[Coco] CoCo 1 'E' board memory upgrade

[Bruce W. Calkins]

Early Radio Shack 32K upgrades were often half good 64K chips, hence the 
high/low jumper.  Personally, I've never seen a half good chip, but I 
recycled many IBM 64K chips which had to be good for 64K.

Bruce W.


On 4/16/24 16:18, Brian Blake via Coco wrote:
> 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)
>