1. Introduction With the rapid development of modern production processes, the requirements for the speed, continuity, and systematic operation of transmission equipment in the field are becoming increasingly stringent. The independent operation mode of a single frequency converter can no longer meet these requirements; multi-unit grouped system operation has become the most prominent feature. All local front-end operations have evolved into a centralized computer-controlled remote automatic control mode. These characteristics are all based on data communication between the computer, PLC, and transmission device. This enables the monitoring of the operating status of field equipment and the issuance of control commands. The invention and application of fieldbus provides the technical guarantee for realizing the above functions. Currently, widely used fieldbuses include PROFIBUS, CANbus, Modbus, DeviceNet, and Lonworks. This article focuses on the communication technology between Siemens SIMATICAL S7 PLC and 6ES70 series frequency converters based on the PROFIBUS fieldbus. 2. Overview of PROFIBUS Fieldbus PROFIBUS is a widely used fieldbus, and its bus standard is an important component of the international bus standard IE61158. It is an open system with a token structure, using the Internet (Open System Interconnection - SIO) as a reference model. It consists of three compatible components: PROFIBUS-DP (Decentralized Periphery), PROFIBUS-PA (Process Automation), and PROFIBUS-FMS (Fieldbus Message Specification). DP is a high-speed, low-cost communication technology used for communication between device-level control systems and distributed I/O. It uses an RS485 data interface, with fiber optic or twisted-pair cable as the transmission medium. The transmission baud rate ranges from 9.6kbps to 12mbps, and the transmission distance can be extended using repeaters. Each DP network can simultaneously configure 122 slave stations, making it a powerful fieldbus. The communication between the PLC and frequency converter described below is also based on PROFIBUS-DP technology. 3. PROFIBUS-DP Data Communication Format The interface between the drive unit and the master PLC via the PROFIBUS-DP network is implemented through the communication module CBP board. S7-300 and 400 PLCs with DP ports can also achieve this through the DP port on the CPU. The system uses an RS485 interface and supports data transmission at baud rates of 9.6kbps to 12mbps (the data transmission structure is shown in Figure 1). The data message header and footer primarily specify the function code, transmission length, parity check, and response information. The core of the data read/write process between the master and slave stations (as shown in Figure 2) consists of the parameter interface (PKW) and process data (PZD). PKW and PZD have five structural forms: PPO1, PPO2, PPO3, PPO4, and PPO5, each with different byte lengths and structural forms. When configuring the communication method between the PLC and the frequency converter, the PPO must be selected. The structural forms for each type are as follows. [align=center]Figure 1 Data Transmission Structure[/align] [align=center]Figure 2 Data Read/Write Process Between Master and Slave Stations[/align] ppo1 4 pkw + 2 pzd (6 words in total) ppo2 4 pkw + 6 pzd (10 words in total) ppo3 2 pzd (2 words in total) ppo4 6 pzd (6 words in total) ppo5 4 pkw+10 pzd (14 words in total) Parameter Interface (pkw): Composed of three parts: parameter ID (pke), index number (ind), and parameter value (pwe). Process Data Interface (pzd): Composed of control word (stw), status word (zsw), main setpoint, and main actual value, etc. It is also necessary to understand the specific meaning of each bit of the control word and status word, and be familiar with the specific application of Siemens inverter parameters. Specific definitions are required when setting communication parameters. 4. Hardware and software requirements and parameter settings for communication (1) Hardware requirements • Programmer with a frequency of 133MHz or higher and a memory of not less than 16MB. • Siemens S7-300/400 series PLC with a RAM of not less than 12KB and a PROFIBUS-DP interface, or S7-400 (RAM not less than 12KB) with a CP443-5 communication board. • Inverter with CBP communication module and VC board with CU2/SC (2) Software requirements • Win 95 or Win NT (v4.0 or above) • Step7 (v3.0 or above) • Install DVA-S7-SPS7 (3) Basic steps for communication settings • Set drive parameters • PLC hardware configuration • Create data block • Write communication program • System debugging (4) Setting drive parameters • P053 = 3 Parameter enable • P090 = 1 CBP board in slot #2 • P918 = 3 Slave address • P554.1 = 3001 Control word PZD1 • P443.1 = 3002 Master setpoint PZD2 • P694.1 = 968 Status sub PZD1 • P694.2 = 218 Actual value PZD2 5 PLC and drive inverter communication program To achieve communication functionality, proper programming is crucial. The following section will use Siemens' S7-416 PLC and 6SE70 frequency converter as examples to illustrate communication programming. (1) Basic Configuration and Definition The basic configuration is shown in Figure 3: [align=center] Figure 3 Basic Configuration Interface[/align] The master station is CPU-416-2DP. The slave station is a 6SE70 drive unit with a PROFIBUS address of 3. Input address: IW256 (2 words PZD); Output address: QW256 (2 words PZD); PPO type: 3; Bus interface: RS485. (2) Function blocks used: ob1 main cycle sfc14 dprd-dat read data system function block sfc 15 dpwr-dat write data system function block db100 data access (dbw0-dbw4 is reading, dbw5-dbw8 is writing) mw200 mw210 communication status display (3) Simple program writing (as shown in Figure 4) [align=center] Figure 4 Program writing interface[/align] ob1 network1: Read data call sfc 14 laddr w#16#100 ret-val mw200 record p#db100.dbx0.0 byte 4 network2: Display data l db100.dbw 0 t mw50 nop 0 network3: Write data lw#16#efff t db100.dbw 5 network4: Send data call sfc 15 The command `laddr w#16#100 record p#db100.dbx5.0 byte 4 ret-val mw210` stores, compiles, and downloads the program. After checking the parameter settings of the transmission device, it can be powered on for debugging. 6. Conclusion Through the above introduction, we can establish communication between the PLC and the transmission device based on the PROFIBUS-DP bus through network configuration and parameter settings. We can then control the device's start/stop, speed adjustment, and other functions by changing the parameters. With the help of a programmer, the PLC can view the status and actual feedback values ​​read from the device online, and the master setpoint can also be viewed on the device's PMU. Communication technology has enabled the wider application of automation. References [1] Yang Xianhui. Fieldbus Technology and Its Application. Beijing: Tsinghua University Press, 2002 [2] Zhou Ming. Fieldbus Control System. Beijing: China Electric Power Press, 2002 [3] Tang Jiyang. Fieldbus PROFIBUS System Integration and Product Development [4] Siemens. Drives Communication for Simatic S7 Author Biography Zhao Shiwei Electrical Engineer Currently engaged in equipment maintenance work.