The page linked to above gets a dozen hits per month or so, and I get an inquiry roughly every 2 months asking if a PCB is available. Because I never did anything with the Z80 SBC afterwards, or pursued the SOAPOC design, I regretfully inform the enquirers that no PCB is available.

In about half of these inquiries, it’s mentioned that they’d really like to be able build a little system that’s easy to wrap their head around, uses through-hole construction, and brings back the fun of PCs from the days before 2.4Ghz processors, 500GB drives, and massive operating systems.

The last week or so, I’ve started thinking about revising the Z80 SBC to satisfy these people. I find myself thinking “Adding Ethernet would be pretty easy. And VGA video would be nice…” And before you know it, you’re looking at a BGA FPGA, and something that’s completely incomprehensible to someone who has an interest in computers but hasn’t kept up with all the latest techologies.

Once video was there, it seems only logical to add a PS/2 keyboard. If you have video, you don’t want to have to hook up a serial terminal, which requires another monitor (at least if you want to see both at the same time). And if you’ve got hardware support for a PS/2 keyboard, adding mouse support isn’t much of a step… Before you know it, what you’ve wound up with is a Z80-based IBM PC.

And wouldn’t bank switched memory be nice? Now we could run MP/M, or CP/M 3…

I decided I needed to focus on the core of the project. A Z80 with 64K of usable memory, maybe bank switched memory, two serial ports, CTC, and CF/IDE storage. This is the minimum really necessary to have a usable system to store and retrieve data.

It’s getting harder to find 64K memory, so likely something larger would be used, and the rest wasted, unless bank switching is implemented. The boot ROM gets similar treatment, although perhaps some jumpers or switches to allow different booting (CP/M boot in one bank, maybe a simple Forth interpreter in another). No Ethernet, no video, no keyboard, no…

Now that I’ve simplified the SBC back to it’s essence, can anything else be reduced? The GAL is somewhat essential, as it reduces the gate count dramatically. It’s true that the majority of people can’t program them at home, but on the flip side, the equations and JEDEC file are available, and surely SOMEONE can be found that program the part (I’ll offer them for little more than the cost of the part and mailing).

The current design uses a traditional LM7805 regulator, a very inefficient device. I’m unsure how much current the 2.5″ drive might pull (200ma to 400ma), but I do know that if we use a 12V input voltage, we’re dropping 7 volts at somewhere around 500ma, that’s 3.5 watts. That’s smokin’ hot, and if we wanted to use a battery to power the SBC, it’s grossly inefficient. One of the design constraints will be to use readily available parts for the switcher, preferably from a supplier like Mouser or Jameco.

In regards to the compact flash slot, I first considered putting the CF socket on the SBC itself. I nixed that idea for two reasons. It makes the SBC PCB larger by about 2.25 inches, and it introduces a surface mount part. The better solution is to put a 40 or 44 pin connector on the SBC, and provide either a kit or an assembled PCB with the CF socket that plugs into it. This solution is desirable for several reasons. It allows a small CF-only solution, plugging an IDE drive directly to the SBC, or providing another PCB that supports both CF and IDE (so programs may be copied about).

However the design finalizes, one thing it will have available is an expansion connector. If a video + keyboard + Ethernet daughter card is ever designed, it’ll have a place to plug in. And if the user wants to control something in the real world (and not just reminiscence by running WordStar), there’s a place to connect a little home-brew board.

At this point, I think I’ve boiled it down to the Z80, 64K, 2 serial ports, CTC, GAL, CF and/or IDE, expansion connector, and switching power supply. And that’s it.

Really.