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512K upgrade WITHOUT a board

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Marty Goodman MD KC6YKC

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May 4, 1997, 3:00:00 AM5/4/97
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Upgrading a CoCo 3 to 512K without a 512K memory board


As of this writing (February, 1993), the CoCo 3 has long been
orphaned, and support for it is now nearly non-existant. As part of
this situation, it has become impossible to buy 512K memory upgrade
boards for the CoCo 3. Well, if you have a 128K CoCo 3 and want to
upgrade it to 512K, it still is possible to do this without a 512K
memory board. The good news about this approach is that all that is
needed are 16 41256 DRAM chips and four .1 mfd to .47 mfd miniature
capacitors, and four 18 pin DIP headers. The BAD news is that this
approach is very tedious, and requires considerably skill and experience
with find soldering, and a lot of patience.

Materials:

16 41256 (256K x 1) DRAM chips These should preferably be 150 or
120 ns acess speed, although faster chips (100 ns or faster) will likely
work just fine, too.

4 18 pin DIP headers, of a size that will plug into the four 18 pin
memory sockets on the CoCo 3. A Machine Pin style 18 pin socket may
work fine for this purpose. Machine Pin style 20 pin sockets can, of
course, be hack-sawed into 18 pin sockets.

4 .1 to .47 mfd (I recommend .33 or .47 mfd) capacitors, physically
very small in size (the size of a match-head is best).

One short piece of wire.


Approach:

What you will do is make four QUADRUPLY STACKED sets of memory
chips. That is, you will be making four, four-chip-high piggybacks on
top of the four 18 pin headers. Most of the pins will go straight thru
and be soldered to the pins of the chip or header below them. However,
a few pins will be re- routed.

Bend straight out (horizontally) pins 1,2, and 14 of a 41256 DRAM
chip. Bend pin 16 of that DRAM chip "forward" so it reaches just a bit
outward in the direction of the length of the 41256 chip.

Line up the chip over the 18 pin header, so that pins 1 and 16 of the
DRAM chip lie over pins 2 and 17 of the header. See to it that pin 16
of the 41256 is bent in such a way that it can make contact with pin 18
of the header.

Solder pins 3,4,5,6,7, and 9 of the DRAM chip to pins 4,5,6,7,8, and 9
of the header. These pins will be going STRAIGHT DOWN, unbent.

Solder pins 9,10,11,12,13, and pin 15 of the DRAM chip to pins
10,11,12,13,14, and pin 16 of the 18 pin header. These pins will be
going straight down.

Solder pin 16 of the DRAM chip to pin 18 of the header. This requires
that pin 16 be bent a bit "forward", as noted above. You might want to
put a "dog leg" type bend in it to keep it from contacting pin 17 of the
header.

Prepare three more 41256 chips in a similar fashion, EXCEPT don't bend
forward or out pin 1 or 16. Leave pin 16 alone. Now stack those three
chips on top of the chip and header you prepared, one on top of the
other. Attach pin 1 of the chips to pin 1 of the lowest chip, and
attach pin 16 of the chips to pin 16 of the chip below it. Note that
all the pin 16's are routed to pin 18 of the header, and that all the
pin 1's are routed to the pin 1 of the lowest chip, which in turn is NOT
hooked to the header, but instead is bent out.

Now, using wire wrap or other small gauge wire, join on each chip pins
2 and 14. Run wires from the joined pins 2 and 14 of each individual
chip to pins 2,3,15, and 17 of the 18 pin header. It does not matter
what pair of joined pins goes to what pin of the header... just be sure
that each of those four header pins is connected to a pair of joined
pins 2 and 14 from one of the four DRAMs in the stack.

Now solder on top of the stack the little capacitor, hooking one side
of the capacitor to pin 8 of the DRAM chip and the other side to pin 16
of the DRAM chip at the top of the stack.


Make up a total of four such stacks as above.

Note that you will likely find it easiest to clip off the narrow part
of the horizontally bent out pins, leaving just the fat part of them to
solder to. This may in fact be necessary to keep the pins from hitting
each other when the stacks are installed.

Now plug each of the four stacks into the four 18 pin DRAM sockets in
the CoCo 3.

Solder short wire jumpers between the joined pin 1's of each of the
four stacks.

Solder a wire from the joined pin 1's of all 16 DRAMs to a plate thru
solde r pad just in FRONT of pin 11 of CN4 on the CoCo 3 mother board.
This is the missing address line used in the 512K upgrade.


The plate thru pad in question is located very near CN4, between CN4
and CN 2 CN 4 is a 12 pin socket used for part of the 512K memory board,
and CN 2 is th e socket for the keyboard of the CoCo 3. Be SURE to
remove the keyboard of the CoCo 3 before doing this, else you risk
destroying the ribbon cable of the keyboard when you try to solder to
the CoCo 3 mother board.


Your CoCo will look a bit weird, as if there are four "high rise
towers" in the memory chip sockets. However, if you carefully follow
these instructions, you SHOULD get out of this a 512K CoCo 3.

Reference Information:

Pin out of 41256 chip compared to pin out of 4464 chip socket in
CoCo 3:


41256 4464
------------
1 *OE (not used by CoCo 3 tied high Ignore
extra adr A8 1 2 I/Oa bi directional I/O line
Data In Din 2 3 I/Ob bi directional I/O line
*WE 3---4 *WE write enable
*RAS 4---5 *RAS Row Address Strobe
adr 5---6 adr address line
adr 6---7 adr address line
adr 7---8 adr address line
Vcc 8---9 Vcc +5 volts

adr 9--10 adr
adr 10--11 adr
adr 11--12 adr
adr 12--13 adr
adr 13--14 adr
data in Din 14 15 I/Oc bidirectional I/O line
*CAS 15--16 *CAS Column Adress Strobe
ground gnd 16 17 I/Od bidirectional I/O line
\-18 gnd ground


What is going on here is that the Din and Dout of each 41256 is
connected t o each other, and those two joined pins are then connected
to one of the bidirectional I/O lines of the header. The one extra
address line (pin 1 of a ll 16 chips) is hooked to the mother board so
that it can connect to that address line of the GIME chip's memory
management section. Finally, pin 1 of the 4464 chips is not actually
used, for it is an enable line that is grounded all the time.
Therefore, we can ignore the fact that the 41256 lacks such a line.


It is a great pity we could NOT use four 44256 chips to upgrade the
CoCo 3 to 512K memory. This would have eliminated the hassel of staking
chips four high . However, the refresh cycle for 44256 chips is 512
cycles, and the durn GIME ch ip in the CoCo supports only 256 cycle
refresh. One would need special extra circuitry (like that on the Disto
2 meg upgrade) to use such chips.

I have successfully upgraded one CoCo 3 to 512K in this fashion.
It's AWFULLY tedious.


---marty MARTYGOODMAN on Delphi
MARTYG...@DELPHI.COM on Internet

Please share this article freely with all.


P.S. minor typo: pin 14 of the 41256 is a Dout, not a "Din" line.
All Din lines of a given 41256 chip are shorted to the Dout line
of that chip in the CoCo 3 memory circuit.

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