Homebrew 8008 Computer Schematics – 1974

8008 Front PanelI unearthed the original schematics of the homebrew computer I built long, long ago… an 8008-based machine that resides in the Computer History Museum (that link will open in a new window and show you four professionally done, zoomable photos). Before sending these six 11×17 hand-drawn documents to be with the machine, I scanned them for this archive.

The system predated those newfangled S-100 Altairs and IMSAIs that kicked off the personal computer phenomenon, flickering to life on October 31, 1974 after six months of obsessive design and fabrication with only the cryptic Intel 8008 databook for reference. In some primal male response to those first blinkies, I started my beard that day.

Building the machine in mid-1974

Building the machine in mid-1974

Microprocessors were bizarre back then, and at first I considered the project just a break from the ALU-based mini I was designing (but never finished). More than with any system since, I dove very deeply into this, feeling the bits between my toes. I still shudder at the 4-phase 600 kHz clock and the odd things that took place during transition boundaries… like the magical fleeting appearance of the content of a register if you tried to move it to itself (“load B with B” and the like). This sort of thing enabled a number of outboard enhancements that let me overcome the 8008′s intrinsic shortcomings… in particular, a hardware hack that added a data stack in outboard TTL RAM in addition to the limited 7-level return stack in main memory, allowing me to save context on a subroutine call.

The machine had 4K of 8-bit static RAM made of 2102s… I originally dedicated a wirewrap board to a mere 2K, but wanted MORE… MORE! So one night, I folded out all the chip-select leads of another 16 expensive 1024-bit DIPs, then soldered them on top of the first batch and wired the floating selects back to the address decoder. Wowzers! A whopping 4K. I was ready to compute the world.

Homebrew 8008 Wirewrap Panel

Part of the CPU board… I used 60-socket Augat panels and a venerable Gardner-Denver 14XA2 wirewrap gun.

There were 8 interrupt channels, 64 bits each of input and output, a graphics subsystem using a pair of 8-bit multiplying DACs outputting X-Y to my Tek 7504 oscilloscope from a DMA display list, and a painfully slow math co-processor made from a Taylor-Series calculator chip with kluged BCD and 7-segment interface (mostly because I was intimidated by writing floating-point routines). I remember the night it drew a lovely (sin X)/X curve… took HOURS and I had consumed half a bottle of Jack Daniels by the time it was done, but it was a thing of beauty, I tellya what.

Over its useful life, this machine also implemented Walsh-function waveform synthesis and a top-octave synthesizer for music projects, a hardware polyphonic music keyboard interface that I published in Byte, a one-shot Hollerith card reader that allowed me to boot-load with a multi-punched image instead of wearing my fingers out with deposit-next, Friden paper tape reader and punch to handle my Cybertronics mailing list, newfangled 1200-baud cassette interface hacked out of a Bell 202 modem, and right outta the gate, a hardware driver for the marvelous Model 28 Baudot teletype that I still recall with wistful fondness. (This latter circuit was my first published magazine article… in the July 25, 1974 issue of Electronics magazine.)

8008 System, Teletype, and Tektronix 7504 Oscilloscope
I sure do miss front panels. This one got a lot of use, and my early “screen saver” was a 555 (associated with the black knob) that allowed variable-speed single-stepping and a corresponding hypnotic blinking of the address and data bus LEDs. I remember one debugging night when address bit 14 appeared to be always on. I popped off the wire at the driver chip (a 7404) and it was still on, so obviously there was a short to ground between there and the LED pin (the other side went to a pullup to +5). So I casually removed the wire.

1974 photo with the homebrew 8008 system

The light was still on… with only one wire connected. I poured myself a drink and stared at it for a long time. It turned out that on my hobbyist budget, I had acquired a batch of surplus LEDs that failed some parametric test along the way… and this one had been dipped crooked in the plastic. The cathode metal was thus flush with the outside of the body, and made contact with my aluminum front panel (forever being known as the infamous “field-effect LED”). Ahhh, memories. In a masturbatory paroxysm of gratuitous acronymism, my friends and I dubbed the machine BEHEMOTH… for “Badly Engineered Heap of Electrical, Mechanical, Optical, and Thermal Hardware.” Fifteen years later, a much larger BEHEMOTH would roll out of the lab, this one the “Big Electronic Human-Energized Machine… Only Too Heavy.” (It now resides in the Computer History Museum as well.)

The Schematics

I didn’t make any effort to write text documentation to go with these, since, you know, the design was intuitively obvious to the casual observer. For the moment, I’ll just title the pages; perhaps some lazy evening at anchor, warmed by the wood stove, I’ll go back and try to recall what some of the more obscure random logic was doing.

CPU and Support Logic CPU Interrupt Logic Interrupt Logic 8 Output Ports and TTY Interface Output Ports and TTY Interface Front Panel Front Panel RAM and DMA Board RAM and DMA Board 8 Input Ports and 80-Column Card Reader Interface 8 Input Ports and Card Reader ControllerLooking at these really takes me back to a time of deep obsession and clear focus. We had no idea then how crazily the computer industry would take off, and could barely even imagine “a megabyte,” much less the impossibly magical things we now take for granted in our smart phones. Drawing tools were primitive (logic template, pencil, and taped-together 11×17 sheets), and to go from this to the wirewrap panels I created a book-length wire list… ticking off connections on photocopies of these drawings. Passionate times indeed. It’s good to remember this as I dive into the Nomadness project. It took many an all-nighter to pull this off, and there was no Facebook or looming IN-basket to constantly distract me with micro-interruptions. That, and I wasn’t yet technically jaded at the tender age of 22… everything I was playing with was amazing. Now we shrug at 32 Gigabyte SD cards…

The front panel of my 8008 system (Oct, 1974) as seen in the Computer History Museum during a May, 2015 visit.

The front panel of my 8008 system (Oct, 1974) as seen in the Computer History Museum during a May, 2015 visit.

The sign under my 8008 system in the Computer History Museum

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