Oscilloscope Video Trigger for Retrocomputer Builds

Working with vintage computers is often difficult, because they have little to no compatibility with modern hardware. If you’ve noticed that prices for old CRT (cathode-ray tube) TVs and monitors have skyrocketed lately, you can thank the retro gaming and retrocomputing communities. Not only do low-resolution graphics look better on a CRT, many vintage systems simply cannot produce a video signal that modern displays accept. One workaround is to use an adapter to convert old RF or composite videos signals to something more modern. To help design such adapters, dmcintyre built this video trigger for oscilloscopes.

While working on video conversion, dmcintyre ran into a problem: TMS9918 video chips cannot reliably trigger oscilloscopes. That makes it almost impossible to analyze the video signals, which is necessary for someone trying to convert those signals. The Texas Instruments TMS9918 line of VDC (video display controller) chips was very popular and used in vintage systems like the ColecoVision, MSX computers, Texas Instruments TI-99/4, and many more. This video trigger device provides composite video signal throughput and a USB interface for oscilloscopes. The USB connection allows for fast waveform capture on many oscilloscopes, including the Hantek ‘scope that dmcintyre owns.

The video trigger device contains mostly discrete components and only needs a few IC chips: a Microchip ATmega328P microcontroller, a 74HC109 flip-flop, and an LM1881 video sync separator. All of the components solder onto a standard protoboard. After flashing dmcintyre’s code onto the ATmega328P, use is straightforward. Plug in cables from the system to the video trigger input and from the video trigger output to a compatible display. Then plug the USB cable into an oscilloscope’s input. Set the ‘scope to trigger on negative edges with a threshold around 0.5V.

With this setup, you can now see the video signal on your oscilloscope. Pushing the rotary encoder on the video trigger device switches between the rising edge and falling edge of the trigger signal. Turning the encoder moves the trigger line and long-pressing the encoder resets the trigger line to zero.

This doesn’t actually perform any video conversion; it just gives users the ability to analyze the video signal coming from TMS9918 chips. But that analysis should help people develop compatible video converters to connect vintage computers to modern displays.