[Coco] Re: Color Computer 3 prototype

[John Kowalski]

At 12:49 PM 21/01/2005 -0500, James Daggett wrote:
>From the block diagram in the service manual, my guess is that if 8 
>bit color mode was available, it would be in the lower resolu tion 
>modes. Say 160x192 or some combination that would not exceed a 
>32 K ram block. 

I definately agree here.  There is no way it would have been 320x200 *and*
256 colors.  My guess is that the mode would have bypassed the bit shifter
and palette lookup RAM altogether, limiting resolution to 160, or even just
80 horizontal pixels.

>It is possible that circuitry on this board may have been left out in 
>silicon. Possibly due to area constraints or other political moves 
>within Tandy.

I agree too.  The SBP28S42 may even be a key example of something missing
from the final production version.

There may actually be some evidence of what *might* have been the "256"
color mode even in production CoCo 3s.  Here are two examples:

10 POKE 59078,33:HSCREEN2
20 POKE 65433,252
30 GOTO 30

The first poke is just to stop the screen from clearing so you can see the
random junk in memory display.  What you see is incomplete/unresolved
graphics mode that appears to use 16 bits *PER* pixel - well, sort of..
Only one out of 16 bits is actually active and the other 15 are ignored.

This shows that the pixel data bit shifter can be disabled.

Example two:

10 POKE 59078,33:HSCREEN2
20 POKE 65433,255
30 GOTO 30

The one shows you a clean blank screen.  There is where a 256 color mode
would have logically been mapped had it existed.  The GIME hardware is still
addressing video RAM as expected - just not displaying any of it on the screen.

This shows that normal graphics output can be disabled.


Neither of these two examples bypass or disable normal palette lookup,
though.  A 256(+) color mode might have required one of the above steps ...
*plus* a way to bypass regular palette lookup.

Which leads us to our mystery SBP28S42 chip.. maybe?

>I leave you with one interesting IC that is down near the dual RCA 
>jacks on picture 011705-006. It is labeled TBP28S42. It is a 512 bit 
>by 8 bit(4096 bits) PROM.Commonly used as lookup tables. Could 
>this be the 256 color lookup table???????

Check out the "COLOR COMPUTER CUSTOM VIDEO PROPOSED FEATURE LIST" document
on this page:
http://www.nickm.launch.net.au/ProjectArchive/256mode.html

It specifically states that there is a total of 512 colors out of which 256
may be displayed.  The TBP28S42 holds 512 bytes of data...  It may very well
be an alternate palette lookup!

Maybe one of the graphics mode examples above, used in conjunction with some
other trigger, may enable an *alternate* (256/512) color set on this hardware?

                                         John Kowalski (Sock Master)
                                         http://www.axess.com/twilight/sock/