The January 1975 issue of Popular Electronics magazine had a breakthrough project on the cover. The Altair 8800 -- World's First Microcomputer Kit to Rival Commercial Models". (Click on the link to read the original article.)
One thing that struck me about microcomputers back then was how big they were. Big impressive boxes (with big impressive marketing claims) seemed to be the norm -- even when the boxes (and marketing claims) were mostly full of hot air. Conventional wisdom says that they *had* to be big, due to the limitations of the technology at the time.
But, is that really true? We had pocket calculators before the Altair. They are all microcomputers, too. The HP-35 scientific calculator came out 3 years before the Altair, and had considerably more computing power at a lower price. AND it fit in your shirt pocket!
So I got to wondering... Could we have built an 8080 microcomputer, like the famous Altair 8800, but in a pocket-sized package? No modern parts; no PICs or Atmels or other modern micro "wizard behind the curtain" to make it work. It's got to be built with vintage parts and through-hole technology; just as it would have been back then. It has to be user programmable; not simply running a fixed program like the HP-35. And, it's got to have a real front panel, like the Altair with its classic switches and blinkie-lights! I set out to try.
I had previously built a reproduction of the August 1976 Popular Electronics "COSMAC Elf microcomputer" in an Altoids tin. I called it the 1802 Membership Card. It used the RCA 1802 microprocessor, which is a natural for battery operation. That project turned out very well. I made it available in kit form, and hundreds have been sold (click the link for details).
But the 8080 proved to be a much harder "nut" to crack. It isn't quite a complete CPU by itself; it needs a few support chips to finish the job. It also needs three supply voltages; +5v, +12v, and -5v. That makes it tough to power it from a single-voltage power supply or battery.
I couldn't find a way to pack it all in without "cheating" (using modern parts or construction technology). I wound up backing off, and designing a version of the Membership Card around the Z80 instead. The Z80 is a derivitive of the 8080 (runs the same software) and is much simpler to use hardware-wise. The Z80 Membership Card is also available as a kit (click link for details).
But I kept thinking and "schematicizing" at it. I couldn't fit the 8080, its support circuitry, memory, and I/O onto one Altoid-sized board (like my 1802 and Z80 versions). It was Josh Bensadon that provided the "aha!" idea. He said, "You'll need a second Front Panel board anyway. So put the CPU on the front board, so everyone can see that it's a real 8080. It can be the "CPU board" as well as the "Front Panel board". Put the memory and I/O on the second board, below it.
That worked! The "CPU board" has the 8080 and its support chips. There's also a little switchmode power supply to provide the 3 voltages needed. But this didn't quite fill the board -- so there was room to include the "Front Panel" switches and lights. Borrowing from the Heathkit H8 (another classic 8080 microcomputer from the 1970's), I used software to drive the Front Panel switches and lights for improved functionality and to save parts. I managed to lay it out, and get it built.
The second board has the memory and I/O. The bus interface between the two is the same as the Z80 Membership Card. Like the old Altair S-100 bus, cards can be interchanged between them, and new cards can be designed that will work with both.
This card has the rest of the circuitry needed to make a complete working computer like the original Altair 8800. As you can see, it's a work in progress. At the time the photo was taken, a few parts still hadn't been installed.
This is still a "project in progress"; but should give you an idea of where we're at. If you look at the Z80 Membership Card, you'll know where we're going! Feel free to contact us with comments, critiques, or complaints.
By Lee A. Hart. Last update 9/4/2018.
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