[Coco] CoCo 1 'E' board memory upgrade

[Brian Blake]

See Below:

On Tue, Apr 16, 2024 at 11:23 AM RETRO Innovations via Coco <
coco at maltedmedia.com> wrote:

>
>
> 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.
>

I did not believe so either, I just needed a sanity check.



>
> 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?
>

I'm going to double check those against the board to be sure - thanks for
the TSB, I had never seen that. Guess I need to peruse the archive a little
closer.

All I have to go on is the fact that the machine will not load or play the
game Sailor Man by Tom Mix, which requires 64k. It will load and play
Donkey King/The King, though the intro music and 'Mario's' footsteps seems
to play faster than norma, so there may be an underlying issue somewhere.

First thing I guess I'll do is swap out the older RAM chips and put in some
new ones, and make sure the jumper for Hi/Low is set properly (I believe
Low is what the doc I was reading said)

-- 
Ohioan by birth, Floridian by the grace of U-Haul and a bad economy...
--------
Sarcasm is the body's natural defense to stupidity.