introduction
The lighting control system uses a large number of DMX512 communication protocols proposed by USIT T (American Theatre Technology Association), which uses an asynchronous communication format. The luminaire control scheme mainly includes the DMX512 protocol, the DAL I bus scheme, and the SPI shift bus scheme. DMX512 is the most popular control method nowadays, but in practical applications, the controller interconnection is used to control the lighting scheme, and the software is more complicated. The DALI bus is a lighting control bus solution proposed by Europe. Each luminaire in the DALI system has an address and a set of control commands, but the communication speed is not fast enough. The SPI shift bus scheme adopts a cascade method, and the rate of change is high. However, the data amount of this method is relatively large, the lines between the lamps are large, the software is difficult, and the product reliability is poor.
The DMX512 standard requires that each luminaire has a controller, and setting the address for each lamp is cumbersome, and the asynchronous transmission method is also cumbersome. In response to this shortcoming, this paper proposes the design of LED control system based on SD5128 synchronous DMX512. This paper presents the software and hardware design method of synchronous DMX512 implementation. The storage interface chip is added to the hardware to realize the mobile storage of lighting control data, which has better scalability. Synchronous DMX512 is a synchronous serial transmission using a synchronous DMX512 bus (with one line for shifting and placing control data), taking advantage of both DMX512 and SPI shift buses. Synchronous DMX512 has the same electrical characteristics and data structure as the DMX512 protocol. It uses synchronous transmission to perform two-line shift transmission, and adopts SPI one-bit transmission method to avoid the address setting problem of DMX512 protocol. The synchronous DMX512 retains the electrical characteristics of the DMX512, so the RS485 bus is not used when the distance between the lamp and the lamp is not very far.
1 DMX512 Lighting Control Protocol
The DMX512 protocol is a digital dimming protocol that enables digital control of dimmers and other control devices in stage, theater, studio, etc., for point-to-multipoint master-slave control. The interconnection form adopts a multi-point bus structure, and there is no information path blocking problem, and the reliability is high.
The protocol specifies that the transmission of control signal data packets takes place by means of asynchronous communication. A DMX512 packet contains a start code and 512 data frames. Each frame of data includes a 1-bit low start bit, 8 data bits, and a 2-bit high stop bit. One data frame represents one control channel, so the protocol supports 512 control channels. The frame structure and packet structure of the DMX512 protocol are shown in Figure 1.
2 SD5128 and its synchronous DMX512 protocol
The SD5128 is a highly integrated low power LED control chip. It uses single-wire transmission mode, built-in sampling/clock regeneration circuit, digital phase-locked circuit, shift register, pulse width modulation module, cascading capacity over 1024 chips; 3 I/O channels, 8 mA output per channel The current, in an externally driven operating mode, drives the triode and MOSFET. The SD5128 uses 24-bit true color (8R+ 8G+ 8B) in combination with the DMX512 protocol data format. The frame structure and packet structure of the SD5128 are shown in Figure 2.
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