In 1986, at the recommendation of the USITT (US Institute of Theatre Technology), two standard protocols were adopted for board-to-dimmer communication: the first, USITT AMX 192, is an analogue communication protocol for communications for communication between
manual control
boards and dimmers. The second, USITT DMX 512, is a digital communication protocol between computer control boards and dimmers.
The first signal (break) lasts a specific amount of time, and alerts the dimmer system to prepare to receive information. The second signal (header) determines the direction of communication, and whether the dimmer will receive information from the
control board
or transmit to it. A specific number identifies a specific dimmer within the system, and another number specifies its intensity and thus determines the intensity of the light.
This method of communication between computer
control board
and dimmer is called multiplexing. A three or four wire cable can transmit control signals to hundred of dimmers. Groups of dimmers are connected to each other in a daisy-chain, that is, the communications cable runs from the
control board
to the first group of dimmers, from there on to the next group, and so on.
DMX512 Timing diagram.
With the introduction of the multiplexed digital signal it became possible to revise the electric power distribution of the lighting system. Conventional thick electric supply cables ran along the dimmer racks in the dimmer room, and through to the sockets located in various areas in the theatre. Digital communication allowed smaller dimmers to be built. These could now be distributed close to the luminaire sockets in different parts of the theatre. All that was necessary was a slender
control cable
connecting the groups of dimmers, and three phase power supply to each group of dimmers. The DMX 512 communications protocol is only at the beginning of the road, and the USITT is considering new and more advanced protocols.
