Han Zhenlei, Qi Lisen—— Department of Media Technology, School of Computer Science and Technology, Shandong University of Technology

　　 The lighting system based on the DMX512 control protocol for dimming control is called a digital lighting system. At present, various stage lighting equipment including computer lights, dimming controllers, consoles, color changers, electric booms and other stage lighting equipment, with its full support for DMX512 protocol, have fully realized dimming The digitalization of control, and on this basis, gradually tends to be computerized and networked. Therefore, for film and television lighting designers and operators, it is very necessary to understand the program structure, control principles and application points of the DMX512 control protocol.

DMX512 lighting control protocol

　　DMX is the abbreviation of Digital MultipleX, which means multiple digital transmission. The DMX512 control protocol is an industry standard for data transmission between lighting controllers and lighting equipment issued by the United States Stage Lighting Association (usITT) in 1990. The full name is USITT DMX512 (1990), including electrical characteristics, data protocols, data formats, etc. . Each DMX control byte is called a command frame, called a control channel, which can control one or several functions of the lighting equipment. A DMX command frame is composed of 1 start bit, 8 data bits and 2 end bits, a total of ll bits, and adopts unidirectional asynchronous serial transmission.

The basic mechanism of DMX512 protocol

　　One DMX interface can control up to 5l2 channels, because scanners generally have several to dozens of functions, so. A computer light needs to occupy as few as a few to as many as dozens of control channels. Let's take a look at the control process and principle of DMx5l2 through a DMX channel table of small scanners with simple functions and fewer channels. The scanner has eight DMX control channels, a color wheel, and two pattern wheels. It has the functions of dimming, strobe, head shaking and changing light colors and patterns. Its DMX channel serial number, channel code and corresponding functions are shown in Table 2. Show.

　　 The DMX values ​​in Table 2 are expressed in decimal numbers. The binary combination of 0 7 corresponding to 8-bit control data is 00000~0 000001 1. The binary combination corresponding to 232~255 is 11101000~11111111, and so on. Combining part or all of the 8-bit binary code of a certain command frame in the DMX protocol to form a computer lamp-the process of function conversion or state change is decoding and control. It can be clearly seen from the DMX channel table that the scanner 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 starting address code. For example, like this kind of computer light with only 8 channels, the number that can be controlled by a DMX interface is 64 (512/8=64). If the number of DMX channels of another scanner is 20, then the number that can be controlled by a DMX interface is 25 (512/20=25.6, discard the remainder)

Several application problems of DMX5 12 control protocol

　　 When applying DMX512 protocol to control digital lighting equipment, it is also necessary to understand the application characteristics of DMX interface, starting address code, unit load and signal terminator. 3.1 Application characteristics of DMX interface DMX512 standard stipulates that DMX interface uses 5-core bayonet port, of which 1 core is grounded, 2, 3 and 4, 5 cores transmit control signals (2, 4 are inverted ends, 3, 5 are in phases) End), 4 and 5 cores were originally intended to transmit information such as the status and error detection of the lighting equipment, but were then left unused. The reason why it is required to use a 5-core bayonet instead of the more common 3-core bayonet is to prevent accidental connection with the 3-core bayonet commonly used in professional audio, because the audio equipment is connected to the 3-core condenser microphone Kaykou can provide a phantom voltage of 48 v externally. This kind of wrong connection can easily burn out the internal circuit. Nevertheless, many scanners still use 3-core bayonet ports. If two types of bayonet ports coexist, they must be properly connected with an adapter.

All digital lighting equipment has a DMX input interface and a DMX output interface. The DMX512 control protocol allows various lighting equipment to be mixed and connected. In use, the DMX output interface of the previous device can be directly connected to the input interface of the next device. stand up. However, it should be clear that this kind of seemingly series link architecture is actually parallel to the DMX control signal. Because the DMX control signal enters the lighting equipment, "the soldiers are divided into two paths" (see Figure 2). After voltage comparison, amplification and shaping of the op-amp circuit, the command pulse is decoded, and then the stepping motor is controlled by the drive circuit to complete various Control action; the other way is directly transported to the next lighting equipment after buffering and isolation. In addition, it is not difficult to draw such a conclusion from the voltage comparison effect of the op amp in Figure 2: the use of the high common-mode rejection capability of the op amp circuit can greatly improve the anti-interference ability of the DMX control signal, which is why DMX512 control The reason for the balanced transmission of the signal.

