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

This video board was designed for a NorthStar Advantage, but is compatible with a broad array of devices, including the Raspberry Pi Pico.

Gareth Halfacree
1 month agoRetro Tech / HW101

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.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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