Serial communication definition
The serial port is a very common device communication protocol on the computer. Most computers contain two serial ports based on RS232. The serial port is also a common communication protocol for instrumentation equipment; many GPIB compatible devices also have RS-232 ports. At the same time, the serial communication protocol can also be used to obtain data from remote acquisition devices.
Serial communication principle
The concept of serial communication is very simple. The serial port sends and receives bytes in bits. Although it is slower than parallel communication by byte, the serial port can use one wire to send data while receiving another data. It is simple and enables long-distance communication. For example, when IEEE488 defines the parallel traffic state, the total length of the device line must not exceed 20 meters, and the length between any two devices must not exceed 2 meters; while for the serial port, the length can reach 1200 meters. Typically, the serial port is used for the transmission of ASCII characters. Communication is done using 3 wires: (1) ground wire, (2) send, (3) receive. Since serial communication is asynchronous, the port can send data on one line while receiving data on the other line. Other lines are used for handshake, but they are not necessary. The most important parameters of serial communication are baud rate, data bits, stop bits and parity. For two ports to communicate, these parameters must match:
a, Baud rate: This is a parameter to measure the communication speed. It represents the number of bits transmitted per second. For example, 300 baud means that 300 bits are sent per second. When we refer to the clock cycle, we are referring to the baud rate. For example, if the protocol requires 4800 baud rate, then the clock is 4800 Hz. This means that the sampling rate of serial communication on the data line is 4800 Hz. The baud rates of telephone lines are usually 14400, 28800 and 36600. The baud rate can be much greater than these values, but the baud rate is inversely proportional to the distance. High baud rates are often used for communication between instruments placed close together. A typical example is the communication of GPIB devices.
b. Data bit: This is a parameter to measure the actual data bit in communication. When the computer sends a packet, the actual data will not be 8 bits, the standard values ​​are 5, 7 and 8 bits. How to set depends on the information you want to send. For example, the standard ASCII code is 0 to 127 (7 bits). The extended ASCII code is 0 to 255 (8 bits). If the data uses simple text (standard ASCII code), then each packet uses 7 bits of data. Each packet refers to a byte, including start / stop bits, data bits and parity bits. Since the actual data bits depend on the choice of communication protocol, the term "packet" refers to any communication situation.
c, stop bit: used to indicate the last bit of a single packet. Typical values ​​are 1, 1.5 and 2 digits. Since the data is timed on the transmission line, and each device has its own clock, it is likely that there is a small asynchronization between the two devices in the communication. Therefore, the stop bit not only indicates the end of the transmission, but also provides an opportunity for the computer to correct the clock synchronization. The more bits that are suitable for stop bits, the greater the tolerance of different clock synchronization, but the slower the data transmission rate.
d. Parity bit: a simple error detection method in serial communication. There are four error detection methods: even, odd, high and low. Of course, there is no check digit. For the case of even and odd parity, the serial port will set the parity bit (the bit after the data bit), and use a value to ensure that the transmitted data has even or odd logic high bits. For example, if the data is 011, then for even parity, the parity bit is 0, ensuring that the number of logically high bits is an even number. If it is odd parity, the parity bit is 1, so there are 3 logic high bits. The high and low bits do not really check the data, simply set the logic high or logic low check. This allows the receiving device to know the status of a bit, and has the opportunity to determine whether noise interferes with the communication or whether the transmission and reception of data is not synchronized.
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