1. Introduction Although frequency converters are widely used in various industries, their simple display panels and weaknesses in data processing, calculation, and storage have limited their application in complex control systems. However, combining communication technology with frequency converters can overcome these shortcomings. Remote control can be achieved using the communication function between the PLC and the frequency converter, while also enhancing the frequency converter's data processing and fault alarm functions. This paper takes a wastewater treatment control system as an example to introduce how to use the Modbus communication protocol to control three frequency converters with a Yonghong PLC, making the control of motor speed, direction, torque, and frequency converter operating parameters much easier and more precise. The previous control method used three frequency converters to control three traction motors respectively. The speed was given by rotating the operating potentiometer on the operation panel to give the frequency converter a current signal of 0-10V. The disadvantages of this method are: (1) The operating parameters of each frequency converter cannot be accurately obtained during operation; (2) The frequent operation of the control potentiometer due to frequent rotation increases the failure rate; (3) Since the frequency converter is generally installed in the control cabinet, it is difficult to obtain the frequency information on its panel; (4) Changing its operating parameters requires manual debugging multiple times; (5) The degree of automation is not high. Therefore, it is necessary to improve the control method and adopt PLC control, using Modbus communication to realize the control and monitoring of the frequency converter operation by PLC. 2 Related System Composition and Configuration The system uses the FBS-60MCT type PLC of Taiwan Yung-Hung Motor Co., Ltd. as the main controller; the TVF2000 series frequency converter is used for drive control; and the Hitech PWS3261 touch screen is used for human-machine information exchange. The block diagram of the related system is shown in Figure 1. In this system, PLC Port 2 and the frequency converters form a Modbus bus. The PLC controls three frequency converters to meet system control requirements, enabling speed setting, operational status monitoring, and parameter exchange. 3. Modbus Communication Protocol The Modbus communication protocol is a serial, asynchronous master-slave communication protocol. Only one device in the network can establish the protocol; other devices can only respond to the master's queries by providing data or taking corresponding actions based on the queries. The Modbus protocol defines the format of the master query, including: the master-slave addressing method (or broadcast), the function code requesting the action, data transmission, and error checking. If the master's requested action cannot be completed, a fault will be generated as a response. The Modbus protocol does not require a special interface; a typical physical interface is RS-485. Modbus communication networks generally provide ASCII and RTU communication modes. In this control system, the Yonghong PLC supports the Modbus protocol and can flexibly utilize both ASCII and RTU communication modes. The TVF2000 frequency converter supports the RTU communication mode in Modbus. 4. System Implementation This control system is mainly implemented using Modbus communication control. Therefore, both the master control device and the controlled device must support the Modbus protocol. Yonghong PLC provides a convenient and quick Modbus master table, which is intuitive and easy to understand, as shown in Figure 2. Items 0, 2, and 4 are for writing the control word and setpoint value 1 to inverters #1, #2, and #3, respectively; items 1, 3, and 5 are for reading the status word of inverters #1, #2, and #3. The Shidai TVF2000 series inverters can communicate with external PLC control systems via a serial RS-485 bus. The communication protocol is Modbus RTU format, and all control commands and setpoints come from Modbus communication. Its RS-485 bus structure is shown in Figure 3. Register 40001 of the TVF2000 inverter contains the control word, register 40002 contains the setpoint value 1, register 40004 contains the status word, and register 40005 contains the actual value. The relevant parameter settings are as follows: 5005=2—Modbus communication; 5201=1, 2, 3—Inverter station number; 5202=5—Baud rate 9600bps; 5203=0—No parity; 5003=1.0—Communication timeout; 1001=10—External command 1 is for communication control; 1003=3—Motor forward and reverse control; 1104=0.1—Minimum frequency setting unit 0.1Hz; 1105=50—Maximum set frequency is 50Hz; 1601=7—Communication enabled; 1604=7—Fault communication reset. The menu parameters of TVF2000 are mapped one-to-one to Modbus protocol registers. Modbus communication operations on each register are equivalent to operations on the corresponding menu parameters in TVF2000. The corresponding command register is 40001, and the specific control content of each bit is shown in Table 1. When controlling each frequency converter, start/stop information is given via the Hitech touchscreen. The converter's position information is as follows: First, 40001=0000 0000 0000 0110b The frequency converter enters the communication control ready state; Second, 40001=0000 0000 0000 0111b The frequency converter enters the standby state; Third, 40001=0000 0000 0000 1111b A rising edge pulse is sent to enter the operation allowed state; Fourth, 40001=0000 0000 0010 1111b The frequency converter is started; Fifth, 40001=0000 0000 0110 1111b The frequency converter is operated at the given frequency value. Meanwhile, by reading storage register 40004, information such as the inverter's operating status can be obtained. This information can then be displayed on a touchscreen interface, allowing operators to intuitively understand the inverter's operating status. The address information of register 40004 is shown in Table 2. 5. Programming org m1924 fun 08 7,r1000 fun 08 7,r1100 fun 08 7,r1200 /* Inverter ready*/ org m1962 fun 150p 2,r5800,r5900 /* Modbus communication command settings*/ org m10 fun 08p 47,r1000 fun t10 100 and t10 fun 08p 111,r1000 /* Start inverter #1*/ org m11 fun 08p 7,r1000 /* Stop inverter #1*/ org m12 fun 08p 47,r1100 fun t11 100 and t11 fun 08p 111,r1100 /* Start inverter #2*/ org m13 fun 08p 7,r1100 /＊Stop #2 inverter＊/ org m14 fun 08p 47,r1200 fun t12 100 and t12 fun 08p 111,r1200 /＊Start #3 inverter＊/ org m15 fun 08p 15,r1200 /＊Stop #3 inverter＊/ 6 Conclusion Using Modbus via PLC to control the operation of multiple TVF2000 inverters meets the needs of various occasions and requirements of production equipment. Compared with the original system, the degree of automation has been greatly improved, making the control simple and precise; compared with the traditional control mode of PLC+D/A module, although the control requirements can be met, we generally cannot accept the expensive D/A module from an economic point of view. When controlling multiple frequency converters at the same time, the system will become very large and not conducive to system maintenance; therefore, adopting bus structures such as Modbus to realize the control requirements of production equipment is a trend in industrial control. At the same time, with the combination of frequency conversion technology and computer communication technology, the communication control of PLC and frequency converter will be more widely used in industrial control. In this paper, the author has designed related systems for use in sewage treatment systems, food, pharmaceutical and cosmetic packaging systems, etc. The degree of automation has been greatly improved and the system failure rate has been greatly reduced. It is a transmission control system worthy of promotion and application. References [1] Yonghong Programmable Controller User Manual i and User Manual ii [2] Hitech Industrial-grade Human-Machine Interface User Manual [3] Times TVF2000 Series Frequency Converter User Manual