In modern industrial control system, the comprehensive application of PLC and inverter is the most common. The traditional application is to use the output contact of PLC to drive the intermediate relay to control the start, stop or multi speed of the inverter; More accurately, PLC and D / a expansion module are generally used to continuously control the operation of the inverter or the synchronous operation between multiple inverters. But for large-scale automatic production line, on the one hand, the number of frequency converters is large, on the other hand, the distance of motor distribution is inconsistent. Using D / a expansion module for synchronous motion control is easily affected by the fluctuation of analog signal and the inconsistent attenuation of analog signal due to the inconsistent distance, which reduces the stability and reliability of the whole system. The use of RS-485 communication control, only through a communication cable connection, can complete the inverter start, stop, frequency setting; And it is easy to realize the synchronous operation between multiple motors. The system has the advantages of low cost, long signal transmission distance and strong anti-interference.
Hardware composition and connection of the second system
The hardware composition of the system is shown in Figure 1, which is mainly composed of the following components;
Figure 1: system hardware composition
1. Fx2n-32mt-001 is the core of the system.
2. Fx2n-485-bd is the communication adapter of FX2N system PLC, which is mainly used to send and receive data between PLC and inverter.
3. SC09 cable is used for data transmission between PLC and computer.
4. The communication cable is made of five core cable.
The following describes the manufacturing method and connection mode of communication cable:
The definition of interface terminal block when Pu interface of frequency converter is used for RS485 communication is shown in Figure 2 below: (from the bottom of frequency converter)
Figure 2: definition of inverter interface terminal block figure 3: schematic diagram of communication connection between PLC and inverter
According to figure 3, the user defines that one end of the five core cable is connected to fx2n-485bd, and the other end (as shown in Figure 2) is connected to the Pu port of the inverter by crimping the five core cable with a special interface（ Remove the fr-du04 panel.)
485 communication protocol and data definition between PLC and inverter
For communication between PLC and frequency converter, the communication specification must be set in the initialization of frequency converter. If there is no setting or an error setting, the data will not be able to communicate. After each parameter setting, the inverter needs to be reset. Make sure that the setting of parameters takes effect. After setting the parameters, data communication will be carried out according to the following protocol（ (see Figure 4)
Figure 4: RS485 communication protocol diagram
1 communication request data from PLC to inverter
2. Response data from inverter to PLC when data is written
3. Response data from inverter to PLC when reading out data
4. Send data from PLC to inverter when reading data
The definition of communication data is as follows:
1 control code
2 communication data type
All instruction codes and data are sent and received in ASCII (hexadecimal). For example: (frequency and parameters) according to the corresponding command code to determine the definition of data and set range.
IV. software design
In order to realize the communication control of PLC to inverter, PLC must be programmed; Through the program, the PLC can control the operation of the inverter and collect the data. PLC program should first complete the initialization of fx2n-485bd communication adapter, the combination of control command words, code conversion and the processing of inverter response data. The design process of PLC communication operation program is shown in Figure 5 below
Figure 5: PLC communication flow chart
PLC control inverter operation through RS-485 communication program example: (in the form of instruction list).
When the above program is running, PLC starts the converter by RS-485 communication program, and stops by X3 terminal. The control commands are shown in the table below
This paper discusses the programming and application of RS-485 communication function between Mitsubishi PLC and its frequency converter through examples, which is helpful for readers’ further research and application. The application of this program is easy to connect with Mitsubishi’s f900 series touch exhibition. It will further expand the flexibility of the application.
Editor in charge: LQ