Salamis Tablet - Journal of Computer History


SCELBI, Altair and the Journey to Home Computing

For the first decades of the computer age, the general public largely stood on the outside looking in. Those with an interest could read about computers in books and magazines, but by and large had no direct access to the new technology. Unlike other electronic wonders such as telephone, radio, and television, computer technology did not readily or rapidly translate into consumer products.

Although they were steadily becoming cheaper to manufacture, by the mid-1960s computers were still prohibitively expensive and remained in the realm of governments, universities and businesses. For those workers and students who did have access, their use was strictly controlled.

But behind the scenes, a loose knit fraternity of electronics enthusiasts was galvanizing around the mission of building their own computers. These hobbyists were driven by the desire to own a machine that was theirs to experiment with, and the intellectual challenge of the seemingly impossible task. They formed clubs to share information and equipment, started their own newsletters, and avidly devoured the electronics hobby magazines of the day.

When the first microprocessors became available in the early 1970s, it was this disparate group of engineers and enthusiasts who stood ready to integrate the new devices into their framework. The hobby computers they brought to market were an entirely new class of machine. The hardware profiles, software paradigms and even the business models that coalesced in just a few short years, created fortunes and shaped the personal computer industry well into the twenty first century.

The earliest independent experimenters had a steep hill to climb. To begin with, there did not exist anywhere in any form a complete end-to-end reference design for an aspiring builder to use as a starting point. Hobbyists needed to master a diverse range of disciplines which spanned the mathematical and logical theories of electronic computation, the electrical engineering knowledge to support fickle devices like vacuum tubes and early transistors, and the technical skills to actually build the machines. Beyond that, necessary components were difficult to find and expensive to buy, and used equipment was generally undocumented and frequently unreliable.

Stephen Gray, a computer columnist and early builder, founded the Amateur Computer Society (ACS) in 1966 primarily to provide a venue for experimenters to share knowledge. He wrote letters to various electronics trade journals and hobby magazines inviting interested experimenters to join the society. In fairly short order the ACS took shape with over 100 members from around the world. Their ranks included engineers from major technology companies such as IBM, GE, Westinghouse and Bell Labs, along with college students, high school teachers, and university professors.

The ACS was active until 1976. Its main focus was the ACS Newsletter. The newsletter featured information about key skills and technologies, teletypes and other peripherals, integrated circuits (and later microprocessors), kits, and write-ups of members' computers. It also provided information on contacting manufacturers for schematics and other technical details, sources for new and surplus parts, and surveys of the latest books and articles. Members used the newsletter as a forum to sell and trade parts, and to ask questions.

Although small in number, the ACS membership was visible and influential. Gray himself would go on to be managing editor of Creative Computing, one of the first national magazines dedicated to personal computing. Don Tarbell, an early and active contributor, founded Tarbell Electronics manufacturing computers and components and best remembered for the Tarbell Cassette Interface a defacto tape interface standard for many years. Don Lancaster was an engineer and author of books and magazine articles, best known for his TV Typewriter design, a do-it-yourself computer terminal, published in the September 1973 issue of Radio-Electronics magazine, and the book TTL Cookbook published in 1974. John Titus designed the Mark 8 computer, a landmark home computer project featured on the cover of the July 1974 issue of Radio-Electronics.

Jim Sutherland was an ACS member and Westinghouse engineer whose 15-minutes of fame came early. Sutherland designed and built the Electronic Computing Home Operator (ECHO) IV which was featured in a four-page article in the April 1968 issue of Popular Mechanics. Built largely from obsolete parts purchased as scrap from Westinghouse. The machine had 17 machine language instructions and an 8K core memory. It occupied about 20 square feet of basement floor space. Input was by keyboard, and Sutherland had 16 keyboard access points scattered throughout the house.

Although at the time of the article the ECHO IV had only recently been completed, Sutherland and his wife Ruth described its many potential applications and the programs they planned for such functions as helping out with household budgets and shopping lists, reminding Sutherland of birthdays and anniversaries, and controlling the temperature of the house.

Outside the small, highly driven circle of the ACS and later organizations, was a group of more generalized electronics hobby publications. With names like Wireless World and Radio-Electronics, these magazines had their roots serving the needs of ham radio enthusiasts, but had long since opened the aperture to encompass the wide range of their readers' interests. They featured articles on new technologies, detailed project plans, and plenty of advertisements for parts, kits, and equipment.

