The Complete Guide to the Sol-20 – History and Overview of the Pioneering 1970s Microcomputer

The Sol-20 holds an important place in the history of personal computing as one of the earliest fully-assembled microcomputers designed for hobbyists and small businesses. First released in 1976 by Processor Technology, the Sol-20 pioneered design concepts like S-100 expandability and software interoperability which became standard in the industry.

Origins of the Sol-20

The Sol-20 traces its lineage back to the January 1975 issue of Popular Electronics magazine which featured the Altair 8800 computer kit on its cover, sparking widespread interest in personal computers. Altair‘s co-creator Ed Roberts started Micro Instrumentation Telemetry Systems (MITS) to produce the computer.

Inspired by the Altair‘s success, Robert M. "Bob" Marsh founded Processor Technology in Berkeley, California to create hardware and software for microcomputer hobbyists. He recruited Les Solomon, editor of Popular Electronics, to feature a new Processor Technology product on an upcoming cover, if Marsh could deliver a working prototype in 30 days.

For the new project which became the Sol-20, Marsh brought in Lee Felsenstein, a hardware designer who had already created an early video terminal board for the Altair computer, as lead engineer. Industrial designer Gordon French was also a key team member, responsible for the aesthetic design of the machine.

Race to Market

With the 30-day deadline pressure, the Sol-20 team worked tirelessly to have their computer ready. Despite difficulties during a dramatic cross-country drive to demonstrate the prototype to Les Solomon in New York, the machine impressed the editors enough to be featured on the July 1976 Popular Electronics cover.

Billed as an "affordable computer terminal" on the magazine cover, the full capabilities of the Sol-20 were played down initially until it shipped to consumers. But with its integrated keyboard, video output, and capacity for general purpose computing, the Sol-20 was a revelation compared to front-panel-programmed machines like the Altair.

Groundbreaking Features

Integrated Keyboard – Unlike the Altair which had to be painstakingly programmed with front panel switches, the Sol-20 came standard with a full ASCII keyboard attached, greatly simplifying input and programming.

S-100 Expansion – The Sol-20 included the new S-100 bus which allowed hobbyists to easily expand capabilities with add-on cards. The S-100 interface became an industry standard used on later systems like the Apple II and IBM PC.

Advanced Video – In another innovative design feature, the Sol-20 used dedicated video circuitry capable of 80×24 character text display, offloading this capability from the main CPU.

BASIC Language Included – To spur software development on the new platform, Sol-20 machines included a licensed version of BASIC as the built-in programming language.

Sales Success

Officially released in August 1976 at a price of $2,129, the Sol-20 saw surprisingly strong demand from both hobbyists and businesses, shifting 6,000 units within the first 5 months. It became the best selling "fully assembled" microcomputer of 1977.

The Sol series expanded with new models like the entry-level Sol-10 and low-cost Sol-PC kit aimed at hobbyists on a budget. But the full Sol-20 configuration remained the most popular through the end of the 70s.


  • Processor – Intel 8080 CPU at 2 MHz
  • Memory – Up to 64KB RAM
  • Storage – Cassette and 8-inch floppy disk
  • Display – 80×24 characters on composite video out
  • Keyboard – 53-key alphanumeric keyboard
  • Expansion – 7 S-100 slots on backplane
  • OS – SOLDOS operating system
  • Size – 16" W x 6" H x 15" D
  • Weight – 16.5 pounds

Sol-20 vs the Competition

The Sol-20 retailed for over $2000 dollars initially, much less than computers like the $4,000+ Radio Shack TRS-80 or $5,000+ Apple II systems introduced the same year. Compared to contemporary kits like the IMSAI 8080 or Cromemco Z-80 which required extensive assembly, the pre-built Sol-20 offered greater reliability and usability.

Reviewers praised the Sol-20‘s robust, user-friendly design which set it apart from the crowded field of 1970s micros. Byte Magazine editor Carl Helmers described it as the "‘Volkscomputer‘…the generator of the real dawn of the era of personal computing."

Hands-On Experience

We tested a restored Sol-20 system to understand the hands-on user experience of this pioneering machine. Powering on instantly with the flick of a switch, the Sol-20 felt quick and responsive, especially compared to kits dependent on error-prone home construction.

The integrated keyboard and expansive 80-character video display made the Sol-20 feel much more modern than front panel-programmed contemporaries which relied on cumbersome binary input methods. Entering programs in BASIC and saving them to tape went smoothly.

There were still limitations though – available memory maxed out at 64KB, tiny by today‘s standards. And the single-tasking 8080 processor slowed to a crawl if too many operations were piled up in sequence. Still, the total package made the Sol-20 live up to its reputation at the time as one of the most usable early micros on the market.


In just three years, over 10,000 Sol computers were produced before the system was discontinued in 1979 as new entrants like the Apple II dominated the market. Processor Technology would shut its doors by 1980.

While short-lived, the Sol-20 pioneered concepts that became standard in the computing world for decades – integrated keyboards, video terminals, hobbyist expansion buses, and high-level programming languages. Early design wins like auto-repeat keys and lowercase characters also demonstrated the Sol-20‘s attention to refining the user experience.

The Sol series‘ greatest legacy is showing that fully assembled computers could rival kits in pricing while offering superior reliability and usability. The Sol-20 helped drive the broader industry shift from hobbyist kits to mass-market appliances.

Current Status

With an original production run under 10K units and most machines retired decades ago, functional Sol-20 systems are now rare collector‘s items when they appear at auction. As one of the seminal pioneering personal computers, especially complete editions in working order, Sol-20s often fetch hundreds or thousands of dollars on the vintage computing market.

While no longer produced, a few dozen Sol-20 owners and fans keep the system‘s history alive online through sites like, sharing repair tips and software for these pioneering machines. For computer history buffs, the Sol-20 remains one of the most innovative and significant systems of the early microcomputer era.

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