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The Sol-20 retained badging belying its original design as an intelligent terminal. In the Sol-10, the numeric keyboard would normally not be present, but it was otherwise identical.

The Sol-20 was the first fully assembled microcomputer with a built-in keyboard and television output,[a] what would later be known as a home computer. The design was a combination of an Intel 8080-based backplane, a VDM-1 graphics card, the 3P+S I/O card to drive a keyboard, and circuitry to connect to a cassette deck for program storage. Additional expansion was available via five S-100 bus slots at the back of the machine. It also included swappable ROMs with boot code that allowed it to start up running a selection of programs.

The design was originally suggested by Les Solomon, the editor of Popular Electronics. He asked Bob Marsh of Processor Technology if he could design a smart terminal for use with the Altair 8800 and its increasing number of clones. Marsh contracted Lee Felsenstein, designer of the VDM-1, to create it. They agreed the best solution was to build a complete computer with a terminal program in ROM. Felsenstein suggested the name "Sol" because they were including "the wisdom of Solomon" in the box.[b]

The Sol appeared on the cover of the July 1976 issue of Popular Electronics as a "high-quality intelligent terminal". It was initially offered as a motherboard known as the Sol-PC, offering schematics for free. The company also introduced the Sol-10 and Sol-20 as complete computer systems, differing largely in that the -10 did not include the expansion slots. A Sol-20 was taken to the Personal Computing Show in Atlantic City in August 1976 where it was a huge hit, building an order backlog that took a year to fill. Systems began shipping late that year.

The Sol-20 remained in production until 1979, by which point about 12,000 machines[c] had been sold. By that time, the "1977 trinity" -the Apple II, Commodore PET and TRS-80- had begun to take over the market, and a series of failed new product introductions drove Processor Technology into bankruptcy. Felsenstein would later develop the successful Osborne 1 computer, using much the same underlying design in a portable format.


Tom Swift Terminal[edit]

Felsenstein was one of the operators of Community Memory, the first public bulletin board system. Community Memory ran on a SDS 940 mainframe, and was accessed through a Teletype Model 33 in a record store in Berkeley, California. The cost of running the system was untenable; the terminal cost $1,500 (their first example was donated), the modem another $300, and the mainframe time would normally be dollars a minute. Even the reams of paper output from the terminal were too expensive to be practical.[3] The replacement of the Model 33 with a Hazeltine 1500 helped, but it required constant repairs.[4]

Since 1973, Felsenstein had been looking for ways to lower the cost.[5] One of his earliest designs in the computer field was the Pennywhistle modem,[6] a 300 baud acoustic coupler that was ​13 the cost of commercial models. When he saw Don Lancaster's TV Typewriter on the cover of the September 1973 Popular Electronics, he began adapting its circuitry as the basis for a design he called the Tom Swift Terminal. The terminal was deliberately designed to allow it to be easily repaired by anyone. Combined with the Pennywhistle, users would have a cost-effective way to access Community Memory from anywhere.[7]

In January 1975, Felsenstein saw a post on Community Memory by Bob Marsh asking if anyone would like to share a garage.[d] Marsh was designing a fancy wood-cased digital clock and needed space to work on it.[e] Felsenstein had previously met Marsh at school and agreed to split the $175 rent on a garage in Berkeley. Shortly after, Community Memory shut down for the last time,[10] having burned out the relationship with its primary funding source, and the energy of its founding members.[3]

Processor Technology[edit]

January was also the month that the Altair 8800 appeared on the front page of Popular Electronics, sparking off intense interest in the hacking minded engineers of the rapidly growing Silicon Valley. Shortly thereafter, on 5 March 1975, Gordon French and Fred Moore held the first meeting of what would become the Homebrew Computer Club.[11][f]

Marsh saw an opportunity supplying add-on cards for the Altair, and in April, he formed Processor Technology with his friend Gary Ingram.[11] Their first product was a 4 kB memory card; one was already available for the machine but was almost impossible to get working.[12]

Marsh began offering Felsenstein contracts to draw schematics or write manuals for the products they planned to introduce. Felsenstein was still working on the terminal as well, and in July, Marsh offered to pay him to develop the video portion.[5] This was essentially a version of the terminal where the data would be supplied by the main memory of the Altair rather than a serial port.[13]

The result was the VDM-1, the first graphics card. The VDM-1 could display 16 lines of 64 characters per line,[g] and included the complete ASCII character set with upper and lower case characters and a number of graphics characters like arrows and basic math symbols. An Altair equipped with a VDM-1 for output and their P3+S card running a keyboard for input removed the need for a terminal, yet cost less than dedicated smart terminal products.[15]

Intelligent terminal concept[edit]

