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Fundamentals of Digital Electronics - Lab 11

Serial Communications Nhiều nhạc cụ, bộ điều khiển, và máy tính được trang bị một giao diện nối tiếp. Khả năng giao tiếp với các thiết bị này trên một giao diện nối tiếp sẽ mở ra một thế giới hoàn toàn mới về đo lường và kiểm soát. Các bit định dạng nối tiếp tiêu chuẩn, RS-232, xác định thứ tự bit và hình dạng sóng trong cả thời gian và biên độ. Ở mức tối thiểu, chỉ có ba dòng thông tin liên lạc cần thiết cho giao tiếp giữa máy tính và thiết bị bên ngoài:. | Lab 11 Serial Communications Many instruments controllers and computers are equipped with a serial interface. The ability to communicate to these devices over a serial interface opens a whole new world of measurement and control. The standard bit serial format RS-232 defines the bit order and waveform shape in both time and amplitude. At a minimum only three communication lines are needed for communication between a computer and an external device transmit receive and a reference ground. Transmit Line Receive Line Ground Line Figure 11-1. Serial Communication Lines In serial communications a high level is called a Mark state while the low level is called the Space state. In normal operation the output line is in a high state often denoted as a 1 or in LabVIEW as a Boolean True. The transmitter signals the receiver that it is about to send data by pulling the transmit line low to the space state 0 . This falling edge or negative transition is the signal for the receiver to get ready for incoming data. In RS-232 communication all data bits are sent and held for a constant period of time. This timing period is the reciprocal of the Baud rate the frequency of data transmission measured in bits per second. For example a 300 Baud data rate has a timing period of 1 300 of a second or 3.33 ms. At the start of each timing period the output line is pulled high or low and then held in that state for the timing period. Together these transitions and levels form a serial waveform. Consider an 8-bit data byte 3A or in binary 0011 1010 . For serial communication the protocol demands that the least significant bit b0 be transmitted first and the most significant bit b7 last. By convention time is represented as moving from left to right hence the above data byte would be transmitted as 01011100 in reverse order. National Instruments Corporation 11-1 Fundamentals of Digital Electronics Lab 11 Serial Communications 8-bit data rb0 bn 3A AJ 0 Figure 11-2. Serial Transmitter Sends the