DMX

DMX is a control protocol that was originally developed for the entertainment market. It has now become common place for controlling colour change fixtures in the commercial lighting market for internal and external solutions. The official name for DMX is USITT-DMX512-A or just DMX512-A and is maintained by ESTA (Entertainment Services and Technology Association).

DMX512 can transfer up to 512 individual channels of data through one network or universe. Multiple universes can be used to control many thousands of channels.

Advantages

  • High speed for use with colour control and advanced lighting solutions
  • Industry standard protocol and supported by multiple manufacturers
  • Extremely powerful control systems available to allow advanced control of many thousands of lighting points
  • Full control of every individual device
  • Cable runs can be up to 500m between the controller and the device to be controlled

Disadvantages

  • Must be installed with correct cable to ensure reliable communication
  • Cabling has to be screened from mains supply cables
  • No error checking so can not be used for applications involving safety
  • Maximum of 32 devices per DMX universe

Addressing

The DMX controller transmits commands on any of the 512 addresses per universe. The fixture can be set to 'listen' to any of the addresses. Fixtures usually respond to more than one address. For example, a basic RGB colour change fitting will require three addresses, one for each colour. If the start address is set at 1 then the red colour will respond to data on address 1, green will respond to data on address 2 and blue on address 3.

If there multiple fixtures, they can be addressed in two ways depending on the functionality of the controls and the complexity of the programming required.

If all the fixtures are to perform in exactly the same way, they can be configured to use the same start address. If some fixtures are to perform differently to others, they can be set to use a different start address so the controller can send data to individual fixtures on different addresses.

DMX Addressing
DMX Addressing

Cabling And Connectors

As DMX is based around the RS485 network, the cabling has to be correct to ensure the reliability of the installation. The cable which should be used is 120Ω impedance and can be of various diameters to suit the installation. In the DMX512A standard it states that a 5 pin XLR connector should be used between equipment. It is very common to find equipment using non standards compliant 3 pin XLR plugs and sockets. As audio equipment uses 3 pin XLR cables it is not un-common to find these being used for DMX signals. As microphone cable is rated at 110Ω this can cause interference to the DMX signals. The usual effect of using a cable with the incorrect impedance is either the fixtures being non-reponsive or flickering un-controllably.

DMX Connection

A DMX universe also requires a termination resistor in the end of line fixture. Some fixtures are self-terminating and some have a dip switch to add the termination resistor. Not including a termination resistor can have various side effects but a classic symptom is of un-controllable flickering of fixures.

DMX Termination