DMX512 is a digital dimming protocol, which can be used to digitally control dimmer switch and other control devices in stages, theaters, studios and other places. It is suitable for master-slave control systems with more points. Its interconnection form adopts multi-point bus structure, and there is no problem of information channel blocking. Its connection is simple and its reliability is high.

Although the lighting control system has developed into the networked control system, DMX512 signal system is still the most widely used system at present. Even for large-scale networked lighting control system, DMX512 signal control mode is still widely used at the end of control.

DMX512 protocol is applicable to one-to-many master-slave control network systems. According to the requirements of DMX512 data transmission rate and the characteristics of decentralized control network, its physical layer design adopts RS-485 bus transceiver, and the bus uses a pair of twisted pairs to realize the connection between the dimmer and the dimmer switch. RS-485 bus adopts balanced transmission and differential reception, with high reception sensitivity and strong anti-interference ability, and the signal transmission distance can reach 1000m. If the dimmer switch is too far away from the dimmer, an amplifier or a signal separator can be used to enhance the signal. The DMX512 protocol adopts a bus structure, but the data is transmitted from the dimmer to the dimmer switch in one direction. Therefore, there is no phenomenon that information is blocked due to the competition for bus use rights among various dimmer switch.

With the application and popularization of digital technology and computer technology, computer dimmer has appeared, and analog and digital communication protocols such as D54, AVAB, CMX, PMX, EMX have appeared successively. Since the above protocol standards are the respective protocol standards of each manufacturer, their compatibility is not good and there are problems in the mutual universality of equipment. Later, in order to solve the problem of equipment universality of various manufacturers, the American theater Technology Association (United State Institute for Theatre Technology,USITT) formulated the DMX512 protocol in the early 1980 s. DMX512 is a Digital multiplexing (Digital Multiplex,DMX) protocol. After the protocol was formulated and modified, USITT made the DMX512 protocol more standardized in 1990, forming a DMX512-1990. At present, almost all lighting and stage equipment manufacturers support this control protocol. As a widely used digital lighting data protocol, DMX512-1990 has also become an international standard for lighting control.

The unification of DMX512 protocol enables the devices of various manufacturers to connect to each other, thus greatly improving the compatibility. At the same time, because DMX512 protocol adopts serial mode to transmit digital signals, only one signal line is needed between the console and the equipment, which greatly simplifies the connection line between the console and the equipment.

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

The DMX512 protocol stipulates that data packet transmission must be carried out according to the specified format and transmission baud rate. A complete DMX512 data packet format consists of an MTBP signal, a BREAK signal, an MAB signal, an SC signal, and subsequent data frames. Their meanings are as follows:

(1)MTBPMTBP(Mark Time Between Packages) indicates that a complete DMX512 data packet has been sent. At the same Time, the next data packet is about to start to be marked bit. The high level is valid, indicating that the current transmission line is idle and there is no data transmission.

(2)BREAKBREAK is a revelation control signal of DMX512 data packet, corresponding to the reset stage after a data packet ends. After the reset is completed, the data of the next packet should be sent. The protocol stipulates that the BREAK signal is valid at a low level and the duration is not less than the length of two DMX512 data frames, namely 88us.

(3)MABMAB(Mark After Break) is the sign that a data packet starts to send. Since the first bit of each data frame is low, MAB signals are added to distinguish the low level of BREAK from the low level of the start bit of the data frame. The protocol stipulates that the typical duration of MAB is 8us, that is, two bits of time, and the high level is valid.

(4)SC(Start Code)SC is the starting Code, which is the same as an ordinary data frame, but its 8-bit data bits are all zero, indicating the beginning of the data frame in the data packet.

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

(1) the signal is transmitted based on differential voltage and has strong anti-interference ability;

(2) RS-485 bus transceiver is adopted, and signals can be transmitted over long distances;

(3) The data is refreshed quickly. No matter whether the output of the dimmer switch needs to be changed or not, the host must send a control signal. The time between the data frame and the data frame is less than 1s. If the dimmer switch does not receive a new data frame within 1s, it can be known that the data has been lost;

Several issues to be paid attention to when using DMX512 signals.

① it is very useful to carry a portable DMX512 signal tester at any time. It can check the errors in the system at any time, find the problem parts quickly, eliminate the faults in time, and ensure the normal operation of the lighting control system.

② using qualified signal lines can greatly reduce the trouble in work. According to the requirements of DMX512 protocol standard, select twisted pair shielded lines that meet the specifications to make signal lines, and audio lines must not be used instead. The correct connection of cylindrical plug core wire should also be paid attention. In practical work, many faults are caused by problems such as the rupture of signal lines, the desoldering of signal line joints and the wrong connection of line numbers. Therefore, checking the on-off of signal lines and the phase of core lines before use can achieve twice the result with half the effort.

③ when using three-core DMX512 cylindrical plug, especially when using five-to-three or three-to-five signal adapter, special attention must be paid. Before using, the technical requirements of the equipment on the signal phase should be carefully understood, and the wrong signal connection should be corrected in time.

④ check the ground wire connection between the controlled equipment and the DMX512 cylindrical plug. According To The DMX512 standard, the No. 1 core of the cylindrical plug and the No. 1 line of the controlled equipment are not allowed to be connected to the shell. Only the No. 1 line of the DMX512 output port of the console is allowed to be connected to its chassis. Therefore, after connecting the signal lines of the control system, use the tester or multimeter to check whether there is a shell connection problem of Line 1. Once it is found out, it should be eliminated immediately. Otherwise, the signal system may be damaged due to accidental electrification of the equipment shell.

⑤ when controlling different types of computer lights on the same console, because the equipment of different manufacturers will be different, it is easy to cause problems when mixed installation and use. Therefore, it is better to install isolation amplifiers between them to isolate different types of computer lights. In addition, when renting computer lights or other equipment from other companies, it is also important to install isolation amplifiers between equipment from different companies.

⑥ pay attention to use the power supply PSU of computer lights separately from the power supply PSU of other electrical equipment (such as silicon box and high-power lamps, etc.) in the studio or performance place, otherwise it is easy to cause damage to computer lights caused by overload of other electrical equipment.

⑦ keep in mind that 120Ω terminal resistance should be added to the end of the signal connection line. According to the requirements of DMX512 Signal Protocol, terminal resistance should be connected to the end of each string of signal chains to form a "closed loop" of the control loop. Therefore, in actual work, more terminal resistance should be prepared.

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

⑧ Pay attention to the correct setting of the address code, especially when using computer lights, you should be clear about the number of channels of each computer light and the setting method of the address code. Some computer lights (such as HIGHEND in the United States and SGM in Italy) have their own special setting methods of the address code, which should be paid special attention.

⑩ when laying DMX512 signal cable, try to avoid laying in parallel and close proximity with PSU cable and light line to reduce the interference of PSU and dimming silicon box on control signal