Beginning in August 1967, the long running British publication Wireless World ran a five part series on building and programming a computer. The machine was designed by staff member Brian Crank. The parts required included 400 germanium switching transistors, 66 silicon transistors, 450 diodes, 1,800 resistors, over 300 capacitors and numerous other bits such as switches and lamps. The resulting computer had 28 instructions for arithmetic and moving data in and out of the storage register. It was programmed via a bank of switches and output to a bank of lamps.

The Wireless World Computer was designed to be an inexpensive machine for educational purposes. To keep costs low, it specified "reject" germanium capacitors among other intentionally affordable solutions. Crank estimated it would cost 50-60 British Pounds without cabinet (perhaps $160). With several thousand solder connections and sub-premium parts, the Wireless World Computer would have been extremely challenging to build and troubleshoot. It is unknown exactly how many people undertook the endeavor, but it is documented that multiple machines were completed by schools and individuals.

In the early 1970s things moved very quickly in semiconductor manufacturing. First introduced in by Sylvania in 1963, transistor-to-transistor logic (TTL) integrated circuits (ICs) were becoming common fare. Texas Instruments developed a full line of digital circuits that became the defacto standard in the industry, with multiple manufacturers producing interchangeable, compatible parts. Generally referred to as the 7400 series because the part numbers all begin with 74, the new breed of ICs gave engineers a wide range of prepackaged logic circuits to include in designs.

At $20-$50 each, the first TTL chips were too expensive for most home user projects. But improvements in manufacturing techniques and general economies of scale drove TTL prices down rapidly. Between 1970 and 1972 alone, TTL prices dropped by 75% or more depending on the part. By 1973 basic building block ICs with four logic gates were available for $0.49 at Radio Shack and as low as $0.25 in single quantities from mail order houses.

The commercialization of digital logic ICs turned the corner on one of the home builder's most vexing problems: the virtually overwhelming number of parts and interconnections of a transistor computer construction. Consider Texas Instruments' SN7400 IC. This namesake of the 7400 series puts four digital logic circuits called NAND gates into a single 14-pin package. It includes 16 transistors, 24 diodes, and 28 resistors. With this circuit density, the number of solder connections for a rudimentary computer was a small fraction of the thousands that would be required for a transistor-only machine such as the Wireless World Computer.

The dramatic reduction in parts count and complexity of construction made the prospect of designing and selling computer kits and educational computers much more practical. In 1971, the National Radio Institute (NRI) produced the Model 832 NRI Digital Computer. The Model 832 was a "trainer" that students completed as part of a $538 correspondence course. A subsidiary of McGraw Hill Education, NRI would run elaborate two-page ads in Radio-Electronics, Popular Electronics and other magazines. The kit was designed by ACS member Louis E. Frenzel. The Model 832 was built from 74 digital logic ICs mounted in sockets. It was programmed in machine language, had 15 instructions and memory for 32 8-bit words. Input was by switches on a front panel and output was by lamps.

At the same time that prices were dropping on basic logic chips, the semiconductor industry was steadily increasing the number of components that could be included in an integrated circuit. In March 1970, Texas Instruments released the SN74181N (generically the 74181), a complete 4-bit arithmetic logic unit (ALU) on a chip. The ALU could perform 16 arithmetic functions and 16 logic functions. It was made up of 75 logic gates with a transistor count of 240. The initial price for the SN74181N was $16.50 in quantities 100-999.

In April 1972, just two years after the release of the 74181, Intel announced the 8008 microprocessor. Made up of about 3,500 transistors, the 8008 was a full 8-bit central processing unit (CPU) on a single IC. While a good number of supporting ICs were still required, the 8008 itself included the ALU along with decoding control for 48 instructions, an 8-bit input/output bus, seven 8-bit registers, and an 8-level stack to support subroutines. It could also address 16K of RAM or ROM memory.

SCELBI Computer Consulting began advertising the SCELBI 8-H computer in the March 1974 issue of ham radio magazine QST. Built around the Intel 8008, the SCELBI 8-H was available in kit or fully assembled form. Hobbyists could order a set of circuit boards for $440, with complete kits starting at $695.

