New monitors on old machines

On 11/2/06, Jim Leonard <trixter at oldskool.org> wrote: It depends on the level of precision you require and if the target VGA monitor handles RGBI scan frequencies. So... start with a multisync that can handle the requisite sync frequency from your RBGI-equipped CPU. Something like an old NEC-3D can sync down to the NTSC range (~15KHz horizontal). You might or might not need to invert hsync and vsync (I did to attach an Amiga to an RGBI monitor). Now that you have sync taken care of, you can worry about R, G, and B. In essence, an older VGA monitor is expecting an analog signal on each of 3 color pins, but RGBI is a 4-bit digital interface. It shouldn't be hard to use sets of resistors to make a simple, if imprecise, DAC to turn 4 digital bits into 16 analog levels on the 3 color pins (the "I" bit controls intensity of all three channels simultaneously, such that if your RGBI pattern is 1111 vs 1110, it's white vs gray (I can't remember if "I asserted" means full intensity or half, but worst case, if you have it backwards, it's not hard to invert a TTL signal to flip the sense of the "I" bit before it goes into your "DAC"). Worst case, you could start by ignoring "I" and have an 8-color output TTL levels down to the high end of what the VGA monitor is expecting (0.7VDC == max intensity?) If you want to be able to tweak the color balance, one could just use 3 variable resistors that can span the range near the value you would have used for each fixed resistor. I haven't tried to do this myself, so I can't supply specific values, but an oscilloscope would be a handy tool if you wanted to build a simple 8-color (no "I") resistor-based TTL->VGA analog "DAC" - get your sync issues solved first, then put a variable resistor on, say, Green first, then get the computer to display a green screen, then set the resistor so you have approximately 0.7V showing as the peak voltage on the green line. Repeat for Red and Blue. Voila... digital RGB on an analog VGA monitor. -ethan