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When you should resist

safety

What do a helmet, a seatbelt and a 120Ohm resistor have in common? They are all safety precautions. Nothing may happen if you don't use them, but if something happens, you will wish you had them. OK, you will probably understand helmet and seatbelt, but why the resistor?

DMX-512 is transmitting data at a relatively high speed of 250kb/s. At this speed, the signal source, the transmission line and the load should all have the same impedance. If they don't reflections happen, like in the image below.

DMX_signal

In both images the same signal source and the same transmission line are used, the exact same data were transmitted. So why are there a few pulses marked with red in the left image, but not in the right image? What you see in the left image are reflections. They disappeared in the right image, because a terminator was added.

A DMX-512 controller has an impedance of 120 Ohm. That's why you should use a cable with 120 Ohm impedance. And that is also why you should terminate the last device in a DMX-512 chain/universe with a resistor of 120 Ohm.