So I have recently come across Ben Eater's "World's worst video card" series of videos, and his 6502 oriented computer videos - and it has rekindled an interest in electronics. I haven't done much for many years, but I have been quite interested in the structure of video cards - specifically the VGA because of how easy (mostly) it is to connect to.

But when going over specs on memory access speeds and pixel access rates, it looked like I could get ahold of memory that could do the job but it would be somewhat close - if I wanted to try a full resolution solution. I realize that I can cut down the pixels like Ben Eater does to increase the time I have, or have pixels that take up less than a byte. Which I am considering, I was thinking about doing a text mode first (there are certain aspects about it that fascinate me - trying to figure out in my own mind how certain things can be done).

But then a friend mentioned RAMDACs that were actually use in the before time. I understand with HDMI it isn't really necessary anymore, so they are basically obsolete (even before because they were integrated into the chipsets).

But are there any discrete RAMDACs that are still being made and are accessible to these kinds of projects?

I don't think dedicated devices are still being made, no. Generally if an OEM needs a basic display output, they integrate something like an ATI Rage128 which has a RAMDAC built in (but also a lot of other stuff), or they implement DSI or DPI directly to an LCD panel.

The new RP2040 microcontroller (aka Raspberry Pi Pico), however, has the capability to bit-bang DVI/HDMI signals at modest resolutions through programmable I/O state machines. It has enough internal RAM to hold a text or bitmap display with limited colour. If you can find a way to also interface it to an external RAM, then you could use it as a pseudo RAMDAC.

Welcome zzing! I too would rather suspect no RAMDACs presently being made. There are outlets for used parts, but buyer beware.

The Pico has 26 I/Os so stands a chance of emulating a 28 pin RAMDAC. It's a fair project to do this though. And officially the Pico isn't 5V tolerant.

Thank you both, I have decided to just use a set of parallel resistors for the part of the output.

I have found some really fast ram that works at 5V the 71256SA12TPG ( https://www.digikey.ca/en/products/deta ... PG/2011023 ).

There are a few questions that I haven't worked out the answer yet, mostly because I haven't investigated it nor actually applied to look into it.

The most important question: Is it possible to use this ram with the 6502? (or its 16bit successor)

If the answer to that question is yes, preferably an unqualified yes, then the next question is how to make it work for both video and cpu.

I have seen the idea of dual port memory, but I haven't found anything that is particularly satisfactory. I would like to be able to spec memory that can be as fast as possible to allow for flexibility later in per pixel times.

So one thought I had was to have effectively a double buffering going on. If the video memory is effectively 32kbytes, then I would have two chips, and would swap between them on the low bit of the counter. In the period of time where it isn't accessing one of the chips, another chip can transfer the video memory over from main memory without taking too much cpu time away.

That is the high level idea I am thinking about without specific means for doing any of it yet. So it might turn out that I can't do any of it or some of it for some reason or another.

It's advice rarely taken, but often given, and it's good advice: start simple. Proceed in stages. Learn how to build and how to debug. You'll be much more likely to finish the job if you build up confidence and competence. It's a common failure mode to overreach and become demotivated.

The best place to start is with an existing working design, and build that, and debug your build.

Having said which... figure out the bandwidths you need. Draw the diagrams, do the arithmetic, figure out how many accesses per second are needed, and how wide they are. And draw out the bus structure. Where do addresses come from, where does data go.

I should apologise if my previous post was too harsh - as the idea was to prevent discouragement, it would be pretty bad if it actually caused discouragement.

What I should have tried to do is to encourage you to start by building a simple-as-possible system and work up from there.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Oh not at all. I am working off of two existing projects already (I think I mentioned them up above). Ben Eater's breadboard 6502 and breadboard video card. His 6502 computer is built up rather slowly over a few videos so I will get some familiarity there. His video card is a little bit more basic, so I am probably going to try to adapt it to what I am hoping for.

My direction is to reproduce what he did on the computer using the kit he sells (its one way I can support/thank him for the resources he has provided), and then work from there. I am wanting to look at the larger picture in a theoretical way - it is early to do that yes - but it is one of the parts that is fascinating.

Now I have seen the link to http://wilsonminesco.com/6502primer/ and I am taking some of the information down. I am interested in looking at an oscilloscope sometime after I get some of the kit stuff going.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

RAM-DACs are supposed to be no longer in production.

DigiKey has AnalogDevices RAM-DACs from Rochester Electronics (year 2021), but if the stock is gone, it's gone.
For some chips the minimum quantity is a bit high, some other chips might be a bit too expensive for a hobby project.

EF9369 (PDF page 216) is around $20 at ebay, but wheter chips at ebay are fully functional originals or not would be going to be a different topic.

did you find any datasheets for those chips?

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No simulation survives contact with reality!

The Brooktree RAMDAC's look like they'd be quite easy to interface with. They are out of production but there are still NOS ones around, but the prices are quite high. The BT101 is a VideoDAC, so has no lookup table, The BT450 is a 4-bit RAMDAC, and there are plenty of 8-bit ones, like the BT468. Datasheets are quite easy to find on the internet.

The AnalogDevices homepage still has datasheets of obsolete RAMDAC chips, just use the "search" function.
Also take a look at Bitsavers: 1992 Analog Devices Audio Video Reference Manual, PDF page 522, 6-3.

TI still (year 2021) manufactures video DACs without a lookup table, and Mouser has them on stock:
THS8135: 3*10 Bit 240MSPS.
THS8136: 3*10 Bit 180MSPS.
THS8200: 3*11 Bit 205MSPS.

For a dissection of an old (monochrome) video DAC module containing ECL chips and transistors, go here.

Edit:
Related thread: ANALOG's "COMPUTER GRAPHICS AND RAM-DACs"