Richard Cini wrote:
I do like the VID design, but here are a few things I thought of:
If you don’t have an aversion to using a PLD, a small GAL like a 16V8
might be useful for decoding and would still be period-appropriate.
There are several “new” designs in both the Retrobrew S100 and ECB
designs that use them, and they’re still available online from secondary
suppliers or eBay.
I've used PLDs in other designs for clients. They certainly reduce the parts count, and are good for "get it out the door quick" projects. They're great for commercial products that must fit in a certain size, and to keep your design safe(r) from copycats.
But they don't have the longevity of generic widely-sourced parts. I still have a pile of disk controller boards I can't build because the parts are no longer available.
PLDs also seem less suitable for hobby projects when you *want* people to be able to duplicate it. There are problems finding parts, finding programmers, and writing the equations to program them. They feel like a closed-box solution.
For the RAM, if you don’t need 128k, I might consider something like a
Cypress 32kx8 which is in a 0.300” DIP28 (rather than the normal
0.600”). That saves some space.
Yes, I use them on several of my kits. Once they got used by the millions in early PCs, they became generic. Lots of sources, and still available new.
They enabled another trick: If you look at my 6502 Badge and VIP2K kits, you'll see the 32k 0.3" RAM that hides *under* the 32k 0.6" EPROM. Their pinouts are almost the same, so it's easy to wire up, too.
The reason for the 128K RAM on the Z80-VID was to try to make a CP/M system on two boards. One is the existing CPU board, with Z80, 32K RAM, 32K EPROM, and 8-bit I/O ports. The second is the VID board, with NTSC/PAL video, SPI to run an SD-card for "disk storage", and more RAM so there's enough for 64K for the Z80, plus 20K for the video screen.
Alas, there are still too many parts to fit on a 3.5"x2" card without PALs, surface mount, modern parts, etc. But hope dies hard; maybe there's a way...
Lee Hart
--
A designer knows he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint Exupery
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
Lee Hart