[Coco] Re: [Color Computer] CoCo 2 questions.

Mike Pepe lamune at doki-doki.net
Thu May 12 17:35:02 EDT 2005


Phill Harvey-Smith wrote:
> Hi,
> Just got myself a cheap, (£4.99) CoCo 2 off ebay, not working at the 
> moment, but I intend to fix that :) I have a couple of questions :
> 1) It seems very different from the CoCo 1, much more compact, and seems 
>   to have some different chips. Does anyone know where I can get a 
> circuit sheet for this, I have the CoCo 1 tech ref, but this seems to 
> different.

I might have one, I can check, but it would be NTSC not PAL- not much 
difference I would imagine.

> 2) There are a couple of chips with numbers I don't recognise they are :
> HD61J204P / TCC1000, a 28 pin chip on the bottom left of the board.

That's the BASIC ROM

> SC7331P, a 40 pin chip in the top middle of the board, next to the 6821,
> this seems to be connected to the keyboard, so I assume it's a PIA, but 
> that's a number I don't recognise.

It's roughly the equivalent of a 6822, which was discontinued and 
replaced with this "custom" part. If it's dead, a 6822 or 6821 can be used.

> 3) There seem to be some wire links to select between 16K and 64K rams,
> assuming that I can get some 64K, what type does it need (41464s at a 
> guess) ?

Depends on the board. All can be upgraded with 8 4164 DRAMs. Some can 
take 2 64kx4 chips.

> Also the machine seems to have a 74LS785, rather than a 74LS783, I have 
> tested this in my Dragon test board and it works ok, but wondered what 
> the difference was ?

Functionally identical with the major difference being the 785 supports 
256 column refresh RAMs (like 4164) whereas the 783 has a 7 bit counter 
and only can refresh 128 column (max) rams.


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