Siemens S7-200 series PLC and PC communication program flow chart and working process

In the above communication mode, since only two wires are used for data transmission, the hardware handshake signal cannot be used as a detection method. Therefore, when a bit error occurs in the communication between the PC and the PLC, it will not be possible to determine whether a bit error occurs through hardware, or when the operating speeds of the PC and the PLC are different, a conflict will occur. These communication errors will cause the PLC control program to not work properly, so software handshake must be used to ensure the reliability of communication.

Since the communication is coordinated between the PC and PLC, the communication programs in the PC and PLC must also be coordinated with each other, that is, when one party sends data, the other party must be in the state of receiving data. Figures 7-18 and 7-19 show the communication program flows of the PC and PLC respectively.

Figure 7-18 PC communication program flow chart

Figure 7-19 S7-PLC communication program flow chart

The working process of the communication program: Before sending each byte, the PC first sends a handshake signal. After receiving the handshake signal, the PLC transmits it back to the PC. The PC starts to send a byte of data only after receiving the handshake signal sent back by the PLC. After receiving this byte of data, the PLC also transmits it back to the PC. The PC compares the original data with the data sent back by the PLC. If the two are different, it means that a bit error occurred in the communication, and the PC resends the byte of data; if the two are the same, it means that the data received by the PLC is correct, and the PC sends the next handshake signal. After receiving this handshake signal, the PLC stores the previously received data in the specified storage area. This working process is repeated until all data transmission is completed.

After adopting software handshake, no matter how far the speed of PC and PLC differs, the sender will never be ahead of the receiver. The disadvantage of software handshake is that it greatly reduces the communication speed, because each byte must be transmitted back and forth twice on the transmission line, and the handshake signal must also be transmitted. However, considering the reliability and time requirements of control, sacrificing a little speed is worthwhile and feasible.

The communication program of PLC is only a small part of the whole control program of PLC. The communication program can be compiled into the interrupt program of PLC. When PLC receives the data sent by PC, it processes the received data in the interrupt program. The communication program of PC can adopt VB, VC and other languages, or directly adopt Siemens special configuration software, such as STEP7 and WinCC.