Its looks like it 256 color videocard from early 1990. Just look at DAC - main chip connection 😉 only 8 wires (2^8 gives you only 256 colors). Wrong palette - check the scratches on PCB and DAC soldering.

The datasheet for the bt475kpj80 says its 3 8bit DACs for RGB, so I thought it should be able to handle 64k colors.

mulciberxp wrote on 2022-08-04, 15:00:

The datasheet for the bt475kpj80 says its 3 8bit DACs for RGB, so I thought it should be able to handle 64k colors.

Actually, it can handle even more than 64k colors, namely 256k colors, but only 256 colors at the same time. The Bt475 doesn't have 3 8bit DACs (would be able to produce 16M colors), but only 3 6bit DACs. But as the connection from the video chip to the DAC is just 8 bits wide, the video chip sends an 8-bit color number, and the DAC uses its internal RAM to look up which of the 256k possible colors is mapped to the current pixel color number. The chip is called RAMDAC, because it is an DAC with RAM included.

For future reference: A RAMDAC with "476" in the model number is most likely a clone of the ADV476 original VGA 256-out-of-256k RAMDAC. A direct competitor is the INMOS G171 and G176, so the number "171 and "176" also are typical signs for 256-color-only DACs. Your Bt475 is quoted to be "pin-compatible with the Bt476", so unless the datasheet explicitly mentions high-color modes, the DAC doesn't have any.

Thanks for your insights! Now I just have to wonder what was the purpose of a VLB graphics card that didn't support true color output.

mulciberxp wrote on 2022-08-05, 20:25:

Thanks for your insights! Now I just have to wonder what was the purpose of a VLB graphics card that didn't support true color output.

I don't think true color output on low-end/mid-range systems was mainstream until well into the late 1990s. There's a reason for "web-safe colors..."

Ha! Just look at main chip and JAMMED pins 😉

Quake uses 8-bit colors (with palette swapping) so the Bt475 RAMDAC should be able to display it without this faulty colors.
I have bought a similar card recently. Today I have tested a lot, and it seems that the Bt475 is speed sensitive: if the CPU is faster than 486DX2-50, palette swapping going to erratic. Actually Quake and Doom are still good on 486DX2-66 but other games (like Pehistorik 2) are not. On DX4-100 Quake and Doom also get strange colors at palette swapping. On 486DX2-50 and DX-33 all are OK.