Last Updated on March 1, 2026 by Dave Farquhar
Processor Technology Corporation was a personal computer company founded in April 1975, by Gary Ingram and Bob Marsh in Berkeley, California. Their first product was a 4K byte RAM board that was compatible with the MITS Altair 8800 computer but more reliable than the MITS board. They followed it up with a series of memory and I/O boards including a video display module.
Popular Electronics magazine wanted a feature article on an intelligent computer terminal and Technical Editor Les Solomon asked Marsh and Lee Felsenstein, both members of the legendary Homebrew Computer Club, to design one. Popular Electronics featured it on the July 1976, cover and it became the Sol-20 Personal Computer. Processor Technology shipped the first units in December 1976 and the Sol-20 was a very successful product. However, the company never developed next generation products to succeed the SOL-20, and ceased operations May 14, 1979.
Felsenstein went on to design the Osborne 1 portable computer.
The Processor Technology Corporation SOL-20
Bob Marsh, Lee Felsenstein and Gordon French started designing the Sol-20 between April and July 1975. The Sol-20 used the Intel 8080 8-bit microprocessor chip, running at 2 MHz. The major difference between the Sol-20 and most other machines of the era was its built-in video driver. This allowed it to be attached to a composite monitor for display instead of using a separate terminal.
Inside the case, the Sol-20 had its main motherboard mounted at the bottom containing the CPU, memory, video display, and I/O circuits. A five-slot S-100 cage sat next to the motherboard for expansion. The case also had a power supply, fan, and keyboard mounted inside. The SOL-20 case was blue, an imitation of the color of blue IBM used, and the sides of the case were made of solid oiled walnut they acquired inexpensively from a gun stock manufacturer.
Processor Technology manufactured approximately 10,000 Sol-20 personal computers between 1977 and 1979. Purchasers could buy all Processor Technology products either fully assembled, or as electronic kits. Processor Technology also sold software on compact cassette. One side of the tape contained the software in CUTS format, and the other side in Kansas City standard format. Gary Ingram and Steven Dompier wrote the original software utilities. Lee Felsenstein wrote the original user manuals as a contractor.
Other products
Processor Technology’s S-100 bus boards were meant to be compatible with the Sol-20, but they also worked with other S-100 systems.
The Video Display Module 1 (VDM-1) was the original video display interface for S-100 systems. The board generates sixteen 64-character lines of upper and lower case typeface with a composite output, suitable for display on any standard composite video monitor or a modified TV set. Utilizing a 1K segment of system memory, the VDM-1 provided memory-mapped I/O for high performance. It also included hardware support for scrolling. The VDM-1 Video Board was a great improvement over using a teletype machine or a serial attached terminals, and became popular with owners of other S-100 bus systems such as the IMSAI 8080.
Another popular product was the CUTS Tape I/O Interface S-100 board. The CUTS board offered standard interface for saving and reading data from cassette tape. It supported both the Kansas City standard format, as well as their own custom CUTS format. Lee Felsenstein was key participant of the development of Kansas City standard format, which was the first cross-system data transfer standard for microcomputers.
David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.