How To Upgrade To 512k Without A Commercial Memory Board. Written by Marty Goodman.

Well, a long time ago, just to prove a point, I upgraded a CoCo3 to 512K without using a commercial upgrade board. The way I did it was to make four stacks, four high, of 41256 DRAM chips. These I mounted on 18 pin headers, carefully routing shorted data in and out lines to each of the four I/O lines of the 4464 chips, and running the added address line (joined on all 16 of the DRAMs) to the spot on the CoCo 3 board where this line is available from the GIME chip.

The schematic is trivial... instantly obvious at a glance at the CoCo 3 schematic. The place to find the added line from the GIME chip is a little trickier, but I can describe it. 

What's a MAJOR BITCH... a TERRIBLE TEDIOUS TIME-CONSUMING bit of Work... is making four of the special four high stacks.

(naturally I put pass capacitors on the stacks, too).

If you REALLY want to embark on such a nutty project, feel free to write me for more details.

And/or I'll try to hunt up my article on it.

---marty

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 considerable skill and experience with fine soldering, and a lot of patience. Materials: QUANTITY DESCRIPTION 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). 1 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 solder 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 41256 41256 4464 4464 4464 1 *OE (not used by CoCo 3, tied high, Ignore) Extra Address 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 Address Line adr 10 11 adr Address Line adr 11 12 adr Address Line adr 12 13 adr Address Line adr 13 14 adr Address Line Data In (typo - actually it is Data Out) Din (typo - actually it is Dout) 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 to 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 motherboard 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 stacking chips four high. However, the refresh cycle for 44256 chips is 512 cycles, and the durn GIME chip 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 [email protected] 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.