Sunday, August 23, 2020

Four Chip 512k Upgrade for the Color Computer 3

NOTE: These memory chips require a 512 refresh cycle, while the Coco only provides a 256 cycle refresh for DRAM refresh.  This makes the board only work as a 256k board.  There are many other upgrade options available now and this solution is not optimal.  I am leaving this project up for reference purposes.

The Problem

Back in 2003, I was trying to figure out why one of my 16 chip memory board had stopped working for some reason.  I actually solved my problem by wiring up a couple of 256k SIM modules using a schematic I found on the internet.  That got me thinking about trying to create a whole new solution for memory upgrades for the CoCo 3.

Over the years I have collected many old ISA cards, always thinking about ways to wire them up for use on the CoCo.  This time around I found it easier just to take the memory chips of some of the 512k -1MB boards and using them for a 512 memory upgrade project.

DRAM DIP pinout

Part number GM71C4256A-80

The Solution

I needed a reason to design my own board and have a couple of them made.  Making my own boards sounded like a great challenge as I have never tried anything like this before.  Checking my latest edition of Circuit Cellar I found several programs available for use in creating circuit boards.  A program from Autodesk EAGLE seemed like a good fit for this small project.  It includes a free edition that even handles auto-routing.  The board size in the free edition is rather limited and there is also a 2 side limit.

Board layout

Board schematic

The Result

After having two boards manufactured I was ready to test out the final product.  I pulled the memory chips off of the ISA VGA cards and added them to my new four chip memory board.  I plugged in the memory board into my CoCo 3 and crossed my fingers and, success on the first try!  After further testing, I found the issue where this board is only working as a 256k board.  Due to the board not working as a 512k board I could no longer use it in my system.

Front and rear board photos

Coco 3 with the memory board installed

Note: I have to trim the board a little by hand so it would fit

Link to the EAGLE project to make this board

Autodesk EAGLE design project

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