Before the VDM-1 was launched in late 1975, the only way to program the Altair was through its front-panel switches and LED lamps, or by purchasing a serial card and using a terminal of some sort. This was typically a Model 33, which cost $1,500 if you could get one. Normally you could not, they typically sold only to large customers, which led to a thriving market for broken-down machines that could be repaired.[16] Ed Roberts, whose Micro Instrumentation and Telemetry Systems (MITS) had developed the Altair, eventually arranged a deal to supply refurbished Model 33s to MITS customers who had bought an Altair.[16]

Les Solomon, whose Popular Electronics magazine launched the Altair, felt a low-cost smart terminal would be highly desirable in the rapidly expanding microcomputer market. In December 1975, Solomon travelled to Phoenix to meet with Don Lancaster to ask about using his TV Typewriter as a video display in a terminal. Lancaster seemed interested, so Solomon took him to Albuquerque to meet Roberts.[16] The two immediately began arguing when Lancaster criticized the design of the Altair and suggested changes to better support expansion cards, demands that Roberts flatly refused. Any hopes of a partnership disappeared.[17]

Solomon then travelled to California and approached Marsh with the same idea, stating that if they could produce the design within 30 days, he would put it on the cover of the magazine. Marsh once again hired Felsenstein to design the system.[17]


Felsenstein initially wanted to build it following the model of his earlier Tom Swift design, using discrete electronics.[14] Marsh, in parallel, sketched out a version using the Intel 8080. It quickly became apparent the difference in cost would only be about $10, and from then on the original dedicated terminal concept was dropped.[17][5]

While Felsenstein worked on the design, Marsh continually came up with new ideas that he demanded to be included. This led to creeping featuritis problems and the final design was not delivered until about two months of "frantic" work. The final product consisted of a single motherboard with the 8080, a simplified version of the VDM-1, serial input/output, and 1k of SRAM for the screen buffer. A ROM, the "personality module", would include the terminal driver or other code which would begin running as soon as the machine was turned on.[5]

As the machine increasingly expanded in power, Felsenstein suggested the name "Sol", because they were including "the wisdom of Solomon" in the system. Les Solomon would later quip that "if it worked, they'll say Sol means 'sun' in Spanish. It it don't work, they're gonna blame it on the Jewish guys."[18] Stan Veit later joked to Les that they named it after Solomon in another way, "the LES Intelligent Terminal".[19]

Finalizing the design[edit]

Felsenstein originally thought he was only needed for the initial design, but as the physical layout began it was clear that the layout artist they had hired would not be able to do it on his own. Marsh had a woodworker friend build a large light table and Felsenstein and the layout artist began using it to design the printed circuit board for the motherboard.[5]

Marsh, meanwhile, was working on the physical design. He demanded from the start that it use walnut sides, which he had learned from the woodworker would be practically free if they were smaller than a certain size because they could be made using off-cuts. Beyond that, anything was fair. The deadline for the magazine had been pushed back, but there was still little time to finalize the layout before it needed to be photographed. Marsh decided that the machine should have a cassette deck, so they mocked up a machine with a keyboard on the left and cassette player on the right.[5]

The first motherboard arrived 45 days after the project started, and the first cases and power supplies about 15 days after that. By this point it was clear the system was a usable microcomputer on its own, but "the decision was made to soft-pedal the fact until the last possible moment. Once published, all the fuss possible was to be made about its general-purpose nature; but until it actually saw print, it was to be treated first as a terminal."[5]


The original mock-up of the Sol shows a slimmer case and a cassette deck where the numeric keypad would ultimately be placed.

In February 1976, the first machine, a kludged up box of parts, was readied and flown to New York to show Solomon. When they arrived the machine would not work, displaying unreadably fuzzy images. Marsh and Felsenstein took the train to visit the Boston offices of the newly started Byte magazine. While there, Felsenstein had time to discover the problem was a tiny bit of broken wire that got stuck under a chip, shorting out two of the video lines. They returned to Solomon's house to demonstrate the working unit.[20]

Due to publication timelines, it did not appear in the magazine until the July 1976 issue, where it was described as "high-quality intelligent terminal". The cover image showed the mockup version; it was packaged in a slim case, not unlike the general shape of the TI-99.[21] By the time the article appeared, the design had changed; the new design had a distinct "step" behind the keyboard that rose up over the expansion chassis and power supply at the back of the case. A bent piece of sheet steel formed most of the case, capped on the left and right by the wooden panels Marsh demanded.[22]

The new design was first shown at the Midwest Area Computer Club conference in June 1976. The machine was not ready for sales at this point, but they did a brisk business selling their existing expansion card line.[23] This was followed by the Personal Computing '76 (PC'76) show in late August in the dilapidated Shelburne Hotel in Atlantic City. The order book was officially opened[2] and Sol was a huge hit at this show.[19]

