PLC communication protocol - Solutions - Huaqiang Electronic Network

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In order to transmit reliably and receive data, both parties must have a specified data format, synchronization mode, transmission rate, error correction mode, control characters, etc., that is, a special communication protocol is required. Strictly speaking, any communication requires a communication protocol, but in some cases, its requirements are relatively low, and implementation is relatively simple. In the plc control system, it is customary to simply perform a data exchange, such as "no protocol communication", by simply setting parameters such as the data format and transmission rate of the transmission. The communication that will require the installation of dedicated communication tool software and the special processing of data through the program in the tool software is called "dedicated protocol communication".

(1), special protocol communication

Dedicated protocol communication refers to a communication method in which data exchange between a PLC and an external device is performed by installing PLC-specific communication tool software on an external device.

The advantage of dedicated protocol communication is that it can directly use the external device to read, write, edit, and read and write the status of the PLC programming component, and read and write the buffer memory of the special function module. It can also control the operation and stop of the PLC through remote commands. , or monitor the operating status of the PLC. However, the external equipment should be installed and the special tool software required for PLC communication must be installed. In general, after installing a dedicated tool software, the external device can automatically create a communication application that can communicate directly without PLC programming.

(2), no protocol communication

Non-protocol communication is a communication method that requires only simple setting of data format, transmission rate, start/stop code, and the like, and direct data transmission and reception between the PLC and the external device.

No protocol communication is generally required through special PLC application instructions. In the data transmission process, the data format can be converted by the control of the application instruction, such as ASCII code and HEX (hexadecimal) conversion, frame format conversion, and the like. The advantage of non-protocol communication is that the external device does not need to install dedicated communication software, so it can be used for communication of many simple peripherals such as printers, barcode readers, and the like.

(3), two-way protocol communication

The two-way protocol communication is a communication method of transmitting and receiving data with an external device through a communication interface using an information format of a PLC communication module. Bidirectional protocol communication can only be used for 1:1 connection and needs to be performed by special PLC application instructions. In the data transmission process, the data format can be converted by the control of the application instruction, such as ASCII code and HEX (hexadecimal) conversion, frame format conversion, and the like.

When the two-way protocol communication data is transmitted and received, it is generally required to perform a "sum" check. If an external device that communicates in a two-way protocol can transmit/receive data in accordance with the information format of the communication module, it is not necessary to install dedicated communication software. In the communication process, it is necessary to respond by data transmission response information ACK, NAK, and the like.