The Data Transmission of DMX512 Lighting Protocol

Technical light is a powerful tool used to create different feeling or moods in a space. Recent advances in lighting design, as well as in entertainment and stage lighting, have increased the need of using DMX lighting control protocol. DMX512 protocol is a set of universally used rules, for controlling intelligent fixtures and lighting dimmers.

The protocol specifies the digital data transmission method, between controllers and fixtures. It was design to give interoperability between controllers (concerning both mechanical and communication level), without strangling when using devices from different manufactures. DMX stands for Digital Multiplex and 512 refers to the number of possible control channels that can be transmitted at a single network of devices. This network is called “universe”. Manufacturers and users have many benefits by standardizing the protocol and the equipment. The required development time and the cost of the products is reduced, the quality and safety of each product increases and it makes the communication between devices simpler, thus it protects it against obsolescence. At first the protocol was designed to be used for controlling high tech lights, in entertainment industry (I.e. clubs, theatres, concerts) but nowadays it is widely used in smart homes, commercial and architectural applications.

DMX512 is a unidirectional asynchronous serial transmission protocol, which means that the transmitter cannot get feedback from receivers for possible error messages, connections or auto addressing of DMX decoders in the data bus. This protocol has a transmission rate of 250kHz (maximum 250,000 bits per second). Thus, in order to transmit that rate, an RS485 line is needed. A single transmission is called “DMX packet” and can include channel data and synchronizing elements for maximum 512 channels. Each lighting device is assigned to a channel. The controller is able to control 512 channels, however only 32 devices (network DMX standard) can be connected in a DMX line. If the system consists of more than 32 devices, a DMX splitter must be used, in order to add DMX lines.

Parameter values can take values from 0 (=off) to 255 (=maximum intensity). DMX uses a standard 8N2 encoding and so the signal if each packet must include one start -bit, eight data-bits and two stop-bits. In order to form the DMX signal, bits are sent out every 4μs. Each code is 8 bit (1 byte) long, thus there are 256 different codes. A start bit (low level) marks the beginning of the byte and 2 stop bits (high level) mark where the byte ends. When there is no information (idle) the signal lies on high level. There are 11 bits in total for each byte, which are called “slots” and each slot last for 44μs (4×11). Therefore the form of the DMX signal is:

BREAK – START BIT – BYTE – STOP BIT – IDLE (then repeat).

A complete packet can last for minimum 1024μs and maximum 1 sec.

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