Re: Apple //e's RGB Card To Apple IIgs' RGB Monitor

["Michael J. Mahon" <mjmahon@aol.com>]

Bryan Parkoff wrote:
> Folks,
>
>
>
>      I have seen Apple //e's RGB card is connected to Apple IIgs' analog RGB 
> monitor using resistor network as the alternative option because digital RGB 
> monitor is hard to find.
>
>      Digital CGA monitor is recommended for best fit, but unfortunately, it 
> gives wrong Apple II colors to be true 16 CGA colors.  It is what you don't 
> want 16 CGA colors.
>
>      Resistor network is ideal for Apple IIgs' analog RGB monitor, but it 
> gives true 16 CGA colors!!  Are you satisfied?  No way.
>
>      Look at RGBI to 12 bit RGB table below.
>
>
>
>          RGBI  RED   GREEN BLUE  Hex
>
> Color 0  0000  0000  0000  0000  000
>
> Color 1  1000  0000  0000  1010  00A
>
> Color 2  0100  0000  1010  0000  0A0
>
> Color 3  1100  0000  1010  1010  0AA
>
> Color 4  0010  1010  0000  0000  A00
>
> Color 5  1010  1010  0000  1010  A0A
>
> Color 6  0110  1010  0101  0000  A50
>
> Color 7  1110  1010  1010  1010  AAA
>
> Color 8  0001  0101  0101  0101  555
>
> Color 9  1001  0101  0101  1111  55F
>
> Color A  0101  0101  1111  0101  5F5
>
> Color B  1101  0101  1111  1111  5FF
>
> Color C  0011  1111  0101  0101  F55
>
> Color D  1011  1111  0101  1111  F5F
>
> Color E  0111  1111  1111  0101  FF5
>
> Color F  1111  1111  1111  1111  FFF

Something is mislabeled.  The only difference between black (color 0)
and blue (Color 1) is the R digital output, but the only column that
changes in RGB space is blue.

I suggest that the RGBI column label is actually "BGRI".

The usual IBM-standard arrangement (that corresponds to the color
numbers) is IRGB.

>      RGBI is Apple //e's video port of RGB card.  RED signal, GREEN signal, 
> and BLUE signal are the 4-bit signal for analog RGB monitor for Apple IIgs.
>
>      I got this information from several manufacturers who they manufactured 
> CGA ISA controller cards.  Sometimes, people claim to say wrong color 
> palette for VGA which wrong color palette made an attempt to match 16 CGA 
> colors.  It is not important, but wrong palette seems to be satisfied.
>
>      I borrowed this information and I wrote it into my own table above.  I 
> can guarantee to be correct, but I am not 100% to be correct.
>
>      Get rid of resistor network.  You don't need it.  Here is Apple IIgs' 
> table below.  I got it from my own analysis using oscilloscope and Apple 
> IIgs' hardware manual.
>
>
>
>          RGBI  RED   GREEN BLUE  Hex
>
> Color 0  0000  0000  0000  0000  000
>
> Color 1  1000  1101  0000  0011  D03
>
> Color 2  0100  0000  0000  1001  009
>
> Color 3  1100  1101  0010  1101  D2D
>
> Color 4  0010  0000  0111  0010  072
>
> Color 5  1010  0101  0101  0101  555
>
> Color 6  0110  0010  0010  1111  22F
>
> Color 7  1110  0110  1010  1111  6AF
>
> Color 8  0001  1000  0101  0000  850
>
> Color 9  1001  1111  0110  0000  F60
>
> Color A  0101  1010  1010  1010  AAA
>
> Color B  1101  1111  1001  1000  F98
>
> Color C  0011  0001  1101  0000  1D0
>
> Color D  1011  1111  1111  0000  FF0
>
> Color E  0111  0100  1111  1001  4F9
>
> Color F  1111  1111  1111  1111  FFF

In this case, the RGBI column label appears to be simply a color number,
and not "RGBI" or any permutation of it.

>      How do you do this to make Apple //e's RGB card work with analog RGB 
> monitor using this table above?  Forget about resistor network.  You will 
> need 5 chips.  Simple?  Yes, it does!!
>
>      First of all, you need to program this table above.  RGBI is the four 
> input pins.  RGB is the 12 output pins.  You can write it using EPROM chip. 
> I suppose that 4 bit is to be address bus and 12 bit is to be data bus, but 
> EPROM chip might not be the answer for you.  It may need two or three EPROM 
> chips.
>
>      You can program using two GAL16V8 chips.  Two GAL16V8 chips are enough. 
> Are you satisfied to have 12 bit RGB pins to be 12 digital signals?
>
>      The problem is the analog RGB monitor.  It can't accept 12 digital 
> signals.  You need three chips.  It is 4 digital bits to 1 analog bit 
> convertor.  One for Red chip, Green chip, and Blue chip.

I'd suggest using three 4-resistor networks to do the three 4-bit D-to-A
conversions--four resistors beats a D/A chip for just 4 bits.

And buffer amplifiers may be needed for the RGB signals as well, if the
RGB monitor's inputs are 75 Ohm.

-michael

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