Analysis of Digital Lighting System Based on DMX512 Control Protocol
The lighting system based on the DMX512 control protocol for dimming control is called a digital lighting system. At present, various stage lighting equipment, such as computer lights, dimming controllers, consoles, color changers, electric booms, etc., have fully realized dimming with full support for the DMX512 protocol. The digitization of control, and on this basis, gradually become computerized and networked. Therefore, for the film and television lighting design and operation personnel, it is necessary to understand the program structure, control principle and application points of the DMX512 control protocol. 1, DMX512 lighting control protocol DMX is an abbreviation for Digital MultipleX, which means multi-channel digital transmission. The DMX512 control protocol is the industry standard for data transmission of lighting controllers and lighting equipment released by the US 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 an instruction frame and is called a control channel that controls one or more functions of the lighting device. A DMX instruction frame consists of one start bit, eight data bits, and two end bits of a total of ll bits, using one-way asynchronous serial transmission, as shown in Figure 1. Figure 1 DMX512 timing program frame structure (top) and packet structure (below) S in the control instruction in the dotted line in Figure 1 is the start bit, the width is one bit, which is the start flag of the controlled luminaire ready to receive and decode the control data; E is the end bit, the width is two bits, indicating the end of an instruction frame D0D7 is 8-bit control data, its level combination has 256 states from 0000~l1111111 (corresponding to 0~255 of decimal number). When controlling the brightness of the light, it can generate 256 brightness levels, 0000~(0) The light is the darkest, l1111111 (255) corresponds to the brightest light. The bit width (bit width per bit) of the DMX512 instruction is 4s, the width of each frame is 44 s, and the transmission rate is 250 kbps. A complete DMX512 packet consists of an MTBP bit, a Break bit, a MAB bit, an SC, and 512 data frames. MTBP (MarkTimeBetweenPackets) marks the completion of a complete packet transmission, is the "free bit" of the next packet to start, active high. Break is an interrupt bit, corresponding to the program reset phase after the end of a packet, and the width is not less than two frames (22 bits). The control data should be sent after the program reset ends, but since the first bit (ie, the start bit) of each data frame is low, it is necessary to use a high-level pulse interval before and after the two low-level pulses. The high-level pulse of separation is MAB (Mark After Break). When this pulse arrives, it means that the control of “new round†has started again. SC (STartCode) means to start the code frame (0th frame in Figure 1). Like the subsequent data frame, it is also composed of 11 bits. Except for the two high-level end bits, the other 9 bits are all Low level, usually called 0th frame or 0th channel (Ch~nelNo0), can be understood as a non-existing channel (NON~~istent Channe1). Table 1 DMX512 packet timing table Table 1 is the timing table of the DMX512 packet. The NS in the table means NmSpec~ed. The width is not strictly limited. It is up to the programmer to decide. For example, the width of the MTBP can be between 0 and 1 second. Each time a dimming console sends a packet, it can form a comprehensive control of all 512 controlled channels. The time to send a packet is about 23IIls, and 44 control will be completed for all 512 controlled channels every second, that is, the refresh frequency of the controlled optical path is 44Hz. If the actual controlled channel is less than 512, the refresh frequency will be corresponding. improve. 2. The basic mechanism of the DMX512 protocol A DMX interface can control up to 512 channels, because computer lights generally have several to dozens of functions. A computer light needs to occupy a few, dozens of control channels. Let's take a look at the control process and principle of the DMx5l2 through a DMX channel table with a simple function and a small number of small computer lights. The computer lamp has eight DMX control channels, one color wheel and two pattern wheels, which have functions of dimming, strobe, shaking head and changing light color and pattern. The DMX channel serial number, channel coding and corresponding functions are as shown in Table 2. Show. Table 2 DMX channel table of computer lights The DMX values ​​in Table 2 are represented by decimal numbers, and the binary combination of 07 corresponding to 8-bit control data is 00000~000000111. The binary combination corresponding to 232 to 255 is 11101000 to 11111111, and so on. The process of decoding some or all of the 8-bit binary of an instruction frame in the DMX protocol to form a function or a state change of the computer lamp is decoded and controlled. It can be clearly seen from the DMX channel table that the function of the computer light, the number of channels and their corresponding relationship are 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, a DMX interface can control 64 units (512/8=64). If the number of DMX channels of another computer is 20, then the number of DMX interfaces that can be controlled is 25 (512/20=25.6, the remainder is discarded). 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