VDM-1 (Video Display Module) | 1975

VDM-1

 The VDM-1, Video Display Module, was the first video card for S-100 bus computers, developed in 1975, allowing S-100 computers to display their own output. Using a 7x9 dot matrix and ASCII characters, it produced a 64-column by 16-row text display. When paired with a keyboard and Processor Technology's 3P+S card, it eliminated the need for a separate video terminal.

The VDM-1 was a complex card, and it was soon replaced by many similar products from other companies. An early competitor was the Solid State Music VB-1, which provided the same display but with a much simpler card design. Later cards using LSI chips had enough space to include the keyboard controller as well.

In September 1973, Radio Electronics magazine's cover article featured Don Lancaster's "Build a TV Typewriter," which allowed users to type characters on a keyboard and have them displayed on a standard television. The article received far more interest than expected, leading to the sale of 10,000 copies of the plans. Bob Marsh built a TV Typewriter and showed it to Lee Felsenstein, who noted that the device lacked external memory, meaning that once a page of text was typed, the entire page had to be erased to display more text. Lancaster had not considered using it as a terminal.

Throughout 1973, Felsenstein was looking for a low-cost terminal. He had designed the Pennywhistle modem to provide remote access under $100, but the terminal they were using still cost $1500. He then designed a system combining the TV Typewriter's video output with 1024 bytes of memory to store a page of text in ASCII format and send it to a video monitor. This design became the "Tom Swift Terminal," which emphasized simplicity and openness.

In 1975, Bob Marsh and Gary Ingram formed Processor Technology to sell expansion cards for the Altair. Marsh approached Felsenstein with the idea of modifying the Tom Swift design to work with the Altair, leading to the creation of a display card that allowed both the display and the computer to access different parts of memory simultaneously, so the computer did not have to stop while drawing. The prototype was completed in under three months, and Trek-80, a game port, was soon created for it. The system was sold as a $199 kit version, and an Altair-compatible machine with a keyboard, VDM-1, and an appropriate monitor was less expensive than a typical smart terminal at the time.

The Sol-20, an all-in-one computer, was developed in December 1975 at the request of Les Solomon. He asked Processor Technology to create a system that would combine multiple components into one, and Felsenstein's design resulted in the Sol-20, which used the VDM-1 as its output device.

Felsenstein later designed the VDM-2, a new version that supported 24 lines of 80 columns and added features like split-screen scrolling, smooth scrolling, grayscale, and blinking. However, in 1979, as Felsenstein tried to assemble this new version, Processor Technology had already closed, and he was unable to find a buyer.

The VDM-1 used a single slot in the S-100 backplane but was large enough to cover adjacent slots in most machines. The front of the board was filled with components, including eight 91L02A 1,024-bit static RAM chips, and the required electrical traces couldn't fit, so a ribbon cable was used to connect the two sides of the board. The monitor was connected via a coaxial cable running from the top corner of the card.

The VDM-1 displayed 16 rows of 64 characters on a monitor, with a black-and-white display. The hardware supported inverse video and cursor bytes by setting the high bit of the character byte. This allowed the hardware to display a cursor by setting this bit on a space character. The system's character graphics were stored in ROM, and there were several versions of the ROM with different glyphs, so the user could not know in advance which version they would receive.