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.
- Assembly Directions
- Bill of Materials
- Interactive BOM / Board Layout
- Original VDM-1 Manual (Rev E)
- History of the VDM-1
- Differences Between VDM-1 and VDM-2021
- Dazzler/Dazzler II/VDM-1/VDM-2021 Comparison
- Ordering Instructions
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.
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.
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.
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.