Abstract : This paper briefly introduces the composition and control process of a variable frequency drive (VFD) control system for edible fungi culture medium fermentation. The shortcomings of this control system, including low automation level and poor reliability, are analyzed. Combining current advanced automation control technologies, the optimal control method is selected. A method for serial communication between a Siemens S7-200 series PLC in free port mode and an EDS1000 frequency converter from Yineng Electric is proposed. Applying the principle of multi-machine communication, the PLC acts as the master, and the frequency converter as the slave, with point-to-point communication between the master and slave. The EDS1000 series frequency converter from Yineng Electric supports the RS-485 serial communication standard, and the unique function of the S7-200 PLC's free communication port mode enables the S7-200 PLC and the Yineng EDS1000 series frequency converter to achieve consistent communication protocols. This paper provides the corresponding PLC program using setting the frequency converter's operating frequency and reading its parameters as examples. Keywords: Frequency converter; PLC; Freeport communication 0. Introduction Northeast China is one of the largest edible fungi production, processing, and export bases. With increasing market demand and expanding production capacity, the aging and outdated nature of production equipment has become prominent. Secondary fermentation of the culture medium is a crucial production process for a certain company and a fundamental step in edible fungi production. Currently, the company has six secondary fermentation tunnels. Each tunnel is equipped with eight temperature sensors, located at the air inlet, air outlet, and culture medium, to monitor the fermentation temperature. Each tunnel is equipped with a fan and damper to regulate the temperature of the fermentation tunnel to meet the requirements of the entire fermentation process. Currently, the company manually monitors the tunnel temperature and manually adjusts the fan speed and damper opening. Many problems, such as low automation, high energy consumption, and waste of human resources, urgently need to be addressed. In traditional PLC frequency converter control integrated systems, the start/stop and fault monitoring of the frequency converter are achieved end-to-end control by the PLC through switching signals. The frequency of the inverter is controlled by the PLC through the analog output port outputting 0~5 (10)V or 4~20mA signals, which requires the PLC to be configured with expensive analog output port modules. When the inverter malfunctions, the PLC reads the inverter's fault alarm contact, but does not know the specific cause of the fault. It is necessary to check the inverter alarm information and then read the inverter manual to find out. With the development of AC frequency conversion control system and communication technology, the serial communication between the PLC and the inverter can be used to realize the PLC control of the inverter. In industrial automation control system, the most common application is the combination of PLC and inverter, and a variety of PLC control inverter methods have been developed. Among them, the scheme of using RS-485 communication to implement control is widely used because it has strong anti-interference ability, high transmission rate, long transmission distance and low cost. This article is aimed at the company's automation problem, and uses the serial communication between the PLC and the inverter to realize the frequency conversion speed regulation and remote monitoring of the fan [1]. 1. System Configuration for Variable Frequency Drive Communication 1.1 Variable Frequency Drive Selection E-Power Electric's EDS1000 series variable frequency drives support serial communication technology. It supports various fieldbus methods including standard RS-485, PROFIDRIVE, and LONWORKS. Among these, RS-485 communication provides users with the most cost-effective and practical serial communication method without any additional fees. Communication with the variable frequency drive can be achieved simply by sending data according to the communication data structure, control word, and status word format specified by the EDS1000 variable frequency drive. 1.2 PLC Selection Siemens industrial control products have a high market share in the industrial control application field. The S7-200 series is a small-scale PLC member of the Siemens SIMATIC PLC family. A unique feature of the S7-200 PLC is its free communication port mode, which allows users to define their own communication protocols. Thanks to this free communication port mode, the PLC can be easily connected to the variable frequency drive in this system. The PLC communicates with the frequency converter through the free communication port, controls the operation of the frequency converter, and reads all the parameters of the frequency converter, such as voltage, current, power, frequency, and alarm information such as overvoltage, overcurrent, and overload. This is more reliable than controlling the operation of the frequency converter through an external port, saves the valuable I/O ports of the PLC, and obtains a large amount of information about the frequency converter. In this example, the author will program the free port of the S7-200 according to the free port protocol [2]. 