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 a PLC and a 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 implement the control of three frequency converters by 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 0-10V current signal. The disadvantages of this method are: (1) the operating parameters of each frequency converter cannot be accurately obtained during operation; (2) the failure rate increases due to the frequent operation of the control potentiometer; (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 operation of frequency converters by controlling and monitoring the frequency converters with 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, the PLC's Port 2 and the frequency converters form a MODBUS bus. The PLC controls three frequency converters to fulfill 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 RS485. In MODBUS communication networks, ASCII and RTU communication modes are generally provided. 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 primarily implemented using MODBUS communication. Therefore, both the master and controlled devices must support the MODBUS protocol. Yonghong PLC provides a convenient and intuitive MODBUS MASTER table, as shown in Figure 2. Entries 0, 2, and 4 respectively record the control word and setpoint value of inverters 1, 2, and 3; entries 1, 3, and 5 read the status word of inverters 1, 2, and 3. The Era TVF2000 series inverters can communicate with external PLC control systems via a serial RS485 bus. The communication protocol is MODBUS RTU format, and all control commands and setpoints originate from MODBUS communication. The RS485 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 of 9600bps; 5203=0 — No verification; 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 of 0.1HZ; 1105=50 — Maximum set frequency of 50HZ; 1601=7 — Communication enabled; 1604=7 — Fault communication reset. The menu parameters of TVF2000 are mapped one by one to the registers of the MODBUS protocol. The operation of each register by MODBUS communication realizes the operation of the menu parameters corresponding to the registers in TVF2000. The corresponding command register is 40001. 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 preparation state; Third, 40001=0000 0000 0000 1111B: A rising edge pulse is generated to enter the operation enabled state; Fourth, 40001=0000 0000 0010 1111B: The frequency converter is started; Fifth, 40001=0000 0000 0110 1111B: The frequency converter operates at the given frequency value. Simultaneously, by reading the storage register 40004 (status word), information such as the inverter's operating status can be obtained. This information can then be displayed on the touchscreen interface, allowing operators to intuitively understand the inverter's operating information. 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 Utilizing MODBUS to control the operation of multiple TVF2000 frequency converters via PLC meets the needs of various production equipment applications and requirements. Compared to the original system, it significantly improves the level of automation, making control simpler and more precise. While the traditional PLC+D/A module control mode can achieve the same control requirements, the expensive D/A module is generally unacceptable from an economic perspective. Controlling multiple frequency converters simultaneously would result in a very large system, hindering maintenance. Therefore, adopting bus architectures such as MODBUS to achieve production equipment control requirements is a trend in industrial control. Furthermore, with the integration of frequency conversion technology and computer communication technology, communication control between PLCs and frequency converters will be more widely applied in industrial control. The author has designed related systems for use in wastewater treatment systems and packaging systems for food, pharmaceuticals, and cosmetics, significantly improving automation and reducing system failure rates. This drive control system is worthy of promotion and application.