DMX is an abbreviation for Digital MultripleX, which means multi-channel digital transmission. The DMX512 control protocol is an industrial standard for data transmission between light controllers and lighting equipment released by the American Stage Light Association(usITT) in 1990. The full name is USITT DMX512(1990), including electrical characteristics, data protocols, data formats, etc.. Content.
Each DMX control byte is called an instruction frame, called a control channel, and can control one or more functions of the lighting device. A DMX instruction frame consists of 1 starting bit, 8 data bits, and 2 ending bits, all ll bits, using one-way asynchronous serial transmission, as shown in Figure 1.

图1 DMX512定时程序的帧结构(上图)和信息包结构(下图)

The S in the imaginary line control instruction is the starting bit and the width is one bit. It is the starting sign of the controlled lamp ready to receive and decode the control data; E is the end bit and the width is two bits, indicating the end of an instruction frame; D0D7 is an 8-bit control data. Its level combination has 256 States from 0000 to one l1111111(corresponding to 0 to 255 of the decimal number). When controlling the brightness of the light, 256 brightness levels can be generated, and 0000 ~(0) corresponds to the light. Darkest, L 1111111(255) corresponds to the brightest light. The DMX512 instruction has a bit width(width per bit) of 4 S, a width of 44 MS per frame, and a transmission rate of 250 kbps.
A complete DMX512 packet consists of a MTBP bit, a Break bit, an MAB bit, a SC, and 512 data frames. MTBP(Mark TIme Betweek Packs) marks the completion of a complete packet of messages. It is the "free bit" that the next packet is about to start. The high level is valid. Break is an interrupt bit, corresponding to the program reset phase after the end of an information packet, and is not less than two frames(22 bits) wide. The control data should be sent after the reset of the program is over, but since the first place(ie, the starting position) of each data frame is low, two low level pulses must be used before and after a high level pulse interval. This high voltage flat pulse that acts as an interval and separation is Mark After Break. Once this pulse arrives, it means that the "new round" control begins again. The SC(Start Code) means the beginning code frame(frame 0 in Figure 1). Like the subsequent data frame, it also consists of 11 bits. In addition to the two high end bits, the other 9 bits are all Low level, It is usually called the 0 frame or the 0 channel(Ch ~ Nel No 0), which can be understood as a non-existent channel(NON ~ istent Channel 1).

表1 DMX512信息包定时表

Table 1 is a timing table for the DMX512 packet. The NS in the table means Nm Spec ~ Ed. The width is not strictly limited and is determined by the programmer. For example, the width of MTBP can be between 0 and 1 second.
Each packet sent by the dimming console can form a complete control of all 512 controlled channels. The time to send an information packet is approximately 23 IIls. Each second will complete 44 controls on all 512 controlled channels, that is, the refresh frequency of the controlled light path is 44 Hz. If the actual controlled channel is less than 512, then the refresh frequency will increase accordingly..
2. The basic mechanism of DMX512 protocol
A DMX interface can control up to 512 channels, because computer lights generally have several to dozens of functions, so. A computer lamp needs to occupy only a few, as many as dozens of control channels. The following is a DMX channel table for a small computer lamp with simple functions and fewer channels. Look at the control process and principle of DMx5l2.
The computer lamp has eight DMX control channels, one color wheel, and two pattern wheels. It has functions such as dimming, Strobe, shaking the head, changing the light color, pattern, etc.. Its DMX channel serial number, channel code, and corresponding function are shown in Table 2.
Table 2 Computer Light DMX Channel Table