SCELBI founder Nat Wadsworth was a young engineer at General DataComm Industries when he attended a seminar on the 8008 in late 1972. "There was no doubt in my mind that chips of the revolutionary type represented by the pioneering 8008 would be the wave of the future," he'd recall later in an essay describing the first days of SCELBI. "Indeed I was completely enthralled by the exciting potential of digital computing at the time."

Wadsworth was the enthusiasts' enthusiast. Born in 1943, he had his ham radio license by age 14. In high school he ran his own business assembling Heathkits for others. He dropped out of high school to join the Navy, where he earned his high school equivalency. He would later go on to earn a BSEE from the University of Connecticut, where he met his long-time business partner Bob Findley.

In 1965 Wadsworth was exposed to computers for the first time working with electro-mechanical devices at Bunker Ramo. He became so interested in computing that he bought a used DEC PDP-8 and had it installed in his home.

After learning about the 8008, Wadsworth's first inclination was to use the new microprocessors to improve designs at General DataComm. But when he failed to spark management's interest he turned his attention to the design of a home computer kit. Wadsworth, Findley and a third colleague pooled their resources to acquire the necessary chips and set to work developing a prototype. They ordered 100 circuit boards, drilling the holes by hand, and Wadsworth developed the software necessary for programming the 8008 using his PDP-8.

The SCELBI platform was notably complete. The system was built up from a modular series of printed circuit cards which were inserted into an 8-slot backplane. The five basic cards which made up the core system included CPU, output, input, RAM and a card to manage the front panel.

Like the Wireless World and NRI projects, users interacted with the SCELBI-8H via front panel switches and lights. However, SCELBI offered a line of add-on boards to enable all manner of input and output. An oscilloscope interface ($225) let customers output 8 lines of text on an oscilloscope. Also available were a teletype interface with or without paper tape control ($75/$50), and an audio cassette tape interface ($125) which allowed customers to save and retrieve programs and data using a standard cassette recorder. SCELBI also sold software, mostly utilities for using the computer itself. Code was available as listings, or for an extra fee SCELBI would load the programs onto paper tape or magnetic tape.

Although the SCELBI-8H and later SCELBI-8B were well received and well regarded among amateur builders, SCELBI did not stay in the manufacturing business long. By early 1976 the company had stopped producing computers and shifted its primary focus to software and books. Wadsworth's health certainly was a factor in SCELBI's ability to capitalize on its early move into microcomputer kits. He suffered two massive heart attacks, at ages 29 and 30, while the first kits were going into production. His second heart attack, in May 1974 with ads already running, put him out of commission for five months. Findley was left fielding as many as 1,000 inquiries a week while assembling boards and shipping orders.

Just four months after the first SCELBI ad ran, and while Wadsworth was still recovering, the cover of the July 1974 issue of Radio-Electronics pictured the Mark-8 computer. The Mark-8 was designed by Jon Titus, then a graduate student at Virginia Tech. The Mark-8 was also based on the 8008.

In contrast to SCELBI, Mark-8 was not a kit, but a project that readers could undertake. The Radio-Electronics article gave readers a high level view of the computer. For $5.00, readers could order directly from the magazine a 52-page package of instructions, schematics and information on programming. Titus had also arranged for a company to have unpopulated circuits boards available for $47.50. But aspiring builders were confronted not only with the challenge of soldering the boards, but of procuring components from a variety sources.

The Mark-8 attracted considerable attention among hobbyists. Several hundred of the circuit board sets and about 5,000 instruction packets were ordered. The Mark-8 Users' Group was established by Hal Singer operating out of the Cabrillo Computer Center at Cabrillo High School in Lompoc, California. The group was soon renamed "Micro-8" as it broadened its scope to include other 8008-based computers, add-ons and projects. The Micro-8 newsletter, first issue September 1974, had a national audience and published extensive information on all aspects of working with the 8008.

Among those who took notice of the Mark-8 was Art Salsberg the editorial director of Popular Electronics, a chief competitor of Radio-Electronics. When the Mark 8 hit, Popular Electronics already had a project in the works for computer trainer, which was also based in the 8008. Salsberg tabled the trainer and sent his team in search of a project that would outshine the Mark 8.

