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What is the lighting DMX512 protocol?

The DMX512 protocol is a data dimming protocol proposed by the United States Stage Lighting Institute (USITT), which gives a protocol standard for communication between lighting controllers and luminaire equipment, as it was proposed in 1990, so the full name of the protocol is USITTDMX512 (1990). The proposed protocol provides a good standard for the use of digital signals to control lighting equipment.

Chinese name: DMX512

Foreign name: DigitalMultipleX512

Proposer: American Stage Lighting Association

Date of introduction: 1990

Features: strong anti-interference ability, etc.

Fields：Computer science and technology

I. Introduction

DMX512 is a digital dimming protocol, the application of which can be used on stage, theatre, studio and other places such as dimmers and other control equipment for digital control, suitable for a multi-point master-slave control system, the form of its interconnection using a multi-point bus structure, there is no information channel blocking problems, its simple connection, high reliability.

Although the lighting control system has developed into a networked control system, DMX512 signal system is still the most widely used system, even for large networked lighting control systems, the end of the control is still a large number of DMX512 signal control method.

The DMX512 protocol is suitable for one-to-multipoint master-slave control network systems. According to the requirements of DMX512 data transmission rate and the characteristics of the decentralized control network, the physical layer of the design of the RS-485 bus transceiver, the bus with a pair of twisted-pair cable to achieve the dimmer and dimmer connected. RS-485 bus using balanced transmission and differential reception, high sensitivity of reception, and the ability to resist interference, the signal transmission distance of up to 1000m. If the dimmer is too far from the The DMX512 protocol uses a bus-type structure, but the data is transmitted from the dimmer to the dimmer in one direction, so there is no competition between individual dimmers for the right to use the bus and the phenomenon of information blockage.

II. Background

With the application and popularity of digital technology and computer technology, the emergence of computer dimmers, and has appeared D54, AVAB, CMX, PMX, EMX and other analog and digital communication protocols. Due to the above protocol standards are the respective protocol standards of the manufacturers, and thus the compatibility between them is not good, there is a problem between the mutual use of equipment. Later, in order to solve the problem of commonality among manufacturers' equipment, the United State Institute for Theatre Technology (USITT) developed the DMX512 protocol in the early 1980's. The DMX512 protocol is a kind of digital multiplex (DigitalMultiplex, DMX). DigitalMultiplex (DMX) protocol. After the development of the protocol, after modification, USITT in 1990 will DMX512 protocol more standardized, the formation of DMX512-1990. almost all of the current lighting and stage equipment manufacturers support this control protocol, as a widely used digital lighting data protocol, DMX512-1990 has also become the international standard for lighting control.

The unification of the DMX512 protocol makes the equipment of various manufacturers can be connected to each other, and compatibility is greatly improved. At the same time, because the DMX512 protocol uses a serial mode to transmit digital signals, the console and equipment can be as long as a signal line, greatly simplifying the connection line between the console and equipment.

III. Data Format

The DMX512 protocol has certain requirements for the data transmission rate, the typical transmission rate is 250kbps, the corresponding duration of each bit is 4us, the duration of each data frame is 44us, and the protocol also stipulates that each DMX512 packet supports a maximum of 512 frames of data transmission at a time. the format of the DMX512 packet is shown in Figure 1 below.

The DMX512 protocol stipulates that packets must be transmitted in accordance with the specified format and baud rate. The format of a complete DMX512 packet consists of an MTBP signal, a BREAK signal, a MAB signal and a SC signal, followed by the data frame. Their respective meanings are as follows.

(1) MTBPMTBP (MarkTimeBetweenPackages) marks the completion of the sending of a complete DMX512 packet and also the imminent start of the next packet is marked bit, active high, indicating that the current transmission line is idle and no data is being transmitted.

(2) BREAKBREAK is a DMX512 packet revelation control signal, corresponding to the end of a packet reset phase, the reset is completed then should send the next packet of data. The protocol specifies that the BREAK signal is active low and lasts for no less than the length of two DMX512 data frames, i.e. 88us.

