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DMX in practice

DMX is basically a digital way to control light.
The setup consist of a PC or Console as controller and some light fixture that are to be controlled.

The Controller sends some digital values using a standard named DMX512 and the fixtures then receives these values.
The fixture maps these received values to various functions.

Each physical fixture are set to an address and this address must be the same as the fixture address in the controller.
The address setting for a fixture is so that it will listen on the set address and then the next addresses depending on how many channels the fixture occupies.
How many addresses are stated by the manufacture.
A fixture as shown above uses 12 channels.
This means that if it is set to address 20 then it will receive the values on address 20, 21, 22 … 31.
Fixtures can be set to the same addresses and the fixtures on the same addresses will then do the same think but the fixture has to be the same.
When setting fixture addresses then take care that they don’t overlap.
If a fixture like above starts on address 20 and uses 12 addresses then the next fixture address must be at address 32 to avoid overlapping.

The maximum no of fixtures in a chain are 32 and this is due to load on the driver circuit.

The last fixture in the chain has to be terminated with a DMX end or terminator.

Why DMX end / termination
The DMX termination is a 120 ohm resistor.
DMX cable has a characteristic impedance of 120 Ohms. It is a bit complex to be described here but it is a complex high frequency calculation of induction, capacitance, serial resistance and leakage resistance in the cable itself.
The impedance 120 ohm can not be measured with a multimeter.
It is important to terminate the cable with 120 ohm to avoid reflection in the cable which causes the DMX signal to bounce back to the fixture.
The reflected signal may cause the fixture to malfunction.
See eventually: Measurement on the DMX cable.
Some fixtures have a build in 120 termination that can be activated.

To build a DMX terminator/end see this link DMX Terminator

Additionally, for optimal performance use cable with a characteristic impedance of 120 Ohms.
Microphone cable can be used but for longer distance may course the fixtures to malfunction.
The reason is that audio cables has a higher capacitance and inherent impedance.

This reflection issue is also the reason for not splitting the DMX signal with a simple Y cable.
Keep it in one line and if the signal needs to be divided use a DMX splitter.

Why Splitter
Splitting the signal is the same problem as above.
The load impedance don’t match 120 ohm and this causes possible reflection of DMX signal bouncing forward and backward on the cable.

The benefit of using a splitter that has building line drivers are that it is possible to have 32 fixtures on all the divided outputs.
And it makes it a bit easier when fixtures are at different places. It is then not a puzzle to get one cable string to all fixtures.

Finally if it is a optical/transformer isolated splitter then there are no electrical connection between the paths.
This eliminates problems with ground loops and spikes when lamps are turned on.
The controller is also protected this way.

dmx_basics/dmx_in_practice.txt · Last modified: 2015/05/27 17:19 by russwig

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