Soon after, Marsh was invited to demonstrate the Sol on NBC's The Tomorrow Show. They used a game by Steve Dompier called "Target" to show off the system's capabilities.[24] The show's host, Tom Snyder, ended up playing the game right through the commercial breaks, and they had to force him to give up the machine in order to finish the show.[25]


The Sol was initially offered in three versions. The base motherboard was offered as the Sol-PC, available as a kit for $575, or fully assembled and tested for $745. The Sol-10 added a case, keyboard and power supply, was $895 in kit form and $1,295 assembled. Finally the Sol-20 added a keyboard with numeric keypad, and a larger power supply to feed the five expansion slots and a fan to cool them, for $995 as a kit or $1,495 assembled. Advertising of the time referred to the Sol-20 as "The first complete small computer under $1,000".[26] Most systems would require additional pieces, which they bundled as the "Sol Systems"; the Sol System I consisted of a Sol-20, an 8k RAM card, a PT-872 monitor and the RQ-413 Cassette Recorder, for $2,129.[27]

In keeping with the hacker ethic, the company also offered to send out copies of the schematic for the motherboard for the cost of postage, later estimating that somewhere between 40 and 50,000 copies were sent. Few, if any, Sol-10s were sold,[h] and the company focused on the Sol-20. The first machines shipped in December 1976. These were also available for 3rd party sales, and this began the formation of a dealer network among some of the earliest computers stores. By 1977, Processor Technology had a reputation for quality and was among the best selling computers in the world.[19]

By this time, S-100 machines were beginning to make inroads into business markets. Processor Technology invited all of their dealers to a meeting in Emeryville, California, outside Berkeley, to introduce their Helios floppy disk drive for $1,199, along with their PTDOS system to work with it. They also promised larger memory cards and a color video card. Additionally, dealers could now order 30 days net, as opposed to cash-on-delivery, although to do so they had to put in orders at least once a quarter.[19]


Unfortunately, these plans quickly fell apart. The Helios was based on a new mechanism from Diablo Data Systems. Diablo had been purchased by Xerox in 1972, and shortly after the Helios was announced, Xerox cancelled development of the floppy line. Processor Technology selected the new Persci 270 in its place, a system that combined two 8" drives together with single drive and stepper motors. This was released as the Helios II, at $1,895 for the kit or $2,295 assembled. The company moved to a much larger factory in Pleasanton, California.[19]

It was at about this point that Radio Shack introduced the TRS-80. Like the Sol, it was a complete all-in-one machine but sold for about half the price when packaged with a monitor and RAM. Moreover, it was available at hundreds of Radio Shack stores across North America. Sales of the Sol plummeted. Meanwhile, the company failed to introduce any of the other new products it mentioned, notably the color graphics card. When the Apple II appeared with color graphics, it quickly became a best seller.[19]

To add to their woes, Processor Technology had contracted North Star Computers to write a new version of the BASIC programming language for the machines. North Star began selling the version to other vendors, at which point Processor Technology sued them, saying the contract had been exclusive. The suit dragged on, hurting both companies before Processor Technology ultimately lost. To add to the injury, North Star then released a new 5.25" drive for the system that sold at half the cost. A patch that allowed CP/M to run on the new drives killed off any interest in alternatives like PTDOS, and new business applications like Wordstar and Electric Pencil soon cemented CP/M as the standard operating system for all S-100 machines.[19]

Processor Technology continued with the Helios system and refused to consider replacing PTDOS with CP/M. Helios proved to be highly unreliable and resulted in a lawsuit by those owners that had purchased them. Meanwhile, the company introduced one of its few new products during this period, 32 and 64 KB memory cards based on dynamic RAM which was much denser than the older SRAMs. These began failing at an alarming rate, overwhelming the company.[19]

These problems caused the company to go bankrupt, and the company was eventually liquidated on 14 May 1979.[19]


From the Sol Systems Manual,[22] unless otherwise noted.

Physical layout[edit]

Looking at the Sol-20 from the front, where the operator would sit, the keyboard was in a typical location with the main QWERTY-style layout on the left and the numeric keypad on the right. The wooden sides of the case were close on either side of the keyboard, potentially interfering with the operator's hands.[28]

On the rear right of the case, directly behind the numeric keypad, was the power supply, which also provided a fan to cool the circuitry. The main motherboard sat to the left of the power supply, spanning about ​23 of the case's width. The board extended forward under the keyboard all the way to the front of the case.[28]

Parallel and serial ports extended off the back of the motherboard into holes in the case. To the right, directly below the fan, was an RCA jack that produced composite video output. This could be connected to a monitor, or with a bit of work, a conventional television. The processor was near the back of the machine, with the memory and video circuits at the front. This required the video output to be routed to the back of the machine with a coaxial cable running across the top of the card.[29]

Sol bus[edit]