(3) MABMAB (MarkAfterBreak) is a marker for the start of a packet transmission. Since the first bit of each data frame is low, the MAB signal is added in order to distinguish between the low level of BREAK and the low level of the start bit of the data frame. The protocol specifies a typical duration of 8us for the MAB, i.e. two bits, active high.

(4) SC (StartCode) SC is the start code, it is the same as a normal data frame, but its 8 data bits are all zero, marking the start of the data frame in the packet.

IV. Features

Compared with other protocols, the DMX512 protocol has the following characteristics.

(1) The signal is transmitted based on differential voltage, which makes it highly resistant to interference.

(2) The use of RS-485 bus transceivers, the signal can be transmitted over long distances.

(3) fast data refresh, regardless of whether the output of the dimmer needs to be changed, the host must send a control signal, the time between the data frame and the data frame is less than 1s, if the dimmer does not receive a new data frame within 1s, it can be known that the data has been lost.

(4) simple to implement, does not require special hardware equipment support

V. Precautions for use

There are several issues that need attention when using DMX512 signals.

①It is very useful to carry a portable DMX512 signal tester at all times, it can check the errors in the system at any time, quickly find the problem areas, timely troubleshooting, to ensure the normal operation of the lighting control system.

②The use of qualified signal cables can greatly reduce the trouble at work, according to the requirements of the DMX512 protocol standard, the use of twisted shielded wire to meet the specifications of the signal cable, never use audio cable to replace. The correct connection of the cylindrical plug core wire should also be given attention. In practice, there are many failures are caused by the signal line breakage and signal line connector desoldering as well as the wrong connection of the line number and other problems, therefore, check the signal line pass and core line phase before use, can achieve twice the result with half the effort.

③When using the three-core DMX512 cylindrical plug, especially when using the five to three or three to five signal adapter cable, you must pay special attention to carefully understand the technical requirements of the signal phase of the equipment before use, and correct the wrong signal connection in a timely manner.

④ check the controlled equipment and DMX512 cylindrical plug ground connection problems, according to the DMX512 standard provisions of the cylindrical plug No. 1 core and controlled equipment 1 line are not allowed to connect the shell, only the console DMX512 output port 1 line and its chassis connection. Therefore, after connecting the signal line of the control system, use a tester or multimeter to check whether there is a problem with the connection of the No. 1 wire to the shell, once found to be immediately excluded, otherwise it may cause damage to the signal system due to the accidental charging of the equipment shell.

⑤ When controlling different models of computer lights on the same console, problems are likely to arise when they are mixed due to differences in equipment from various manufacturers, so it is best to install isolation amplifiers between them to isolate different models of computer lights. It is also important to install isolation amplifiers between the equipment of different companies when renting computer lights or other equipment from other companies.

(6) Pay attention to the power supply of the computer lights, and the power supply of other electrical equipment (such as silicon boxes and high-powered lamps, etc.) in the studio or performance venue are used separately, otherwise it is very easy to produce damage to the computer lights caused by overloading of other electrical equipment.

(7) Remember to add a 120Ω terminal resistor at the end of the signal connection line. According to the requirements of the DMX512 signal protocol, a terminal resistor should be connected at the end of each string of signal chain to form a "closed loop" of the control loop, therefore, in practice, several more terminal resistors should be prepared.

⑧ Pay attention to the "unit load" of the DMX512 system, so that the "unit load" of each output port can not exceed 32, if necessary, increase the signal distributor. In addition, according to our experience, the length of the signal line from the console or distributor to the most end controlled equipment should be less than 200m in order to have a better control effect.

⑨ Pay attention to the correct setting of the address code, especially when using computer lights, to be clear about the number of channels of each computer light and the setting method of the address code, some computer lights (such as the American HIGHEND and Italian SGM) have their special address code setting method, to pay special attention to.

⑩In laying DMX512 signal cable, try to avoid laying parallel and close to the power cable line and lamp line, in order to reduce the power supply and dimming silicon box on the control signal interference.

*Note: The above content are from the network, only for learning and exchange, if there is infringement, please contact to delete.

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What is the lighting DMX512 protocol?