[Coco] 64 K upgrade for COCO 1 gives ?OM ERROR

John E. Malmberg wb8tyw at qsl.net
Sat May 2 19:44:28 EDT 2020

On 5/2/2020 5:26 PM, Bruce W. Calkins wrote:
 > "E" and earlier boards needed some added wiring to a previously unused
> gate on a 74LS02 to properly switch the RAM.  Only the "285"/"F"
> boards  were a direct upgrade.

It is definitely an E board.

So I may need more information on how to upgrade this computer to use 
the full 64K.

I would have expected it to at least claim to have 32 K.

 > The high low jumper was for selecting the upper or lower bank of
 > "half-good" 64k chips.
> It begins to sound like a bad RAM chip, or perhaps a bad jumper or
> some  other "little" detail.

I found the 2164b-12 chips in my memory parts storage.  I think they 
were removed when I did a modified upgrade to my Tandy-2000 to 512 K, 
but I am really not sure.

I have tried both positions of the high / low settings.

 > This book link might help; <www.roust-it.dk/coco/littlewonder.pdf>
 > Starting with page 82.

It was a big help, but does not mention what specific additional wiring 
needed to address the RAM on a revision E board.

On 5/2/2020 5:29 PM, Arthur Flexser wrote:
> The ?OM ERROR sounds like the machine is sensing that it has only 4K of
> RAM.  A test would be that certain very simple commands DON'T give the
> error.  If PRINT 5 works correctly, then that"s definitely what the problem
> is.  Sounds like a misplaced jumper.

I had to remove the Extended Color Basic ROM to get the print commands 
to work.  Yes, it is only seeing 4K.

All jumpers listed in the Tandy's Little Wonder book have been checked.

I do notice that the spec sheet for the 2164B-12 has a notes that a 
special power up sequence is needed for it.  I do not see that note on 
the spec sheets for the 4164 parts.

So it looks like at least for now, I need to put the 16K chips back into it.


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