[Color Computer] Re: [Coco] Hidden 256-color mode

j_e_daggett j_e_daggett at yahoo.com
Thu Jul 28 13:27:31 EDT 2005

--- In ColorComputer at yahoogroups.com, "James Diffendaffer"
<jdiffendaffer at y...> wrote:
> After checking the GIME pinouts, pins 65-68 are Composite, Red, Green
> and Blue.  So, does that mean the video DAC is internal???  If so then
> if it's in there it would be 256 colors.
> Since all address and data lines go through the GIME I think the
> shortage of pins doesn't make any sense.  Shortage of bits in
> registers would make sense so my comment was probably wrong.

While the lines may pass through, not all data passes through. When
the MPU writes to ram, the data is written to ram via U10 and U11
external to the GIME chip. 

As for pin count, I can execpt that the GIME chip was pin limited in
the sense that the next standard package up from 68 pins was 84 pins
in PLCC packaging. BAck in 1986 it was around 10 cents per pin to
package an IC. GOing an extra 16 pins would cost RS another $1.60 per
IC without and die size cahnge. ANother thing to ponder is that the
die itself may have more than 68 pins and not all are bonded out. 

> Now, something else to mull over.  There is a pullup on pin 38. 
> Anyone know what this pin actually does???  I'd think it would be +5v
> or GND unless it has some specific function.


Labeled "TEST". Bonded out to test the packaged IC with the same test
program as in wafer test. One day it might be interesting to get a
pair of 86 and 87 chips and decap them and do a die inspection. That
could help in determining whether the mode did  or did not exist. 

> --- In ColorComputer at yahoogroups.com, "James Diffendaffer"
> <jdiffendaffer at y...> wrote:
> > If they left the mode in the die but the control line(s) for intensity
> > off of the chip it should still have a 64 color mode with the
> > intensity bits ignored.  ;)
> > 
> > >> The possibilies I see are:
> > >> yyyyyrgb - 8 colors with 32 levels of intensity
> > >> yyrrggbb - 64 colors with 4 levels of intensity
> > >> 
> > >> I think the last one makes the most sense and would be most usefull
> > >> for games.  It still works within the 64 color palette of the CoCo
> > >> but with intensity.
> > >
> > > Now, that is an interesting idea...  I wonder if they might have
> > > just hooked the two intensity bits to some video amps that might
> > > then have been left-out of the final board design?  Well, it's an
> > > idea anyway... :-)
> > >
> > > I don't suppose there are any unused (or otherwise likely) pins on
> > > the GIME?  Probably not...
> > >
> > > John
> > > -- 
> > > John W. Linville

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