Name     aergb ;
PartNo   aergb ;
Date     11/28/2020 ;
Revision 01 ;
Designer Renee Harke ;
Company  None ;
Assembly None ;
Location None ;
Device   g16v8ma ;


/**
 * This is the PAL from the Applied Engineering "RGB Card", which is a piggyback
 * card for the RamWorks series of cards for the Apple IIe. The card provides a
 * digital RGB output compatible with the AppleColor Monitor 100.
 *
 * This same PAL (along with another one) is also present on the "Digital Prism"
 * and "Color Link" cards, which might be considered as different revisions of
 * the same base card.
 *
 * The function of the PAL is not changed between the cards. It is involved with
 * converting the serial video signal from the logic board into an RGB output,
 * though understanding its exact function would require more research.
 *
 * The PAL is generally (always?) labelled "AERGB Q" and is located near the
 * bottom-left of the card. It is typically an MMI-branded PAL16L8. An ATF16V8
 * in complex mode can emulate the function of this PAL.
 *
 * Thanks to Robert Justice for putting me on to the idea that we have to
 * express things in negative logic in order to fit the equations on the chip.
 *
 * I claim no copyright on this code.
 */


/****** INPUT PINS ******/
PIN 1 = P1;   /* 80VID  */
PIN 2 = P2;   /* SEROUT */
PIN 3 = P3;   /*        */
PIN 4 = P4;   /*        */
PIN 5 = P5;   /* LDPS   */
PIN 6 = P6;   /*        */
PIN 7 = P7;   /*        */
PIN 8 = P8;   /*        */
PIN 9 = P9;   /*        */
PIN 11 = P11; /*        */
PIN 16 = P16; /* FRCTXT */
PIN 18 = P18; /* SEGB   */

/***** OUTPUT PINS ******/
PIN 12 = P12; /*        */
PIN 13 = P13; /*        */
PIN 14 = P14; /*        */
PIN 15 = P15; /*        */
PIN 17 = P17; /*        */
PIN 19 = P19; /*        */


!P12 = (!P1 & P9 & !P16 & !P18) # (!P2 & !P3 & P7 & P9 & P16 & !P18) # (!P2 & P3 & !P7 & P9 & P16 & !P18)
 # (!P4 & P9 & P18) # (!P9 & P11) # (P2 & !P3 & !P7 & P9 & P16 & !P18) # (P2 & P3 & P7 & P9 & P16 & !P18);

!P13 = (!P1 & !P9) # (!P9 & P16) # (P16 & !P18);

/**
 * Feedback is necessary to express P14 in 7 product terms.
 *
 * You can confirm with a logic analyzer that the original PAL does this too;
 * you can see P13 switch first, and then P14 lags one cycle behind.
 *
 * It's possible that P12 and/or P15 were involved in the original equation as
 * well, but it's not necessary so they probably weren't.
 */
!P14 = (!P1 & P5 & P13) # (!P2 & P7 & P9 & !P13) # (P1 & P6 & !P7 & !P16 & !P18) # (P2 & !P7 & P9 & !P13)
 # (P5 & P9 & P18) # (P6 & !P7 & !P9 & P13) # (P8 & !P9 & !P13);

!P15 = (!P1 & !P9) # (!P1 & !P11 & !P18) # (!P1 & !P16 & P18) # (!P1 & P8 & !P18) # (!P9 & P16) # (P16 & !P18);

!P17 = (!P2 & !P6 & P9 & P16 & !P18) # (!P2 & P3 & !P8 & !P9) # (!P2 & P3 & !P9 & !P11) # (P2 & P6 & P9 & P16 & !P18);

!P19 = (!P1 & !P8 & P9 & !P11 & !P16) # (!P1 & P3 & P9 & !P11 & !P16) # (!P1 & P6 & !P8 & P9 & !P16)
 # (!P1 & P9 & !P16 & P18) # (P1 & !P2 & !P9 & !P16) # (P1 & !P2 & !P16 & !P18) # (P3 & P9 & P18);