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IMSAI 8080

Donor: (Looking it up...)
Location: Williamsburg, Virginia

Think of the Imsai as the Altair's better looking and more popular younger cousin. Generally accepted as the second personal computer, after the Altair, the Imsai was much more popular. It also received more fame, with a starring role in the seminal movie War Games.

Visually, nothing beats those funky red and blue toggle switches. As you can see in this particular model, the switches are, in actuality, just plain old metal toggles. The red and blue plastic just snaps over the existing switches. While it's tempting to think that the plastic covers add no functional value, they do. It's much easier to tell which switches are on or off, which is pretty important when that's your main way of entering information!

This particular model has seen some obvious wear, including the loss of several toggles. But it did clean up quite nicely. The dual disk drive unit that came with it, however, has been a bit more of a challenge. Some sort of animal or insect took up residence in it at some time. I've been trying to clean it out, but if I spray air in the front, I'll just push the debris in further. The strong metal casing prevents me from blowing air into the rear of the drives. And the screws holding the drives in the unit casing are rusted tight. I'm still working on getting it opened up and cleaned out. But this might take awhile, as I certainly don't want to damage it while trying to clean it.

Earlier comments

Owner: M. Friese
Location: Orange, California (where it's been for quite awhile)

This is what early personal computing looked like. The IMSAI found a nitch with computer hobbyists that actually wanted a computer that worked. The basic unit came with a front panel, a 8080A CPU card, a six slot motherboard with two connectors. No RAM. No place to plug in a RAM board either. During 1976-1977, I built 22 of these IMSAI kits. I purchased this computer as a kit in 1977. I went for the optional 22 slot motherboard and 22 sockets.

The inside of the IMSAI. This unit is somewhat unusual because all of its aluminum sheet metal is black anodized. Most came with gold iridite. From the right, we have:

  • Front panel board
  • CPU card with ribbon cable to the front panel
  • Wirewrap RAM/ROM/startup board
  • Wirewrap development board for the Western Digital WD2412 time of day clock chip
  • 64K RAM board
  • Time of day board
  • Wirewrap hard disk interface (hooked up to a Western Digital WD1000 and 5 meg Seagate)
  • Tarbell floppy disk controller (hooked up to two 8" single sided floppies)
  • Wirewrap printer controller
  • Tarbell cassette interface
  • 3P+S Serial interface for printer
  • 3P+S Serial interface for terminal
  • VDM-1 Video interface

This is the CPU card. The big white chip is the CPU. It ran at 2MHz. It used a clone CPU chip, the NEC 8080A. This is not the original CPU card. This came out of a BYT-8 computer. I changed the CPUs because this one had an interrupt controller.

This board holds 64K of dynamic memory. Due to conflicts with my ROMs, I only had 48K enabled.

Very popular single density floppy controller based on the Western Digital 1771 controller. It was made by Tarbell, the maker of the legendary cassette interface. Stored 250K bytes on an 8" floppy. Used a CP/M standard 6:1 interleave. I made a special boot disk with 1:2 interleave. Much faster!

The legendary Tarbell cassette tape interface. Recorded at 1500 bits per inch on a regular cassette recorder from JC Penneys.

Processor Technology 3P+S interface card. Provided serial interface up to 9600 baud. It also provided 3 parallel ports that nobody ever used. I used one parallel port in this machine to run a stopclock.

Processor Technology VDM-1. One of the very first video interface boards. It had all of 1K of VRAM. It displayed 16 lines of 64 characters. It was very fast, and even faster when scrolling due to its scrolling hardware. The contents of this board were used in Processor Technology's SOL computer.

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