Help me troubleshoot a DMX issue

Ringing and overshoot. Things that do not make for happy times, digital or analog (audio filters).
The overshoots from improper termination will have voltage maximums and minimums at periodic points along the line. If a maximum happens to land near where a fixture is, then it causes interference and poor decoding. This is basic "transmission line" theory. It's just luck of the draw whether an unterminated DMX system behaves or acts up, and which fixture will have problems isn't intuitively predictable. The same is true when cable of the wrong impedance is used, or if one side of the balanced pair is open at some point. Termination, proper cable type, and continuity checks are the key to reliable DMX operation.

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