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The VDM-2021 is a text-based video output board for the S-100 bus. Using the VDM-2021 you can easily output text-based video straight from an Altair 8800 or other S-100 based system.

The VDM-2021 is electrically nearly identical to the historic VDM-1 board produced by Processor Technologies starting in 1975.

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Basic Theory of Operation

The VDM-2021 generates a video display containing sixteen lines of 64 character each. The characters are directly memory mapped from ASCII data stored in 1024 bytes of on-board memory. The board produces composite-video output through a BNC connector. Compatible monitors include anything capable of displaying composite video: a closed circuit television monitor, a modern LCD with composite input, or a television with an RF modulator.

Because the board is electrically nearly identical to the original VDM-1 board, almost everything contained in the original VDM-1 manual is applicable.

Software Support

Because the interface is so simple, the VDM-1 enjoyed broad software support including BASIC, CP/M, Trek-80, and more. View the software page for more information.

Board Assembly

The VDM-2021 is assembled from a partial kit. The recommended approach is to order a partial kit which contains the PCB as well as hard-to-obtain components, and then order the remaining components from Jameco. A Bill of Materials (BOM) can be found here which includes every jameco part number. Jameco has a “Quick Order” form under the ordering drop down which will allow for quick entry of the BOM part numbers. At the time of writing, the total order from Jameco is $74.55 if you have none of the parts on hand.

Assembly Directions

VDM-1 History

The VDM-1 was one of the first video cards ever made for a personal computer. It was created in 1975 by Lee Felsenstein and sold by Processor Technologies to be used in the Altair 8800 and IMSAI 8080. History of the VDM-1

VDM-2021 Reproduction of VDM-1

In May of 2021 I reached out to Lee Felsenstein asking him for advice on reproducing the VDM-1. He thought it was a fine idea, but strongly suggested that if I were to undertake the project I should not simply copy the layout of the original board. Lee has always been disappointed by the poor quality of the original board layout – as exemplified by its ribbon cable. I underwent the task of capturing the original schematic in KiCad (specifically revision E of the schematic) and then proceeded to produce a PCB layout based on the original schematic.


I’m selling this board for $30 plus shipping for the bare board or $90 plus shipping for a partial kit. Ordering Instructions

Why build a VDM-2021?

The VDM-2021 is the only board (to my knowledge) that can be built in 2021 to provide a terminal-style text output device for an S-100 system using 1970s technology. The VDM-1 also has some great history. Click here for a comparison between the VDM-1, VDM-2021, Dazzler, and Dazzler II.

How is it different than the original VDM-1?

The short version is that the VDM-2021 is nearly electrically identical to the VDM-1, but with an entirely different layout. See here for a detailed comparison.


Contact me at physicsrob followed by ASCII 0x40 followed by the google mail domain name.