1.3 System Hardware Composition The wiring diagram of the R-485 interface of the EDS-1000 series frequency converter and the free communication port 1 of the Siemens S7-200 series 226CPU type PLC is shown in Figure 1. The PLC is the master and the frequency converter is the slave. The master and slave communicate point-to-point. 1.4 Hardware Installation Method (1) Use a special crimping tool to crimp one end of the cable to the RJ45 crystal head; connect the other end to the DB-9 special adapter plug according to the pin arrangement of the Siemens PLC free communication port. (2) Connect the RJ45 cable to the PU port of the frequency converter respectively, and connect the DB-9 special adapter plug to the free communication port 1 of the S7-200 PLC . 2. Frequency Converter Communication Principle The serial communication of the EDS1000 series frequency converter is asynchronous half-duplex and uses byte parity check. The PLC is the master and the frequency converter is the slave. The transmission of system codes is controlled by the master. The master continuously sends codes of a certain address to the slave and waits for the slave's response. The master can support up to 31 slaves. With a repeater, it can be increased to 126 slaves, that is, the slave address can be set up to 126. During communication, the default transmission format and transmission rate are: 8-N-1, 9600bps. The data command frame format is shown in Table 1. In the above data structure: (1) Frame header: is the character "~" (i.e., hexadecimal 7E), single byte. (2) Slave address: the slave's local address, occupying two bytes, ASCII format. The inverter is factory set to 01. (3) Master command/slave response: the slave's response to the command issued by the master. Occupies two bytes, using ASCII format. (4) Auxiliary index/command index/fault index: For the master, the auxiliary index and command index are used to cooperate with the master command to implement specific functions. For the slave, the auxiliary index and command index are used for the slave to report fault status codes. The command index is not modified and is directly reported. The data type is hexadecimal, 4 bytes, ASCII format. The command index occupies the lower two bytes, the auxiliary index occupies the higher two bytes, and the data range is "00" to "FF". (5) Checksum: The data means frame check, which occupies four bytes and is in ASCII format. The calculation method is the sum of the ASCII code values ​​of all bytes from "slave address" to "running data". (6) Frame tail: hexadecimal 0D, single byte [3] 3. PLC programming example In combination with the needs of the fermentation tunnel control system, this article considers its practicality. The main purpose of this system is to set the operating frequency of the inverter and read the parameters of the inverter. 3.1 Inverter operating frequency setting program The PLC executes the initialization subroutine during the first scan to set the communication port. In this example, port 1 is used for communication and the inverter address is 01. For example: the setting value is 40.00HZ, the format is: "~010C00010FA0027C\R", and the program is as follows: Network 1 // Initial scan, perform initialization operation and set the number of bytes to be transmitted. // LD SM0.1 MOVB 18, VB199 Network 2 // If SM0.7=1, enable free port mode // LD SM0.7 MOVB 9, SMB130 Network 3 // If SM0.7=0, enable PPI/slave mode // LDN SM0.7 R SM130.0, 1 Network 4 // Initialize slave operating frequency given command // MOVB 0, MB2 MOVB 18, MB3 Network 2 // Connect character receive interrupt to interrupt routine 0 // LD SM0.7 ATCH INT_0:INT1, 25 ENI Network 3 // If MB2=MB3, then: clear the counter to 0 and restore the initial state // LDB= MB2, MB3 MOVB 0, MB2 MOVD &VB320, VD316 The interrupt data reception program is as follows: Network 1 // Disconnect the interrupt and put the data into the data area // LD SM0.0 DTCH 25 MOVB SMB2, *VD316 INCD VD316 INCB MB2 4. Conclusion Using this method, the PLC with Siemens S7200 series 226 CPU is controlled by the E-Neng EDS1000 frequency converter via free port 1 using the RS-485 protocol, which greatly reduces the complexity of the wiring connection and avoids the influence of various electromagnetic interferences on the control equipment. References [1] Guo Zongren et al. Programmable Controller and its Communication Network Technology [M]. Beijing: People's Posts and Telecommunications Press, 1999 [2] Programmable Controller Principles and Applications [M]. Beijing: Machinery Industry Press, 2004. [3] EDS1000 Series Frequency Converter User Manual [K]. Shenzhen: E-Neng Electric Technology Co., Ltd., 2007.