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The Processor Technology VDM-1, for Video Display Module, was the first video card for S-100 bus computers.[1] Created in 1975, it allowed an S-100 machine to produce its own display, and when paired with a keyboard and their 3P+S card, it eliminated the need for a separate video terminal. Using a 7 x 9 dot matrix and ASCII characters, it produced a 64-column by 16-row text display.

The VDM-1 was a complex card and was soon replaced by an increasing number of similar products from other companies. An early competitor was the Solid State Music VB-1[2], which offered an identical display from a much simpler card. Later cards using LSI chips had enough room left over to include the keyboard controller as well.


TV Typewriter

In September 1973, the cover article of Radio Electronics magazine was Don Lancaster's "Build a TV Typewriter", which allowed users to type characters on a keyboard and have them appear on a conventional television.[3] Given this limited functionality, they initially estimated that the magazine would sell about 20 copies of the plans for $20 each. Instead, they were flooded by requests and eventually sent out 10,000 copies.[4]

Bob Marsh built a TV Typewriter and showed it to Lee Felsenstein. Felsenstein noted that it had no memory, so once a full page of text had been typed, the entire page had to be erased to display additional text. He phoned Lancaster and asked him about this design note, and Lancaster replied that he simply hadn't considered using it as the basis of a terminal, "I don't know – people just want to put up characters on their TV screens".[4]

Tom Swift Terminal

Throughout 1973, Felsenstein had been looking for a low-cost terminal for the Community Memory bulletin board system. He had designed the Pennywhistle modem to address the need for remote access at a price under $100, but the terminal that they hooked it up to still cost $1500.[4]

Felsenstein began designing a printed circuit board that would combine the video output of the TV Typewriter with 1024 bytes of memory so it could hold a page of text in ASCII format and send it to a video monitor. He called the resulting design "The Tom Swift Terminal",[5] after the Tom Swift books.[4] The design manual also had an extended section on the concept of "convivial design" (essentially "friendly")[6], which argued that a device's social utility was inversely proportional to its complexity, and thus devices should be as simple and open-ended as possible.[4]

Felsenstein sold the design document to local hobbyists, and wrote an article on it in the People's Computer Company in early 1974.[7] By 1975, the system had still not been assembled by anyone.[8]


Early January 1976 Byte advertisement for the VDM-1

In April 1975, Bob Marsh and Gary Ingram formed Processor Technology, initially to sell expansion cards for the Altair. Marsh approached Felsenstein with the idea of modifying the Tom Swift design to work with the Altair, which had been released that January. A key aspect of the resulting design was the use of electronic switches that allowed the display and the computer to access different parts of the on-board memory at the same time, which meant the display hardware did not have to stop the computer while it was drawing.[4][a] The prototype was completed in less than three months.[9]

As soon as the prototype was ready, Steve Dompier began porting Star Trek to it, replacing its serial output with a more game-like display to produce Trek-80.[4] The system went on sale soon after at a price of $199 for the kit version. As reviews of the era noted, an Altair compatible machine equipped with a keyboard, the VDM-1, and an appropriate monitor (from Radio Shack) cost less than a typical smart terminal of the era.[10]


Lee Solomon, who's Popular Electronics cover article had launched the Altair, was looking for someone to build an all-in-one machine that avoided the need for multiple cards from multiple vendors. He first approached Don Lancaster, who created the original TV Typewriter, and introduced him to Ed Roberts of MITS, the creator of the Altair. The two immediately began fighting and any idea of a partnership ended.[9]

In December 1975, Solomon approached Marsh, asking him if he could make this all-in-one "intelligent terminal" design. If he did it within 30 days, Solomon would put it on the cover of the magazine. Marsh again turned to Felsenstein to design it, and as soon as they began discussing it, it was clear the best solution was a complete computer system. This would basically be a low-end computer with the VDM as its output. Felsenstein initially wanted to use a different processor, but eventually concluded the Intel 8080 was the best solution.[9]

As Felsenstein worked on the design, Marsh continually added new required features, leading to a case of feature creep.[11] The design was finally completed after two months. The result was the Sol-20, one of the earliest all-in-one computer designs.[4]


Sometime later, Felsenstein returned to the design in order to make a new version that would have 24 lines of 80 columns, which was becoming a standard. The VDM-1 had 16 rows simply because 64 x 16 = 1,024, the amount of memory on the card.[12] It also added a split-screen feature that allowed the upper and lower sections to be scrolled independently, and place the split at any line. It also added smooth scrolling, greyscale and flashing.[13]

By the summer of 1979, Felsenstein had a partially assembled version and took it to the Javits Convention Center in New York City to show it to the Processor Technology people. After looking for the booth for some time, he found that the company had closed. He could not find a buyer for the design.[14]


The VDM-1 used a single slot in the S-100 backplane, but was so large it covered adjacent slots in most machines.[10] The front side of the board was crammed with components, including eight 91L02A 128-byte static RAMs, to the point that there was not enough room left for the required electrical traces. This was solved by running a ribbon cable from one side of the board to the other.[15] The monitor was connected via a coaxial cable running off the top corner of the card.[10]

The board generated 16 lines of 64 characters[16] on a monitor or a conventional television that was slightly modified[10] to bypass the radio frequency section. The display was black and white, and the hardware included the ability to support inverse video, which they referred to as "cursor bytes", by setting the high bit on the character byte. With the appropriate switch set on the settings DIP switch, any such character would blink. This supplied the hardware cursor display by setting this bit on a space character.[17]

One oddity of the system was that the character graphics were supplied in ROM, and there were several versions of the ROMs with different glyphs. The user could not know in advance which version he would receive.[18]


  1. ^ Today this concept is implemented using dual-ported RAM.



  1. ^ Processor Technology (November 1975). "8800 Hardware advertisement". Byte. Green Publishing. 1 (3): 75.
  2. ^ "S100 Computers - Solid State Music VDB-1". www.s100computers.com. Retrieved 2018-09-19.
  3. ^ Lancaster, Don (September 1973). "TV Typewriter". Radio Electronics. New York: Gernsback Publications. 44 (9): 43–52.
  4. ^ a b c d e f g h Social.
  5. ^ Manual 1976.
  6. ^ wiktionary:convivial
  7. ^ Felsenstein, Lee (1974). "Tom Swift Lives!" (PDF). People's Computer Company. pp. 14–15.
  8. ^ Levy 2010, p. 201.
  9. ^ a b c Levy 2010, p. 202.
  10. ^ a b c d Anderson 1976, p. 36.
  11. ^ Levy 2010, pp. 202, 203.
  12. ^ History 2008, p. 18.
  13. ^ History 2008, p. 20.
  14. ^ History 2008, p. 21.
  15. ^ See image here.
  16. ^ Manual 1976, p. I-1.
  17. ^ Anderson 1976, p. 37.
  18. ^ Anderson 1976, p. 38.


  • VDM-1 Video Display Module (PDF). Processor Technology. April 1976.
  • Felsenstein, Lee. "The Social history of the VDM-1".
  • Felsenstein, Lee (7 May 2008). "Oral History of Lee Felsenstein" (PDF).
  • Anderson, D. (December 1976). "Product Review: Processor Technology VDM-1" (PDF). BYTE. pp. 36–39.
  • Levy, Steven (2010). Hackers. O'Reilly Media.

Further reading

  • Felsenstein, Lee. The Tom Swift Terminal (PDF) (Technical report).
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