Video DAC

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Resistor array forming the video DAC on a 1FZS.

The NeoGeo uses a discrete video DAC made of precision resistors, and two 74LS273 latches controlled by NEO-B1, NEO-GRC or NEO-GRZ.

Video DAC on a AES board.

Aes viddac schematic.png

Each color component has its own set of 7 resistors. 6 of them are used with the actual color value stored in the palette RAM (5 + 1 common bit). The last (SHADOW signal) is related to the REG_DISPDISABLE and REG_DISPENABLE registers (74HC259 latch).

The output voltage is then divided by 5 to obtain a 1Vp-p max signal, which can be fed to the CXA1145 encoder.