【導(dǎo)讀】使用任何PIC微控制器提供一個(gè)通用異步收發(fā)器模塊。特別需要指出的是，通用設(shè)備。它提供了1024字節(jié)的數(shù)據(jù)存儲(chǔ)器,它允許演示代。碼來存儲(chǔ)數(shù)據(jù),為整個(gè)512通道緩沖。只有一個(gè)外部RS-485兼容。的收發(fā)器必須完成應(yīng)用程序的框架。DMX解決方案分為兩部分：。這部分將解釋如何生成和傳輸標(biāo)準(zhǔn)DMX512數(shù)據(jù)包。一個(gè)演示程序，說明如何發(fā)送命令到一個(gè)標(biāo)準(zhǔn)DMX512調(diào)光接收器。這需要長期在臺(tái)上線供電的燈具，整個(gè)團(tuán)隊(duì)將。需要手動(dòng)控制變壓器。最后，模擬控制采取了自耦。來自不同制造商的設(shè)備進(jìn)行互操作的需要。DMX512協(xié)議濃度，后來擴(kuò)大并于1990年改善。該電流的發(fā)展標(biāo)準(zhǔn)DMX512-A是目前管理的娛樂服務(wù)。已獲得了很大的普及。芯或XLR三針連接器標(biāo)準(zhǔn)DMX512接收器。這個(gè)說明書規(guī)定的2屏蔽電纜對應(yīng)該使用。為了避免任何的時(shí)間問題上，應(yīng)稱為狀態(tài)。這可以大大減少為降低發(fā)射功能的微型程序控制器。這就相當(dāng)于微控制器的引腳對應(yīng)于模擬輸入通道0。

　　

【正文】 n the Break condition, a frame error is reported. Since there is no way to predict at which point of a transmission sequence the receiver will be activated, during this phase the UART is polled continuously in a loop to discard any data received until a first framing error is detected. 2. Once the Break condition is identified, the receiver needs to wait for the line to return to the Idle state (mark) and a first byte of data to this phase the UART is polled continuously as frame errors continue to be the first byte received correctly is interpreted as the Start code. In this simple application only frames with a Start code of 0 are received, frames beginning with a different Start code (DMX512 extensions) are ignored. 3. The last part consists, once more, of a loop where the receiver captures up to 512 bytes of data and stores them sequentially in the receiver buffer. A 12bit pointer, available in the PIC18 architecture, is used to provide linear memory access to the RAM memory space. APPLICATION DEMO In the previous section we saw how to get the DMX512 data for 512 channels and to store them into a receiver buffer. In this section we will use the received data to control the PWM module of a PIC microcontroller. Connecting a LED to the PWM output pin we will observe the LED brightness change in response to DMX512 dimming mands. The PIC18F24J10 Capture Compare and PWM (CCP) module offers 10bit resolution. When used in PWM mode, it uses Timer2 as its time base and the PR2 register determines the PWM period. Since the DMX512 protocol provides only 8bit of resolution for each channel, setting the PR2 register to ‘0xFF’ allows us to use just the 8 Most Significant bits to control the duty cycle while still providing a PWM output frequency of approximately 16 kHz. This value greatly exceeds the minimum requirement, of approximately 100 Hz, usually considered sufficient to eliminate any visible flicker of the LED. Since the Most Significant 8 bits of the PWM duty cycle are controlled by the CCPR2L register, it is sufficient to periodically update it copying the contents of the location corresponding to the desired DMX512 address (defined by the constant CHANNEL) from inside the receive buffer. In the demonstration code, the CCPR2L register is updated every time a plete DMX512 frame has been received. 15 FIGURE 7: DMX512 RECEIVER CIRCUIT SCHEMATIC In the schematic, the EUSART receiver pin is connected to the RS485 transceiver’s receiver output 120Ω, 188。 W resistor should be connected between DMX and DMX+ data link as a line terminator. Figure 7 shows the line terminator between pin 2 (DMX data link) and pin 3 (DMX+ data link) of an XLR3 Termination greatly reduces signal transmission problems. SETUP To test the DMX512 transmitter and receiver, a separate pair of PICDEM? 2 PLUS demo boards was used. The PICDEM 2 PLUS can be used to demonstrate the capabilities of 18, 28 and 40pin PIC16 and PIC18 devices. The board has a small prototyping area where the transmitter and receiver transceiver circuits can be built. In order to take advantage of the (4) LEDs available on the board for the receiver demo, the output of the PIC18F24J10 CCP2 module can been redirected to PORTB output pin RB3 by modifying the microcontroller nonvolatile Configuration register CONFIG3H,‘CCP2 MUX’ bit. The provided transmitter and receiver demonstration code uses the polling method to transmit and receive the DMX512 packets. The CPU is waiting for a timer to expire to generate the mark and the Break signals or for the EUSART to transmit or receive the data. To reduce the CPU polling time, the provided code can be written using interrupts. This application note presents a very simple software solution to generate, transmit and receive the DMX512 signals using a lowcost MCU.