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PLC serial communication principle

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PLC serial communication principle


All PLC mainframes have built-in standard configuration of 2 communication interfaces, namely an RS232 and an RS485 communication interface. The RS232 interface is mainly used for uploading and downloading programs or for communicating with the host computer and touch screen, while the RS485 interface is mainly used for building Use the network of RS485 protocol to realize communication control.


1. RS232 interface RS232-C interface connector generally uses a 9-pin socket with model DB-9, and only needs 3 interface lines, namely "send data", "receive data" and "signal ground" to transmit data


RS232-C interface connector definition


In the specification of RS232, the voltage value between +3V~+15V (usually +6V is used) is called "0" or "ON". The voltage between -3V~-15V (usually -6V is used) 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 working mode, and the voltage of its signal is obtained by referring to the ground wire, which can transmit and receive data at the same time. In practical applications, the RS232 interface is used, and the signal transmission distance can reach 15m. However, RS232 only has a single station function, that is, one-to-one communication.


2. RS485 interface RS485 uses positive and negative signal lines as transmission lines. The voltage difference between the two lines is +2V~6V, which means logic "1"; the voltage difference between the two lines is -2V~6V, which means logic "0".


RS485 is a half-duplex working mode. Its signal is obtained by subtracting the positive and negative line signals. It is a differential input method. It has strong anti-common mode interference ability, that is, good anti-noise interference; in practical applications, its transmission distance can reach 1200 meters. RS485 has multi-station capability, that is, one-to-many master-slave communication.
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 methods: simplex, half-duplex and full-duplex, as shown in Figure 2 shown.


Briefly describe the principle of RS232 and RS485 communication interface in PLC


Simplex communication uses a single wire, and the sender and receiver of the signal have clear directionality. That is, the communication only takes place in one direction.


If the same transmission line is used as both the receiving line and the sending line, although the data can be transmitted in both directions, the two communicating parties cannot send and receive data at the same time. Such a transmission method is called half-duplex. When the half-duplex mode is adopted, the transmitter and receiver at each end of the communication system are time-divisionally transferred to the communication line through the transceiver switch to switch directions.


When the data is sent and received by two different transmission lines, both parties in the communication can send and receive at the same time. This transmission method is full-duplex. In full-duplex mode, each end of the communication system is provided with a transmitter and a receiver, so that control data can be transmitted in both directions at the same time. 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 a group, so that the characters can be transmitted one by one, but a synchronous character should be added at the beginning of each group of information. No gaps are allowed. When using asynchronous communication, the transmission interval between two characters is arbitrary, so some data bits are used as separation bits before and after each character. In comparison, when the transmission rate is the same, the information efficiency in the synchronous communication mode is higher than that in the asynchronous mode, because the proportion of non-data information in the synchronous mode is relatively small. However, on the other hand, the synchronization method requires that both parties in the information transmission must use the same clock to coordinate, and it is this clock that determines the position of each information bit in the synchronous serial transmission process. In this way, if the synchronization method is adopted, the clock signal must also be transmitted at the same time as the data is transmitted. In the asynchronous mode, the clock frequency of the receiver does not have to be exactly the same as the clock frequency of the sender, but only needs to be relatively close, that is, not exceeding a certain allowable range.

 
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