Evan Allen's TTL-to-Composite Adapter Board Aims to Give Video Outputs to Retro Gear — And More

Maker and vintage computing enthusiast Evan Allen has built a compact add-on board that takes in Transistor-Transistor Logic (TTL) video signals and turns them into a composite video output — making it possible to replace missing video outputs on retro systems like the NorthStar Advantage.

"In an effort to make up for having two and a half NorthStar Advantages," Allen explains, in a post brought to our attention by Adafruit, "I wanted to have some solution for the video that the third one is missing. Enter the ZRT-80 and its simple composite generation circuit. I did some reverse engineering and repair of this back in 2020 and during that ordeal I remembered that it has a nice simple example of a composite generation circuit. Being a 5V logic based device I decided to investigate the circuit to see how the stages leading up to the combination differed from the Advantage."

The ZRT-80 is a single-board computer and serial terminal designed by William White and released by Digital Research Computers in 1983. The NorthStar Advantage, meanwhile, is a 1982 microcomputer built around a Zilog Z80 processor and featuring for-the-time high-resolution 640×240 graphics capabilities — something you can't really make use of if your particular Advantage, like Allen's, is missing its video output.

By digging into how the ZRT-80 generated its composite video output, Allen was able to figure out how a handful of parts, including a transistor, a handful of resistors, inverter gates, took the TTL signals from the board and generated a choice of composite or separate video-and-sync signals. The trick: finding scanned schematics for the NorthStar Advantage and figuring out how to apply the same techniques there.

"I based the circuit off the one in the ZRT-80," Allen says of his resulting "generic" adapter, "but I made each input individually polarity selectable like the advantage has on its outputs. That makes this design fairly generic and able to be used on a variety of computers to replace monitors that may have failed or video outputs that did not fir the composite video voltage levels. You could even hook this up to a Raspberry Pi Pico or an FPGA and use it to generate a good solid composite signal as long as your video timings were correct."

THe full project write-up is available on Allen's blog, while the KiCad project files have been uploaded to GitHub under the permissive Unlicense public domain license.