In serial communication, data is usually transmitted between two stations. According to the transmission direction of data on the communication line, it can be divided into three basic transmission modes: simplex, half-duplex and full-duplex, as shown in Fig. 1. -1 is shown.
Figure 1-1 Simplex, half-duplex, and full-duplex communication
Simplex communication uses a single wire, and the transmitter and receiver of the signal have a clear directionality. In other words, communication takes place only in one direction.
If the same transmission line is used as both the receiving line and the transmitting line, although the data can be transmitted in both directions, the communication parties cannot simultaneously transmit and receive data. Such a transmission method is called half duplex. When the half-duplex mode is adopted, the transmitter and the receiver at each end of the communication system are time-divisionally transferred to the communication line through the transceiver switch to perform direction switching.
When data is transmitted and received, which are transmitted by two different transmission lines, both communication parties can perform transmission and reception operations at the same time. This transmission mode is full duplex. In the full-duplex mode, the transmitter and the receiver are provided at each end of the communication system, so that data can be controlled to be transmitted in both directions simultaneously. The full-duplex mode does not require direction switching.
Serial communication can be divided into two types, one is synchronous communication and the other is asynchronous communication.
When using synchronous communication, all characters are grouped into one group, so that characters can be transmitted one by one, but synchronization characters are added at the beginning of each group of information, and when there is no information to be transmitted, a null character is filled because of synchronous transmission. No gaps are allowed.
When asynchronous communication is used, the transmission interval between two characters is arbitrary, so some data bits are used as separator bits before and after each character.
In the industrial consumer and other application fields, we often see more types of asynchronous communication, such as the common RS-232, RS-485 interface.
1, RS232 interface common RS232-C interface connector generally uses the model 9 DB-9 9-pin connector, but only 3 interface lines, that is, "send data", "receive data" and "signal ground" To transfer data, the definition of its 9 pins is shown in Figure 1-2.
Figure 1-2 RS232-C interface connector definition
In the RS232 specification, the voltage value is +3V~+15V (usually +6V) is called "0" or "ON". The voltage is -3V~-15V (usually -6V) is called "1" or "OFF"; the RS232 "high potential" on the computer is about 9V, and the "low potential" is about -9V.
RS232 is a full-duplex mode of operation. The voltage of the signal is obtained by reference to the ground. It can transmit and receive data at the same time. In the practical application, the RS232 interface is adopted, and the transmission distance of the signal can reach 15m. However, RS232 only has a single station function, that is, one-to-one communication.
2, RS485 interface common RS485 interface connector is mainly based on 3-hole terminal block, as shown in Figure 1-3:
![[Science] Common two kinds of serial communication](http://i.bosscdn.com/blog/13/50/45/b13_0.png)
RS485 uses two positive and negative signal lines as transmission lines. The voltage difference between the two lines is +2V~6V, indicating logic "1": the voltage difference between the two lines is -2V~6V, indicating logic "0".
RS485 is a half-duplex working mode, and its signal is obtained by subtracting the signal levels of the positive and negative lines. It is a differential input mode, which has strong anti-common-mode interference capability, that is, good anti-noise interference; in practical applications, its transmission distance is up to 1200 meters. RS485 has multi-station capability, that is, one-to-many master-slave communication.
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