Popular Electronics technical editor Les Solomon, knew that Ed Roberts, owner of Micro Instrumentation and Telemetry Systems (MITS), was working on a project that could potentially fit Salsberg's vision. MITS was an established manufacturer of kits sold mail order to hobbyists through magazine advertisements. Roberts already had some history of successful collaboration with Popular Electronics. The November 1971 issue had featured Roberts' kit for an electronic desk calculator. The kit was described with good detail in the magazine and readers could order the calculator in kit form for $179 or fully assembled for $275.

Both kits and assembled calculators were hugely successful. MITS produced increasingly capable calculators for the next few years, at one point reaching $100,000 in monthly sales. However, as quickly as MITS' calculator business had shone, it became eclipsed by larger corporate entities as the calculator market matured and became commoditized.

The changing fortunes of the calculator market had left MITS in pretty serious financial trouble. Roberts was already staking his company on evolving MITS kits to the next technology level, and had a vision for a complete computer project. He was more than happy to work with Salsberg and Solomon to try for a repeat of the successful tie-in to a Popular Electronics article.

What followed were several high pressure months, as Roberts and his small staff worked to complete the design and develop a prototype in order to meet Salsberg's deadlines for the January 1975 issue.

Roberts' design, the Altair 8800, would be based on a new Intel microprocessor, the 8080 which had been announced in April 1974. The 8080 brought some critical improvements over the 8008. It could directly address more memory, 64K vs 16K, it had a larger instruction set, and more flexibility in the architecture of its registers. Most significantly for the growing momentum in hobby computers, it was much easier to interface with and required fewer supporting ICs to build a basic system.

Hobbyists and engineers were well aware of 8080 and anxious to get their hands on a system which used the new chip. Unfortunately, its retail price of $360 was three-times the price of the 8008. Roberts was able to negotiate a deal with Intel to purchase the chips in bulk for $75 each. The record is a little unclear as to how Roberts acquired the chips so inexpensively. In some tellings, it was simply business savvy and negotiating skill while in other versions the chips had cosmetic defects. There was even some unsubstantiated speculation at the time that the 8080s in the Altair were chips rejected by Intel because, although functional, they did not meet full specifications for speed or temperature performance. Regardless, Roberts' pricing coup would enable him to price the barebones kit at $397, selling a full computer for a modest premium over the retail price of the 8080 chip alone.

The Altair 8800 was the feature story of the January 1975 issue of Popular Electronics pictured under the headline "Project Breakthrough - World's First Minicomputer Kit to Rival Commercial Models." The Popular Electronics article is almost universally viewed as a watershed moment in the history of personal computing. The reaction among hobbyists was virtually immediate. Orders for machines began coming in immediately. By the end of February more than 2,000 Altairs had been ordered, and by mid-April the number had reached 4,000.

Given its historical significance, the Altair 8800 is probably as notable for what it lacked as for what it brought. Like its immediate predecessors, the basic Altair used toggle switches for input and blinking lights for output. Programming was a tedious and error-prone process of entering machine language code in binary form one line at a time. As with SCELBI, an expansion card system promised the ability to supplement the 256-byte memory and add features such as a monitor display and keyboard.

Adding to the challenges for early Altair owners, MITS was overwhelmed with the response to the Popular Electronics article. Although assembled computers and accessories were advertised, for many months MITS only shipped kits. Building an Altair from a kit required considerable skill, and even an experienced hobbyist was likely to face some frustrations.

The burning desire of hobbyists to join the computer revolution, MITS' pricing breakthrough, and the hyperbolic enthusiasm of the Popular Electronics article were more than enough to overcome any headwinds caused by supply problems and the technical hurdle of actually getting an Altair up and running. If anything the hobbyists were energized by the challenges. Enthusiasm for the Altair brought a new wave of computer clubs, the first dedicated personal computing magazines, and countless entrepreneurial ventures aimed at providing expansion cards, accessories, software and services for Altair users.

The iconic Homebrew Computer Club held its first meeting March 1975 in Gordon French's garage in Menlo Park, California. The star attraction was one of the first Altairs shipped. In attendance was Steve Wozniak who would soon go on to design the first Apple computer and co-found the Apple Computer Company. The Homebrew Computer Club championed the free exchange of ideas and the quest to develop affordable computing for the general public. Its biweekly meetings featured formal presentations followed by informal swap meets where engineers would buy and sell components often of their own design. The organization became a focal point for the Silicon Valley personal computer industry, and its membership counted some of the most influential thinkers of its time.