Start address code

　　 Each lighting device based on the DMX512 control protocol needs to be assigned a digital start address number, which is the address code of the lighting device. The address code is used to address the DMX512 control signal to ensure that the device only responds to the "own" control signal. The address code is actually the starting serial number of the control channel of the lighting equipment. When the channel number (frame sequence number) of the DMX512 information packet is the same as the address code of a certain lighting device, that is, the channel starting sequence number assigned, the device starts to decode the DMX512 control signal and generate a control action. At the same time, other lighting devices on the same link do not respond to the DM~12 control signal. Until the control channel number of the DMX512 information packet transitions to the same channel starting number assigned by the next lighting device, the device stops controlling, and the address code of the next lighting device plays a role and is in a controlled state. Take scanners as an example, suppose a DMX control port drives several scanners, the start address code of the first scanner is 001, and the start address code of the second scanner is 001 plus the DMX channel of the first scanner Number, and so on. For example, if the channel numbers of the first and second scanners are 16 and 20 respectively, the start address code of the first scanner is 001, the start address code of the second scanner is 017, and the third computer The starting address code of the lamp is 037. The sum of the starting address code of the last scanner and the number of channels cannot exceed 512. If there are remaining scanners, the next DMX control interface of the console should be activated. There are two ways to set the start address code of digital lighting equipment: digital and dial code. Generally speaking, the setting method is relatively simple. Different brands of products may have different operating methods, which can be set according to the manual.

Units of Load (Units of Load)

　　According to the DMX512 protocol standard, each DMX interface can only control a maximum of 32 unit loads under the premise that the total number of channels of the controlled lamps does not exceed 512. When there are more than 32 computer lights, silicon boxes, color changers or other lighting devices that support DMX512 control protocol, but the total number of control channels is far from 512, a DMX splitter can be used to divide one DMX signal into multiple DMX supports On the one hand, it is convenient to connect the various lighting equipment on the lamp holder nearby, and on the other hand, each branch can drive 32 unit loads. However, the total number of channels controlled by each DMX branch on the same DMX link still cannot exceed 512.

DMX Terminator (fDMX Terminator)

The DMX terminator is a XLR connector connected to the DMX output interface of the last lighting device of each DMX branch. The connector should be connected to a resistor with a resistance of 120 Q and a power of about 1 W, so that each The end of the DMX branch is closed. Due to the high frequency of the DMX control pulse, it has the nature of returning to the original path when the transmission line is blocked. In this way, the signal returned from the original path will be superimposed with the later signal, which can easily cause errors in the DMX control command and make the scanner malfunction. Decoding, there is a phenomenon of operation error or control failure. Therefore, connecting a terminal to the DMX output interface of the last scanner will help ensure the stable operation of the scanner.

　　 Compared with the traditional analog dimming system, the digital lighting system based on the DMXS12 control protocol has brought earth-shaking changes to the lighting effects of large and medium-sized film and television studios and variety shows with its powerful control functions. However, the DMX512 lighting control standard also has some shortcomings. For example, the speed is not fast enough, the transmission distance is not far enough, the wiring and initial settings become too cumbersome as the system scale increases, and the control data can only be transferred from the control end to the controlled unit. One-way transmission, unable to detect the working condition and online status of the lamp, and it is prone to transmission errors. Later, the revised and perfect DMX512-A standard supports two-way transmission, can return information such as the error diagnosis report of the lamp, and is compatible with all lighting equipment that conforms to the DMX512 standard. In addition, the decoding circuit of some lighting equipment supports 12-bit and 12-bit data expansion mode, which can obtain more precise control.