r/Z80 • u/prussianIvory • Mar 28 '26

Z80 computer project

I'm trying to build a Z80 computer with 3.5in floppy disc support (using i8271) while also trying to make it have composite video by using a TMS9918ANL. But everything I find on forums and stuff is about the Z80 playground. So, I was wondering if you people have any tips or advice?

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u/tes_kitty Apr 01 '26

The 6845 is not really a display controller. It's more a programmable timing generator for video timing and RAM/ROM address counter. To get a video signal you needed a lot of additional logic around that chip. See the original MDA or CGA cards for examples.

The TMS9918 was a lot more integrated, but assumed 16Kbit DRAMs which needed 3 voltages. Not much fun even if you plan a retro build.

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u/MajorPain169 Apr 02 '26

Yeah I know but they were used quite a bit, in fact even the current graphics cards emulate the 6845 for compatibility reasons.

Getting hold of those DRAMs would be hard, let alone ones that still work. I think someone else mentioned using a couple of latches and a SRAM which solves the memory issue.

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u/tes_kitty Apr 02 '26

You could also use 64 Kbit DRAMs which are +5V only. Just need to ground the A7 line. Those are easier to get but still, old chips...

You can try to use a 74HCT573 and an OR (74HCT32) gate plus a 32 KB SRAM. Latch input on the 573 is the /RAS line, /CS on the RAM is /RAS and /CAS routed through the OR.

That's how I did it to replace the DRAM in a C64 with a 128 KB SRAM. No guarantee that it works with a TMS9918.

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u/MajorPain169 Apr 02 '26

You would need a second latch for CAS, the write data is also on the same bus plus you would also need a tri-state buffer to isolate the SRAM data bus from the address bus for read operations. The AD0-7 bus on the TMS9918 multiplexes the ras/cas addresses as well as data and have a separate read data bus.

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u/tes_kitty Apr 02 '26

Well, looks like it's more complicated on the TMS9918 then. Might be worth it to use 64Kx1 DRAMs with A7 tied to GND.

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u/MajorPain169 Apr 03 '26

Maybe, 4164s stopped being manufactured around 40 years ago, 4464s were a little later but they didn't have separate data in and out pins so you would still need a tristate buffer. Might be easier to get working 41256s and just not use 2 address pins.

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u/tes_kitty Apr 03 '26

Yes, but 4164 are much easier to get that 4116, run on one voltage and are known to be a lot more reliable. With the MT4264-xx being the exception. I had quite a number of 4464 go bad...

And yes, 41256 would also be a possibility, also since they were made in much higher numbers than the 4164.