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Z280 homebrew board plan

My Z280 now has a 1MB Trident SVGA card. Photo of hardware: http://www.hyakushiki.net/misc/z-svga1.jpg Photo of amazing monitor test pattern: http://www.hyakushiki.net/misc/z-svga2.jpg

Getting that to work was harder than I expected. I had first tried a CL-GD5x0 VGA card, then briefly a CHIPS 82C43x EGA card (though this was wildly impractical due to being a full length card), then the Trident 8900D card, and finally a Western Digital 90C00 VGA card. Then I found the datasheet for the TKD8001 DAC and decided to try the Trident card again. At best I had been getting some nice hsync/vsync signals and a blank screen showing color 0, despite being able to read and write the video memory at $A0000. It turns out the key thing I had missed (and finally ran across in the WD90C00 manual on bitsavers) was bit 5 in $3C0. This register looked like it was just a place where you write your index to access some other register. But if you do that, and don't go back and set bit 5 when you're done, you get a blank screen. Huh.
 
I've been slowly building up the low-level code needed for a functional system. Changing screen modes, converting keyboard scancodes to ASCII characters, printing strings on the screen, etc.

There's some voodoo involved with scrolling the text upward when the cursor reaches the bottom of the screen. My VGA card is in an 8-bit slot and the Z280 does not have dynamic bus sizing. Writing a byte is no problem, since the CPU puts the data on both halves of the bus. Reading a byte from odd addresses works, since the CPU expects data on D0~D7, which is actually connected. Reading a byte from an even address doesn't work. But it's still possible to read even bytes by reading a word instead, from an even address, and getting the data in the high byte of the register.

Scrolling happens at a decent speed though. Better than the typical 8088 DOS machine, I think.

With a vga-to-dvi cable I can capture video from the system with my DVI2USB: http://www.hyakushiki.net/misc/z280cap.avi

I'm thinking about eventually setting up a CP/M 2.2 disk image and trying to boot it from RAMdisk (or ROM).
 
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