8bit: How to generate video output?
In general, 6845 systems were not intended to drive standard color TVs or displays with composite video. Rather, they were originally intended to drive dedicated monochrome CRTs which required only 3 inputs: HSync, VSync, Video, and Video was normally a TTL logic level. With a bit of extra circuitry you could get the cursor highlighted, but that's it. Plus, of course, it was text-only. The 6845 does not have sufficient speed to do worthwhile non-text display. Maximum clock frequency is 2.5 MHz. Even assuming 48 usec of horizontal display time (standard for 60 Hz displays) that only gives you about 120 pixel horizontal resolution. For text displays, of course, the pixel clock would run about 8 times that, and be divided down to give the CRTC clock.

These CRT monitors, of course, are long gone, although you can still find them on eBay. In general, any modern monitor with analog VGA inputs will accept non-composite video, although not all will operate at the low scan speeds which the 6845 will produce. Also, VGA interface wants to see 1 volt p-p video levels. 

In addition to your 6845, you'll need a static RAM to actually hold your screen data. You'll also need a character ROM on the output of the RAM, and a shift register to serialize the ROM output. Plus, of course, bus control logic to determine which element (CPU or CRTC) is actually accessing the RAM.

Making a color output from this setup requires a distinctly non-standard approach. Either you encode your video data with a non-ASCII format, putting the character data in the lower 6 bits and using the upper 2 bits as color select, or your video RAM will need to be more than 8 bits wide, with the attendant complexity involved in writing data to it from an 8-bit data bus. You can do it, but I suspect you need a bit more logic design chops than you have. If I'm wrong, have fun.

If you have 2 bits of color data, the simplest use is to use a 2-4 line decoder to select which primary color (R,G or B) you want the character to appear in. If you've done something non-standard and want better color resolution, you can use the 2 bits to address a lookup table which will convert the 2 bits to 3 sets of RGB values (2 or 3 bits each will fit in an 8-bit ROM, or you can even do something creative with high-speed RAM, although then you'll have to figure out how to access it.

Making a 2-bit video DAC is not hard, but you've got to keep in mind that VGA video inputs are nominally 75 ohm. Something like







simulate this circuit &ndash; Schematic created using CircuitLab

should work, with the buffers just being any 7400 TTL series gates.

If you're determined that you've got to have multi-bit color (which in your timeframe was only available on very high-end monitors), you'll need higher-res video DACs. I'd recommend trying to find old Brooktree DACs, which sometimes show up on eBay. The BT103 has 3 4-bit DACs, and the Bt102 and Bt106 handle 8 bits. All of them have TTL inputs and will give 1 volt into 75 ohms. You can find a 1991 Brooktree catalog on this site and it will give you all the technical information you need.