he DMX values in Table 2 are represented by decimal numbers, and the binary combination of 07 corresponding to 8-bit control data is 00000 ~ 00000001 1.1.
The binary combination of 232 to 255 is 11101000 to 11111111, and so on. The process of combining some or all 8-bit binary of a certain instruction frame in the DMX protocol to form a computer lamp's functional conversion or state change is decoding and control.
From the DMX channel table, it can be clearly seen that the computer lamp function, the number of channels, and their corresponding relationships are an important basis for calculating the number of unit loads carried by a DMX interface and setting the initial address code. For example, for a computer lamp with only 8 channels, a DMX interface can control 64 units(512/8 = 64). If the number of DMX channels for the other computer lamp is 20, then the number of DMX interfaces that can be controlled is 25(512/20 = 25.6, eliminating the remainder).
3. Some Application Problems of DMX512 Control Protocol
When using the DMX512 protocol to control digital lighting devices, it is also necessary to understand the application characteristics, starting address codes, unit loads, and signal terminals of the DMX interface.
3.1 Application characteristics of the DMX interface
The DMX512 standard stipulates that the DMX interface uses 5 core cards, of which 1 core is grounded, 2, 3 and 4, and 5 core transmission control signals(2, 4 is the reverse phase end, 3, 5 is the same phase end), 4, The 5 cores originally intended to transmit information such as the state of the lighting equipment and error detection, and were later idle. The reason why it is required to use a 5-core card instead of a more common 3-core card is to prevent accidental connection with the 3-core card commonly used in professional audio. Because the 3-core card connected to the capacitor microphone on the audio device can provide an external image voltage of 48V, this wrong connection can easily burn the internal circuit. In spite of this, many computer lights still use 3 core cards. If there are two types of card ports that coexist, use a adapter to transfer them correctly.
All digital lighting devices have a DMX input interface and a DMX output interface. The DMX512 control protocol allows various lighting devices to be mixed and connected. During use, the DMX output interface of the previous device and the input interface of the next device can be directly connected.. However, it needs to be clear that this seemingly connected link architecture is actually parallel to the DMX control signal. Because the DMX control signal enters the lighting device and is "divided into two routes"(see Figure 2), the voltage is compared and amplified and shaped all the way through the discharge circuit. After decoding the instruction Pulse, the driving circuit controls the stepping motor to complete various control actions.; The other road is delivered directly to the next lighting device after being cushioned and isolated. In addition, it is not difficult to draw a conclusion from the voltage comparison effect of the operational amplifier in Figure 2: Using the high common-mode suppression capability of the operational amplifier circuit, the anti-interference ability of the DMX control signal can be greatly improved, which is why the DMX512 control signal is used. The reason for balanced transmission.

图2 灯光设备DMX接口简化电路

3.2 Initial address code
Each light device based on the DMX512 control protocol needs to be given a digital boot address number, which is the address number of the light device. The address code is used for addressing the DMX512 control signal to ensure that the device only reacts to "own" control signals.
The address code is actually the starting number of the light device control channel.
When the channel number(frame number) of the DMX512 packet is the same as the address code of a light device, that is, the given channel start number, the device begins to decode the DMX512 control signal and generate control actions. At the same time, other lighting devices on the same link did not respond to the DM ~ 12 control signal. Until the control channel number of the DMX512 packet transitions to the same number as the channel start number given by the next lighting device, the device stops being controlled, while the address code of the next lighting device functions and is in a controlled state.
Taking a computer lamp as an example, assuming that a certain DMX control port drives several computer lights, the starting address code of the first computer lamp is 001, and the starting address code of the second computer lamp is 001 plus the first lamp. The number of DMX channels, and so on. For example, the number of channels for the first and second computer lights is L6 and 20, respectively. The starting address code for the first computer lamp is 001, the starting address code for the second computer lamp is 017, and the starting address code for the third computer lamp is 037. The starting address code of the last computer lamp can not be added to its channel number by more than 512. If there are remaining computer lights, the next DMX control interface of the console should be enabled.
Digital lighting device starting address code settings are digital and dialing two types, the overall setting method is relatively simple.
3.3 Unit load of computer lights
According to the DMX512 protocol standard, each DMX interface can only control 32 unit loads if the total number of channels controlled by the lamp does not exceed 512. When there are more than 32 computer lights, Silicon boxes, color converters, or other lighting devices supporting the DMX512 control protocol, but the total number of control channels is far from 512, the DMX distributor can be used to divide the DMX signal all the way into multiple DMX branches. On the one hand, it is easy to connect each light device on the lamp stand nearby. On the other hand, each branch can drive 32 unit loads. However, the total number of channels controlled by each DMX branch that belongs to the same DMX chain can not exceed 512.
3.4 DMX terminal(fDMX terminal)
The DMX terminal is a card port connector connected to the last DMX output interface of each DMX branch. The connector is connected to a resistor with a resistance of 120 Q and a power of about 1 W. The end of each DMX branch is closed. Due to the higher DMX control pulse frequency, when the transmission line is blocked, it has the nature of returning the original route. In this way, the signal returned by the original route will be superimposed with the later signal, which will easily cause the DMX control instruction to generate a wrong code, making the computer lamp unable to properly decode. The occurrence of movement errors or control failures.
Therefore, connecting a terminal to the DMX output interface of the last computer lamp is conducive to ensuring the stability of the computer lamp.