[Coco] CoCo 2 64K upgrade blues

[Dave Philipsen]

Dave Philipsen
> On Aug 4, 2017, at 8:34 PM, Carlos Camacho <idevgames at gmail.com> wrote:
> 
> I have a CoCo 2 with 16KB RAM. I decided to upgrade it to 64KB.
> 
> Since I have a bunch of P21256-12 chips, I decided to go with them.
> 
> This CoCo is a 26-3026 using 8709416 motherboard.
> 
> My first step was to solder a wire between the two pads labeled W1.
> 
> Looking at the 21256 datasheet, Pin 1 is A8 and Pin 16 is VSS.
> 
> When I switched the machine on, it booted fine. A PRINT MEM showed it now
> was a 64KB machine.
> 

Just FYI, I don't believe PRINT MEM can show it's a 64K machine since BASIC uses a signed 16-bit integer for MEM which cannot express a number as high as 64K.


> It was recommended in the Facebook group that I ground Pin 1 to avoid
> memory issues as the machine is used.

What does the machine being 'used' have to do with anything?


> 
> My first attempt, I connected each chip's Pin 1 to Pin 16. This didn't
> work.
> 
> Next, I tried connecting all the Pin 1s together and go to ground on the
> motherboard. I tried to tap into the ground of the 6883 but it didn't boot.

I suppose the best way would be to tie Pin 1 to the ground of the chip which is pin 16. If you are saying that the machine runs ok with Pin 1 left floating but it does not when you tie it low that's kind of strange.

I don't remember what type of memory chip/pinout is used in the 16k CoCo 2 so I'm assuming the pinout is virtually identical to the new 21256s you're putting in.  Does each chip have a bypass cap?


Dave


> 
> So, I tried to ground it on the RF box. It still didn't boot.
> 
> The memory is fine because when I remove the grounding wire going to all
> Pin 1s, it boots fine.
> 
> Summary: No wires on the RAM chips and machine boots fine. Try to follow
> advice to ground Pin 1 and machine doesn't boot.
> 
> I'm really scratching my head on this one. Hate to have to buy 4164s when I
> have a bunch of the 21256s.
> 
> Domo arigatou,
> 
> Carlos Camacho
> 
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