I am attempting to re-create the video drive circuit from an IBM 3101 terminal. I just have the monitor, and I thought it would be really cool to drive it from something like an Arduino. I found some pictures online with the help of a kind person on Reddit who sent me some extremely high resolution photos of the terminal's logic board. I found that it uses a 14.1528 clock oscillator for the scan frequency. I was lucky enough to find this exact crystal on eBay, so I bought it. This is where the trouble starts.

The crystal seems to output a signal akin to a crystal oscillator, with a sine wave and a peak of around 2.5 volts. I thought this wasn't a problem as I have made circuits with the two pin crystal oscillators before. However, those use both their pins as connections to two points of a circuit, while the one I have only has one. There is a pin for 5 volts and ground, and the output pin. The fourth pin is unused. The trouble is I cannot find a circuit that will make use of a crystal with a single pin. I have tried numerous things, including schmitt trigger chains and many other designs. I am not super knowledgable about the actual workings of passive components as I usually stick to digital electronics like retro CPUs.

I should also mention that I want this ~14mhz crystal to drive a TTL load. I am using 74HCT and 74LS logic gates to generate the horizontal and vertical timing pulses. I have attached the crystal and the waveform on my oscilloscope. The circuit is just a crude probe of the chip and nothing else. For whatever reason my scope would not properly read the sine wave so it looks like there are two, one 180 degrees out of phase. Any help or insight would be greatly appreciated!

crystal circuit on breadboard.

osciliscope waveform

  • \$\begingroup\$ Most likely the module is outputing 5V square wave, but your measurements are improperly set up for measuring it. First of all there is a breadboard, long ground wire loop, no bypass caps, and who knows if the probes are at 1x setting while they should be at 10x setting. Fix or verify these issues and it should look like a square wave. \$\endgroup\$
    – Justme
    Commented Mar 9, 2022 at 5:42

1 Answer 1


What you have is a crystal oscillator module, not a bare crystal. The oscillator module will output a sine wave or square wave signal.

If you adjust your scope trigger level you should be able to get a single sine or square(ish) signal displayed. The ground clip of your probe should be connected close to the ground terminal of the oscillator module.

  • \$\begingroup\$ Yes, it was quite a crude experiment. I fixed some of the issues by including some bypass caps and fixing the ground cable for the probe. I cannot do much about the breadboard because its all I have for designing. I set the probes to 10x and saw something that vaguely resembled a square wave. I fed it through some TTL logic gates and nothing happened. Even the 74HCT14 schmitt trigger did nothing. So I am still lost on how to drive a TTL load with this crystal module. The signal still peaks around 2.8 - 3 volts and cannot drive even a single TTL load. \$\endgroup\$
    – user302167
    Commented Mar 9, 2022 at 18:07

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