyright message, then shut off power immedieatly
and go back and check all your work.
If every thing looks fine so far, run the Disto 1-Meg Memtest
program. It will tell you 2-meg RAM Good if every thing went well.
Otherwise, you will have to locate possibly a bad chip or bad
connection somewhere.
SOFTWARE:
The following changes will allow OS9 Level 2 to to be able to
fully use your newly installed 2-meg of RAM.
You will need some sort of disk editor (dEd) or you can probably
use modpatch.
Patch location $09 if the "Init" module to $1F. Remember to
re-verify the CRC.
Patch the "mega" command from Disto at locations $1F,$20 from
$00,$80 to $01,$00. Remember to reverify the CRC
... OR you can ...
Obtain the "krnl.ar" file. This file contains a Ipatch file that
patches os9p1 so it will automatically boot up in 2-meg mode and
also so OS-9 will support "OSK-like filenames" (ie, start with
numbers and some other "special" characters) You will also need
the KUtil program wich allow you to extract/save the kernal
from your boot disk. Both these files are available on Delphi,
the SandV BBS and will most likely make it many other BBSes soon.
Final note: My CoCo is in a "Baby AT" style case. It has the
normal fan that is on the powersupply, and also a SECOND fan
mounted right next to the RAM. Runs pretty cool that way. Also,
all the memory is powered by the AT powersupply.
Welp... That about wraps it up. It took me about 2-3 hours to
get all the work done, but it worked PERFECTLY the first time
I started up the machine and has been working without ANY
problems since June 21, 1991 (24 hours a day). Good luck!
Paul Jerkatis - Call the SandV BBS - 708-352-0948 (OS9/CoCo 3 support)
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