[Abstract] This paper introduces a multi-branch communication network composed of a PLC and a variable frequency drive (VFD), clarifies the advantages of this network-controlled speed regulation system compared to a general analog control speed regulation system, and provides a system block diagram and PLC program. [Keywords] PLC, Variable Frequency Drive, Multi-Motor Control Network, Communication Protocol I. Introduction Synchronous control systems, proportional control systems, and speed-matching systems using VFDs as speed controllers are widely used in industries such as metallurgy, machinery, textiles, and chemicals. Taking a proportional control system as an example, a typical system configuration is shown in Figure 1. During operation, the operator sets the proportional operating parameters through a control unit (which can be a PLC or an industrial PC). Then, the control unit sends speed commands to the VFDs via a D/A conversion module, causing each VFD to drive the motors to operate at a certain speed ratio. This scheme is suitable for applications with a small number of motors and relatively concentrated motor distribution. However, for large-scale automated production lines, on the one hand, the number of motors is large, and on the other hand, the motors are distributed over long distances. When using this control scheme, the attenuation of the speed command signal during long-distance transmission and external interference reduce the overall system's stability and reliability; simultaneously, the large number of D/A conversion modules increases system cost. Therefore, we propose a multi-branch communication control network consisting of a PLC and a variable frequency drive. This system has lower cost, longer signal transmission distance, and stronger anti-interference capabilities, making it particularly suitable for long-distance, multi-motor control. II. System Hardware Composition The system hardware structure is shown in Figure 2, mainly composed of the following components: 1. The FX0N-24MR is the basic PLC unit, executing the system and user software, and is the core of the system. 2. The FX0N-485ADP is the communication adapter for the FX0N system PLC. Its main function is to act as a substation in a computer-PLC communication system to receive information sent from the computer to the PLC, or as a network adapter when multiple PLCs form an n:n network. Generally, it is only used as a receiving unit for a specified protocol. Based on its structure analysis, this paper uses it as a communication master station to complete the transmission of variable frequency drive control signals. 3. FR-CU03 is the computer connection unit for the FR-A044 series proportional speed controller, conforming to RS-422/RS-485 communication standards. It is used to network the computer with multiple variable frequency drives (VFDs). This unit enables network control of the VFDs (such as start, stop, and frequency setting), parameter setting, and status monitoring; it serves as the network interface for the VFDs. 4. The FR-A044 VFD controller enables motor speed control. In a 1:n (1:3 in this article) multi-branch communication network, each VFD is a substation, and each substation has a station number, pre-set by the parameter setting unit. During operation, after the PLC sends relevant command information via the FX0N-485ADP, each substation receives this information. Each substation then checks if the station number address matches its own. If they match, the information is processed and a response is returned; otherwise, processing is abandoned. This ensures that only one substation exchanges information with the master station on the network at a time. III. Software Design 1. Communication Protocol FR-CU03 specifies the communication process between the computer and the frequency converter, as shown in Figure 3. This process consists of up to five stages: 1. The computer sends a communication request; 2. The frequency converter processes and waits; 3. The frequency converter responds; 4. The computer processes and waits; 5. The computer responds. The corresponding process is completed according to different communication requirements. For example, writing the frequency converter start/stop control command completes three processes (?), while monitoring the frequency converter operating frequency completes five processes (?). Whether writing or reading data, the computer sends a request, and the frequency converter passively accepts the request and responds. The data format differs at each stage. Figure 4 shows the data format for writing the frequency converter control command and the frequency converter operating frequency, respectively. 2. PLC Programming To control the frequency converter, the PLC must be programmed to control the information exchange between the PLC and the frequency converter. The PLC program should complete the initialization of the FX0N-485ADP communication adapter, the combination of control command words, code conversion, and the processing of frequency converter response information. The PLC ladder diagram program (partial program) is shown in Figure 5. The communication transmit buffer in the program is D127～D149; the receive buffer is D150～D160. Motor 1's start and stop are controlled by the rising and falling edges of X0, respectively; motor 2's start and stop are controlled by the rising and falling edges of X1, respectively; motor 3's start and stop are controlled by the rising and falling edges of X2, respectively. The program initializes the FX0N-485ADP communication protocol with the system start pulse M8002; then it processes the start and stop signals. Taking motor 1 start as an example, when X0 rises, M50 is activated, the inverter 1 station number is sent to D130, the run command word is sent to D135, and ENQ, the control word for writing the run command, and the waiting time are pre-written by the programmer to D131, D132, and D133; then the checksum is calculated and sent to D136 and D137; finally, M8122 is set to enable the RS instruction to send control information. Upon receiving a signal, the frequency converter immediately returns a response message. This message is received by the FX0N-485ADP and then transmitted to M8132. The PLC processes the information accordingly and terminates the program. IV. Conclusion 1. Practical application shows that this solution enables the PLC to control the operation, set parameters, and monitor the operating status of the frequency converter via a network. 2. This system can control up to 32 frequency converters, with a maximum distance of 500m. 3. Controlling multiple frequency converters is significantly less expensive than D/A control. 4. As the number of frequency converters increases, communication delay increases, and the system response speed is slower than that of D/A control.