In the field of industrial automation, PLC (Programmable Logic Controller) plays a vital role. And Modbus communication protocol, as a common communication standard in industrial automation, provides a reliable way for data exchange between PLCs. In this article, we will analyze how to realize Modbus communication in PLC programming through a specific case in detail.
I. Hardware Setting
In this case, we will use two S7-200 CPU for Modbus communication. Each CPU has two communication ports, we mainly use their communication port 0 for communication. At the same time, we also need a Profibus cable to connect the two CPUs to port 0. The cable should be connected in a specific pin-to-pin relationship, i.e., 3 to 3, 8 to 8. In addition, it is very important to make sure that the logical ground is connected.
After the hardware setup is complete, we also need to program and parameterize the PLC through the Micro/WIN software.
II. Parameter Matching
In order to realize Modbus communication, master and slave need to load different program libraries respectively. The master side needs to load "MBUS_CTRL" and "MBUS_MSG" libraries, while the slave side needs to load "MBUS_INIT" and "MBUS_SLAVE" libraries. "MBUS_SLAVE" libraries on the slave side.
In Micro/WIN, we need to create a new project for the master and the slave, and program and parameterize it accordingly. When setting parameters, it is necessary to make sure that parameters such as Baud rate (Baud) and Parity bit (Parity) are the same for both master and slave. In addition, the slave address (Slave) in the master block "MBUS_MSG" must be the same as the address (Addr) in the slave block "MBUS_INIT".
It is important to note that the baud rate of communication port 0 set in the "System block" of Micro/WIN is independent of the Modbus protocol. This is because Modbus communication is realized through a specific program library, not directly through the PLC communication port.
III. Programming Realization
After the parameter matching is completed, we can start to write the program for Modbus communication. The master and slave programs are written slightly differently, but the general idea is the same.
The master program is mainly responsible for initiating the communication request and handling the response from the slave. By calling the relevant functions in the "MBUS_CTRL" library, the master can specify the slave address to be read or written, function code, start address and data length and other information. The master then sends a request through the functions in the "MBUS_MSG" library and waits for a response from the slave. Once the response is received, the master can parse the data and process it accordingly.
The slave program is mainly responsible for listening to the master's request and returning the corresponding data. By calling the functions in the "MBUS_INIT" and "MBUS_SLAVE" libraries, the slave can configure its own address, communication parameters and function codes to be processed. When the slave receives a request from the master, it will process the request according to the function code and data address and return the result to the master.
IV, debugging and testing
After the programming is completed, we need to debug and test the Modbus communication. First of all, make sure that the connection between PLCs is normal and the Micro/WIN software can communicate with PLCs normally. Then, we can gradually test the various functions of the master and slave to ensure that they can send and receive data correctly.
During the debugging process, we can use the monitoring function of Micro/WIN to observe the execution of the program and the change of data. If we find any problem or abnormality, we need to carefully check the program logic and parameter settings, and adjust and optimize them accordingly.
Through the above steps, we can realize Modbus communication in PLC programming. Of course, in the actual application, we also need to adjust and expand according to the specific project requirements and hardware environment. But in general, mastering the basic principles of Modbus communication and programming methods will provide great convenience and flexibility for our work in the field of industrial automation.