[Coco] CoCo 1 'E' board memory upgrade

[RETRO Innovations]

On 4/16/2024 7:53 AM, Brian Blake via Coco wrote:
> The instructions from Rainbow: https://cmtekwerx.com/rainbow-upgrade.jpg
>
>
> I'm  pretty sure the upgrade is the Radio Shack 32k upgrade, with the
> following mods done (see coco1-pcb image llnked above):
>
> U29 pin 1 to U11 Pin 5, and the connections between U4 & U8; U29 Pin 2 to
> pin 8; and U29 pin 3 to U8 pin 21.
>
> This is where things get fuzzy - fresh 4164's and the 64k addressing
> modification per the May '84 Rainbow, and noticed U29 pin 1 is connected to
> U11 pin 5.
> The article states U29 pin 4 should be connected to U11 pin 5. I'm guessing
> this is in addition to the wire that's already running from U29 pin 1? Or
> should that be removed?

I've never done this mod, so feel free to ignore, but I thought I'd map 
it out and see if I can help answer the question

U29 is 74ls02, and pin 4 is an output (it's a quad 2 input NOR gate, so 
NOT(A or B))

U29:pin 1 is also an output

U11 is 74LS138 and pin 1 is an input (it's the A of the 3 inputs [a,b,c] 
that make up the 3 bit number, so to speak)

pin 5 is Also input, it's a enable input !G2B (it's an active low input, 
meaning that the '138 outputs will be valid only when this pin is at ground)

So, U29:1 is connected to U11:5, output to input.  Output to input is good.

Now, the instructions are noting connecting U29:4 (output) to U11:5 
(input).  Output to input is good.

But, then that means two outputs (U29:4 and U29:1) are connected to 1 
input (U11:5).  Two outputs to 1 input is bad.

So, no, you should not connect both U29:4 and U29:1 to U11:5.

So, I checked the actual technical bulletin for the CoCo 'E' upgrades, 
and what I saw was the 64kB upgrade, which appears to make the same mods 
shown on your PCB:

https://colorcomputerarchive.com/repo/Documents/Manuals/Hardware/Color%20Computer%20Technical%20Bulletins%20(Tandy).pdf 
It's bulletin 15, which starts on page 28, and the Rev E instructions 
start on page 33.

Can you verify the mods shown on that TB are applied as per the 
instructions?  If so, are you sure you don't already have 64kB in place?


>
> A little education and clarity would be greatly appreciated!
>
>
>
> Best regards,
>
> Brian
>

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