1. Introduction As mainstream products in the industrial control industry, PLCs and frequency converters are ubiquitous in various mechanical settings. A common method is to use analog signals (usually voltage) to control the frequency converter. The disadvantages of this method are high cost, susceptibility to interference (voltage-based), and difficulty in achieving high control accuracy. Communication methods can effectively avoid these drawbacks. This article introduces the communication between the V80 series PLC and the frequency converter and its implementation method. 2. Communication Features of the V80 Series PLC The V80 series PLC is a general-purpose, high-performance, small programmable controller (PLC) manufactured by Shenzhen Detian'ao Technology Co., Ltd. It uses a 32-bit high-performance CPU chip and a high-speed logic parsing ASIC chip. Compared to ordinary small PLCs, it has the following characteristics in communication applications: 1. It has dual serial ports: one is an RS-232 programming port, operating in MODBUS slave mode, generally used only for programming and connecting HMIs. The other is an RS-485 interface, which, in addition to having MODBUS slave mode functionality, can also operate in MODBUS master mode and free communication mode, possessing powerful communication capabilities. 2. With 48K of program space, 9000 intermediate links, and 9999 internal registers, coupled with extremely powerful application instructions, it can easily write very complex programs and is well-suited for complex communication protocols. 3. The default communication protocol for the V80 PLC is the MODBUS slave protocol (RTU), and the addressing method of internal variables also follows the protocol, giving it unparalleled advantages in MODBUS communication applicability. 3. Variable frequency communication with compatible standard MODBUS RTU protocol: For devices using the MODBUS RTU slave protocol, the V80 series PLC's serial port 2 can be set to MODBUS master mode for direct interconnection. The following uses the Teco 7200MA frequency converter as an example to illustrate the communication between the V80 series PLC and the frequency converter, and its implementation method: The Teco 7200MA frequency converter adopts the MODBUS RTU slave protocol. The communication data format is described as follows: In the MODBUS RTU mode communication protocol, a message consists of four parts: Slave address, function code, data, and CRC-16 data checksum, which are sent in sequence. The start and end of each message are identified by a 3.5-character interval. Only three commands are used: read, loop test, and write. The Teco 7200MA frequency converter adopts part of the MODBUS slave protocol, supporting two commands: 03 and 10H. For the V80 series PLC, this means the read/write register 4XXXX command, which can be directly connected to the V80. The V80_PLC's M_BUS instruction has already encapsulated these data processes. Therefore, only the following parameters need to be considered: 1. Read/write type and device address. 2. Target device register (coil) number, local data storage register (coil) number, and information length. 3. Communication interval, i.e., the time required to complete two communications. 4. Communication parameters, such as baud rate, parity bit, and communication timeout. The following program can be written to achieve communication with the Teco 7200MA frequency converter. Assuming the device address on the Teco frequency converter is 1, the program in the above figure performs the following function: writing the contents of variables 41100~41115 inside the PLC to registers in the Teco frequency converter register area offset from 00 to 015; the intermediate relay 01000 changes from 0 to 1 each time, and the two parties communicate once. Note that the PLC parameters need to be set beforehand; refer to the V80 series PLC software manual. 4. Communication with frequency converters using other protocols: For frequency converters that are not compatible with the standard MODBUS RTU protocol, free communication function blocks can be used. Compared to M_BUS instructions, free communication is much more flexible, similar to the programming mode of some high-level languages ​​(such as C). Users are advised to have some experience in communication programming and to have debugging tools such as a 232->485 converter and serial port monitoring software readily available. The following uses the SINE003 series frequency converter as an example to illustrate the free communication protocol programming method for V80 PLC. The communication format of the SINE003 series frequency converter is as follows: Data packet format XOR checksum data meaning: The XOR result of the data frame slave address to the data information. That is, the result of XORing the 2nd byte with the 3rd byte, then XORing it with the 4th byte, and so on up to the 13th byte. Data type: Hexadecimal, single byte. Transmission method: The high 4 bits and low 4 bits of the checksum byte are split and converted to ASCII code, and sent high first, then low. Result processing: When the checksum result is less than or equal to 1FH, the checksum result is incremented by 20H. This communication protocol uses ASCII code to represent the transmitted content, using STX (02H) and ETX (03H) as start and end markers. First, the data is converted into ASCII code. The V80 series PLC provides the ASCBIN and BINACS instructions to convert the sixteen ASCII numbers "0~H" to hexadecimal codes. When a transmit pulse is generated, each data is converted into ASCII code and stored in the transmit buffer. Each binary code occupies two words after conversion. SINE003 uses XOR verification, only for the ASCII code part, as shown in the following program: Then, the XOR result is judged. If it is less than 1FH, 20H is added. Then, the word group splitting instruction "PACK" is used to split it into two bytes, which are filled into the transmit area in high-to-low order. The following are the transmit and receive programs. The transmit program only needs to specify the transmit length. Reception is enabled simultaneously with transmission. The receive program enables the start and end characters and fills in the start and end characters. Refer to the V80 series PLC software manual. In this way, when "02H" is received, it is considered the start of a frame, and when "03H" is received, it is considered the end of a frame. Other conditions can also be combined for judgment, such as character timeout, etc. Various states can be reflected in the status bits. After receiving a complete frame, the status bit will display a status indication. The received data is then verified and converted back to hexadecimal for further processing. 5. Conclusion This paper uses the Teco 7200MA frequency converter and the SINE003 series frequency converter as examples to illustrate the programming methods of the MODBUS RTU protocol and free communication protocol of the V80 series PLC. It also details the communication between the V80 series PLC and the frequency converter and its implementation method. This approach can significantly reduce costs and provides high reliability.