© Mark Ollig
In 1976, Sol-20 revolutionized personal computing with its self-contained design, making computing accessible to everyone.
While small microcomputers such as the build-it-yourself Altair 8800 were built with switches and blinking lights, a new fully-assembled model with a built-in keyboard and attachable display monitor marked a dynamic shift toward user-friendly home computers.
Lee Felsenstein designed the Sol-20 microcomputer with the help of Bob Marsh and Gordon French.
They were all important figures at the Processor Technology Corporation and members of the Homebrew Computer Club in Menlo Park, CA, where Felsenstein served as the acting president.
The Homebrew Computer Club was active from 1975 to 1986 and included notable members Steve Jobs and Steve Wozniak, co-founders of Apple, and Bill Gates, co-founder of Microsoft.
The Sol-20 was a personal, independently operating microcomputer, a departure from the era of remote data terminals connected to large mainframe computers like the IBM System/370.
At the 1976 Personal Computing Show in Atlantic City, NJ, the Sol-20 received positive feedback and garnered much attention.
Popular Electronics magazine’s July 1976 cover featured the Sol-20 computer and dubbed it as a “highly intelligent terminal.”
Processor Technology, based in Emeryville, CA, manufactured and sold the Sol-20.
This computer featured a sleek blue metal case, optional walnut side panels, a full-sized keyboard, a power supply, and a cooling fan.
The Sol-20 could be purchased either as a kit for $995 or fully assembled with a monitor for $1,495.
Under the computer’s hood, there was an 8-bit Intel 8080 microprocessor with a clock speed of 2 MHz.
It also included a S-100 bus with five expansion slots (a popular interface used with microcomputers), along with serial, parallel, and cassette ports.
A model called the Sol-10 was available without the S-100.
The Sol-20 reportedly shipped with base configurations ranging from 8 KB to 48 KB of RAM.
While these RAM numbers changed throughout its production run, even a minimal configuration, such as 1 KB to 2 KB, was significant in the late 1970s.
The Sol-20 computer could be expanded to a whopping 64 KB via S-100 printed circuit boards.
The S-100 bus expansion allowed users to add memory, graphics, floppy disk drives, printers, and modems.
A few years earlier, Lee Felsenstein designed the PennyWhistle 103, which was one of the first modems designed for computer hobbyists.
It was a 1200-baud acoustic coupler modem that could connect to other computers or community dial-up computer bulletin board services.
The PennyWhistle 103 transmitted and received data over telephone lines using a standard telephone handset and was priced at $109.95 plus $2.50 for postage and handling.
Before affordable hard drives and modern operating systems, early Sol-20 versions relied on non-volatile memory (NVM) read-only memory (ROM) plug-in modules.
NVM ROM is a type of computer memory that stores data permanently on a chip using binary code containing firmware or software that the user can’t modify.
These modules retain data permanently, even when the power is off, unlike volatile memory.
They also contain programming that initiates a computer’s startup process every time it is turned on (analogous to a boot-up).
These modules provided necessary instructions for starting the computer and controlling the keyboard, display, and cassette interface.
The Sol-20 computer also relied on cassette tapes for program input and data storage.
It utilized the Kansas City Standard (KCS) for encoding, a design intended to store digital data on inexpensive cassette tapes for early microcomputers.
People connected a regular cassette tape recorder to the computer’s cassette port to save data and load programs.
The Sol-20 computer’s five S-100 bus slots were used for expansion options like memory, graphics, audio, storage and memory devices, and printers.
Storage formats included eight-inch disk floppies and the smaller 5.25-inch minifloppies, as they were called then.
One popular peripheral expansion option for the Sol-20 was the Helios II Disk Memory System, which features dual eight-inch drives.
The Helios II Disk Memory System typically uses single-sided, double-density (SSDD) eight-inch diskettes, with each holding an average of 384 KB of data.
The Sol-20 computer’s cassette interface supported both the KCS 300-baud rate and the Computer Users Tape Standard (CUTS), with its optional 1200-baud mode (note: in this instance, the baud rate is equivalent to bits per second).
Games for the Sol-20 included a race-driving game, the action game Target, backgammon, Trek-80 (a text-based space adventure inspired by Star Trek), and GAMEPAC 1, an arcade compilation featuring Pong, chess, and checkers.
Users would input programs into the Sol-20 by loading it from pre-coded cassette tapes, purchasing commercial modules, or manually typing in programming code usually found in computing magazines.
Although nearly 12,000 Sol-20 computers were sold from 1976 to 1979, Processor Technology ended its production in May 1979 due to increased competition in the rapidly-evolving computer industry.
The Sol-20 microcomputer model served as a stepping-stone for the new personal computing enthusiasts, programmers, and engineers.
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| The Sol-20 microcomputer (PC) from 1976. |
