The correct reads and writes response, responds to the host computer data. When the upper computer receives the response, it sends a confirmation application command, and the PLC completes. The steps of one read/write operation include: first, the upper computer issues a read/write command, and the PLC makes a correct response. Point) communication protocol adopts master-slave communication mode. The protocol monitored by the communication is the PPI protocol. The monitoring software of the computer is compiled by VB, and the serial communication is accomplished by the MSComm control. When the timing time is up, the data acquisition or PID control task is executed to supplement the on-site signal control. The S7-226PLC is completed, and the PLC works in a cyclic scan mode. Measurement and control tasks in the system by SIEMENS In this way, the host computer follows the PPI protocol, and can conveniently read and write the data inside the PLC to realize the man-machine operation function of the host computer. How to get the PPI agreement? When the PLC programming software reads and writes PLC data, the third serial port can be used to listen to the PLC communication data, or the software method can be used to intercept the data of the port that has been opened and is communicating, and then summarize and parse the data of the PPI protocol. If the upper computer follows the PPI protocol to read and write the PLC, the communication code of the PLC can be shown. The programming communication interface of S7-200PLC, the internal curing communication protocol is PPI protocol.

In this way, PLC programming debugging is more cumbersome, consuming PLC software interrupts and code resources, and when the PLC communication port is defined as a free communication port, the PLC programming software cannot monitor the PLC, which brings inconvenience to the PLC program debugging.

In this communication mode, the programmer needs to first define his own honest communication format, write code in the PLC, and use the interrupt mode to control the data transmission and reception of the communication port. In the field application, when the PLC is needed to communicate with the host computer, more custom protocols are used to communicate with the host computer. Point) protocol, which can be utilized to transfer and debug PLC programs. S7-226’s programming port physical layer is RS-485 structure, SIEMENS provides MicroWin software, using PPI (Point to Compared with the usual free communication protocol, this communication method omits the programming of the PLC communication program, and only needs to write the communication program resources of the upper computer. Through hardware and software listening methods, the PPI communication protocol inherent in the PLC is analyzed, and then the upper computer adopts VB programming, follows the PPI communication protocol, reads and writes PLC data, and realizes man-machine operation tasks.

Ppi rs485 siemens driver