Micro-Expander Model 1

Expander image


The Micro-Expander Model 1 is a Z80-based S-100 bus computer system that is in some ways similar to the Processor Technology Sol-20. This is no coincidence - the Expander, like the Sol, was designed by Lee Felsenstein. In an interview with Kip Crosby of CHAC, Lee Felsenstein had the following to say about the Expander:

Berkeley, CA, circa 1980

...I took on a walk-in client, a Swedish guy, who wanted to design and build something that looked like an Apple II, but ran CP/M, had a Z-80 in it, and would also handle Radio Shack graphics. I should point out that by the fall of 1977, the SOL design was imitated fairly well by the Radio Shack TRS-80 Model I, although it didn't use an S-100 bus. It used dynamic memory for storage and, I think, static memory for the display, but it was the same memory space, which was important, because they were still doing a basic memory access to get something on the screen.

John French, who designed that - if I'm remembering his name correctly - credits the SOL as his inspiration, and I'm happy enough with that. That's what happens when you do a design that has legs. So Radio Shack-style graphics were one thing this Swedish guy, Mats Ingemanson, wanted to include along with color - European color, PAL. We called the computer the Expander. I bashed my way through that. Eventually I charged too little money for it and turned it over to Bob Marsh to try to get it into production. About 200 of them were produced, but they were never paid for, so only a couple of them got out and I don't know that the European color on it ever really worked.

from CHAC's Analytical Engine 3.2, February, 1996


The Expander is based on the Zilog Z80 microprocessor. The main system unit is a single piece with an integrated Swedish keyboard. The power supply is an external unit containing the power supply, rectifiers, and filter capacitors. The linear voltage regulators are located inside the main system unit. Display circuitry is integrated on the mainboard, which contains 2 kilobytes of static RAM for the display. Video output is 80x24 NTSC composite color, with PAL selectable by changing DIP jumper blocks on the motherboard. There are serial and parallel I/O ports as well as a cassette interface integrated into the system, and 5 S-100 slots are available for adding memory and peripherals.
internal image

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