[Coco] CoCo 2 64K upgrade blues

[Carlos Camacho]

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.

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

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.

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