Of the many engineers and entrepreneurs who recognized the sea change represented by the appearance of the Altair, one pair in particular stands out. Bill Gates and Paul Allen had been friends and business partners since high school. When they saw the Popular Electronics article they immediately contacted Roberts to pitch a custom version of the BASIC programming language. Gates and Allen argued that the availability of a programming language would broaden the appeal of Altair and make it more accessible to hobbyists who might be struggling with machine-level code. Roberts agreed, and in April 1975 Gates and Allen officially founded Micro-Soft (later Microsoft) to develop the BASIC interpreter. MITS was soon offering Altair BASIC on paper tape or cassette for $200 - discounted to $75 for those who purchased the memory upgrade board needed to run the software.

For a variety of reasons, MITS itself was never able to keep pace with the enthusiasm created by the Altair. The firm struggled to expand and fill orders while simultaneously working to bring new peripherals to market and develop the next generation Altair. In May 1977, Roberts sold MITS to the Pertec Computer Corporation, a California company that had seen some success selling disk drives to Altair owners. Pertec was oriented to business users and after introducing several Altair versions of its own eventually phased out the line.

Inevitably, the age of the computer kit was brief. Altair's success did inspire imitation, and for a short time hobby magazines were peppered with advertisements for mail order kits. Most offered fully assembled systems for an additional cost, and soon many systems were only available assembled.

By the time MITS was sold in mid-1977, a new era of home computing was unfolding with Apple, Commodore and Tandy offering complete systems with monitors, keyboards and other peripherals, built around the next generation microprocessors such as the MOS Technology 6502 and the Zilog Z80. These substantially more capable and user friendly machines opened computing to a much wider audience of consumers. Nonetheless the new home computers bore the indelible mark of the hobbyist culture from which they emerged.

The hobby market demonstrated that computer buyers were willing to put in substantial effort to get a computer working, and would tolerate a product that was less-than perfect if the price was right and underlying technology pushed the envelope. The hobbyists themselves created an impromptu ecosystem in which software and technical knowledge were as likely to be given away as sold, laying the foundations for the open source and free software movements. Perhaps most significantly the success of the first hobby computers revealed the thirst among the general public to participate in the computer revolution for no other reason than the thrill of exploration and discovery, shaping a market for entertaining and educational home computers that virtually nobody in academe or industry anticipated.


Freiberger, Paul and Swaine, Michael (2000). Fire in the Valley: The Making of the Personal Computer. McGraw-Hill. ISBN: 978-0071358927

Ceruzzi, Paul E. (2003). A History of Modern Computing. The MIT Press. ISBN: 978-0262532037

Ditlea, Steve (ed.) (1984). Digital Deli: The Comprehensive, User-Lovable Menu of Computer Lore, Culture, Lifestyles and Fancy, Workman Publishing Company. ISBN: 978-0894805912

Gray, Stephen B. (1966-1976). Amateur Computer Society Newsletter. Vol. 1-4.

Gray, Stephen B. (1984) The early days of personal computers. Creative Computing. Vol. 10 No. 11

Mims, Forrest M. (1984) The Altair story; early days at MITS. Creative Computing. Vol. 10 No. 11

SCELBI Computer Museum.

Roberts, H. Edward; William Yates (January 1975). Altair 8800 minicomputer. Popular Electronics. Vol. 7 No. 1.

Titus, Jonathan (July 1974). Build the Mark 8 Computer. Radio Electronics. Vol. 45 No. 7.

Infield, Glenn (April 1968). A Computer in the Basement? Popular Mechanics. Vol. 129 No. 4.

Crank, Brian et. al. (August-December 1967) Wireless World Digital Computer, Wireless World Magazine. Archived at South Manchester Radio and Computing Club:

Other Articles in the Series:

The Antikythera Mechanism and the History of Clockwork

The Vacuum Tube in Computer History

From Boole to Bits - Claude Shannon's Digital Revolution

George Stibitz and the Bell Laboratories Relay Computers

Grace Hopper - Matriarch of Programming

The Commodore VIC-20 - The Friendly Computer

Computer History Timeline