Z-80 Homebrew Computer - Dirt Cheap Bootstrap Loader
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The inspiration for this was an article in Byte magazine around '79 or so called "Build a Dirt Cheap Booststrap Loader", or somesuch. I've been struggling to get a complete homebrew computer running and documented since about that time. The original Byte magazine article used a printed circuit board that had been scored. The squares were used with a S/R latch to tap out instructions one at a time and load them into an 8085 system using a single-step signal. Load up an instruction to write the next fetch to memory, single-step, load up the bus with the data, single-step again, and the 8085 would load your byte into memory. All you needed was eight bits in, and the single-step function.
Now, the beautiful thing about the Z-80 is that you can bring the BUSREQ line (pin 25) low, and the Z-80 will essentially disconnect itself from the bus and put a low out on the BUSACK line (pin 23). All you have to do, then, is program the RAM/EEPROM directly. At one point (1984), my only electricity was via a motorcycle battery and a solar panel. I'd recharge the battery during the day while I worked selling CP/M and PC clones. I would use just a single ACE Superstrip, a Z-80, an 8 bit latch (probably a 74ls374), some LEDs/330 Ohm resistors, and a 6116 to bring up a functioning microcomputer. I could bring BUSREQ low, and simply wire the address/data, and program the memory by bringing RD low.
This device is a little more sophisticated, but not much. When I built it I had a $10 budget and a few spare parts:
You tap out the data bits using set/reset touch posts on the device in the lower part of the picture, and latch the byte. You can see the red wire sticking out that is used to tap out the data. I do have an increment address using cascaded TTL counters. Here is a schematic of my front-panel-on-a-budget (click for a larger version):
This is connected to the homebrew via a db-25 connector:
A0-10 = 1-11
D0-7 = 14-21
RD = 22
WR = 23
BUSACK = 24
BUSREQ = 25
This goes straight through to the corresponding pins on the Z-80.
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