Originally, expansion was going to be handled through an external cage that connected to the main console using two 50-pin ribbon cables. However, the original Altair bus design lacked signal ground pins in order to fit into a 100-pin connector. This led to noisy signals as they all shared a common ground, a topic of considerable derision by many earlier users. When extended into a 50-pin ribbon cable this was too noisy, and Marsh demanded that there be additional ground pins spread across the cable to reduce this noise.[5]

The Sol solved this problem by supporting only one of the two data busses at a time, allowing input or output and switching between them by signalling with the DBIN pin on the 8080. Since only one bus was being used at a time, this allowed eight less pins, which allowed those eight to be used as grounds instead. Ultimately, however, the idea of using an external chassis was dropped. By this time the decision to use the additional lines for ground had been made, which had the desirable side-effect of making the board easier to design.[5]

Instead, this same basic idea was implemented in an internal expansion chassis, the Sol-BPB. This extended vertically upwards from roughly the center of the motherboard. It had five horizontal connectors, and a metal framework on either side to mechanically support the cards. The chassis also had another edge connector at the top, but it is unclear whether this could be used for further expansion.[30] The BPB retained the ground pins of the early design, and quickly became a de facto standard.[5]


Three "personality modules" were released with the original systems.[31] CONSOL provided a simple terminal emulator function, along with a small number of additional commands to load and run programs from tape using TLOAD. SOLOS added names to the files on the cassette, the TSAVE command for saving data to the tape into a named file, and TCAT to print out the details of a named program. TXEC loaded and executed a named program in one step. SOLED included block-mode editing, used on some mainframe systems, but it is not clear if this was actually available.[32]

One commonly used software for the Sol-20 was the BASIC-5 (or BASIC/5) language. This was able to run in even a minimal machine with a 4 KB expansion, but in order to fit it had only single-precision floating point numbers and lacked strings. An Extended BASIC that ran in 8 KB added strings and other functions. Processor Technology also sold a wide variety of other programs, including many games, on cassette format for the Sol, or on punch tape for other S-100 machines.[33]


  1. ^ Earlier machines like the Micral N or Altair required a separate terminal for interactive use. The Sol-20 required only a television for output, everything else needed was built in.
  2. ^ The name is pronounced "saul twenty", not "soul twenty".[1]
  3. ^ Some sources put it at 5,000 kits and 5,000 assembled machines, but Felsenstein puts it at 12,000.[2]
  4. ^ There are minor variations on the theme; in his 2008 interviews, Felsenstein suggests this took place in 1974.[8]
  5. ^ Marsh ultimately gave up on the clock design.[9]
  6. ^ Here too the stories vary somewhat; Felsenstein's 2008 interviews state they went to the first meeting together, while other sources suggest Marsh attended the second meeting in early April.
  7. ^ 16 x 64 = 1,024, the number of bytes in a 1 KB SRAM.[14]
  8. ^ As Felsenstein later put it, "Nobody ever bought one".[20]



  1. ^ "".
  2. ^ a b Felsenstein 2008, p. 20.
  3. ^ a b Levy 2010, p. 148.
  4. ^ Levy 2010, p. 145.
  5. ^ a b c d e f g h i j Felsenstein 1977.
  6. ^ Felsenstein 2008, p. 17.
  7. ^ Levy 2010, p. 156.
  8. ^ Felsenstein 2008, p. 16.
  9. ^ Swaine & Freiberger 2014, pp. 117-118.
  10. ^ Levy 2010, pp. 156-157.
  11. ^ a b Levy 2010, p. 174.
  12. ^ Swaine & Freiberger 2014, p. 120.
  13. ^ Levy 2010, p. 201.
  14. ^ a b Felsenstein 2008, p. 18.
  15. ^ Anderson, David (December 1976). "Product Review: Processor Technology VDM-1" (PDF). Byte. pp. 36–39.
  16. ^ a b c Levy 2010, p. 202.
  17. ^ a b c Levy 2010, p. 203.
  18. ^ Levy 2010, p. 204.
  19. ^ a b c d e f g h i Veit 2002.
  20. ^ a b Felsenstein 2008, p. 19.
  21. ^ Popular 1976.
  22. ^ a b Systems 1977.
  23. ^ Swaine & Freiberger 2014.
  24. ^ Younger 1986, p. 45.
  25. ^ Levy 2010, p. 205.
  26. ^ "The first complete small computer under $1,000" (PDF). Ham Radio Horizons. March 1977.
  27. ^ PriceList 1977, p. 1.
  28. ^ a b Systems 1977, p. VI-10.
  29. ^ Systems 1977, pp. VI-10, VI-103.
  30. ^ Systems 1977, p. VI-16.
  31. ^ Barbour 1978, p. 127.
  32. ^ "Personality Modules".
  33. ^ PriceList 1977, p. 2.


External links[edit]

  • Sol-20, web site with may programs and information about the Sol-20.