The Paleotechnologist

The EET325 class is wrapping up this week, but not before I got Bill’s prototype Z80 core working with the original protoboard control panel. Not only that, but two of the students (Mike and Austin) have finished, working Z80s. So now there are four of them! Can a Beowulf cluster of Z80s be far behind?

To paraphrase Emperor Palpatine…

“And now, witness the power of this fully ARMED and OPERATIONAL … Z80 computer!”

OK, so running at the current 40kHz system clock, it’s got more milliamps than megaflops — but at least it’s running properly on one board (CPU, memory, controls, and LEDs all together.)

More about the milliamps to come: it’s currently drawing over half an amp(!) at 5V — almost all of which is going into keeping the 74LS245s warm. The parts should be in tomorrow to give it a quick “Energy Star” upgrade.

Click the image for a larger version. (The dials on the right are, from top to bottom, address-high-byte, address-low-byte, and data.)

The Z80 computer now has its name in lights! (See video; I apologize in advance for the horrible low resolution and mp4 artifacts.)

Dr. Rosen and I got approval to buy an Alpha 215R single-line LED display as a demo for the Z80 computer. While it was on the way, I built a second peripheral for the Z80: a specialized RS232 port that accepts strings and reformats them for output to the LED display. (It’s an interesting challenge to build a peripheral based solely on information from a datasheet, without having the actual device available.)

When I finally got to work with the display on Thursday, the interface actually worked — but various electrical gremlins were causing all sorts of problems, when the Z80, interface, control panel, and sign were all connected. The problems eventually turned out to be mostly due to insufficient voltage — as it turns out, 5.5V (rather hot for TTL circuits) at the power supply ends up being a nice, clean 5.0V to 5.1V at the Z80. Those 40-pin ribbon cables keep finding new ways in which to be a Dumb Idea(tm)…

At any rate, it started running very reliably Friday afternoon. The bad news was that this meant that it was time to start programming it. Writing Z80 code to do a demonstration script isn’t too bad; a bit tedious to enter the strings, but still fun.

Toggling some 200 bytes of machine code in, byte by byte, however, is mind-numbingly boring — even if (like me) you actually enjoy programming in assembler. (Note to self: there has GOT to be some source for knobs for those rotary switches SOMEWHERE!)

The script is running, though. Hopefully it will prove inspiring to the EET325 students. (How many college courses out there lead you through building your own computer, chip by chip and wire by wire?)