This is a small measurement to show what happens when a cable in not terminated with a 120 ohm resistor.
The measurement is performed with a Eurolite DMX512 dongle supplied with 16m cable and a Rohde & Schwarz, “RTO 1004” oscilloscope.
Additional a couplings network is used to connect the oscilloscope between the dongle and the cable.
The images bellow shows that the cables are to be terminated correct and that audio cables are to be avoided.
The oscilloscope is connected so channel 1, the yellow trace measures from GND to Data+ and channel 2 the green trace measures from GND to Data-.
The blue trace is generated in the oscilloscope showing the difference between Data+ and Data- which is the DMX signal that the fixture will see.
On the first image the timing is set so a frame 44uS can be seen. The trigger point 0uS where the blue trace goes from high to low is the start bit and the frame shown here only contains 8 x “0”.
The high part is the two stop bits.
On the zoomed image bellow the cable is terminated with 120 ohm. The blue trace look nice and smooth.
There are a small discontinuity 150nS after the signal goes high. This is probably because the cable is 110 ohm and the terminator is 120 ohm. It is 10 ohm mismatch.
In the image bellow shows the 16 m cable which is not terminated and on the blue trace a relative high peak can be seen Digital like to have a clear high and low signal this may course wrong detection of the bit.
The peak will in worst case be 2 x the normal voltages.
It can also be seen that there are more peaks next to each others and this indicates that the signal is bouncing forwards and backwards on the cable.
The time from the raising edge to the reflection comes are depending on the cable length. Here 16 m gives round 150nS and 30m will delay it to round 300nS.
The last image is an audio cable terminated with 120 ohm.
The audio cable used for DMX is just as bad as running without termination. One of the reasons is that audio cable is not 110 ohm and therefore the 120 ohm termination don’t have any effect.