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

Under the communication mode, data transfer due to using only two wires, so will not be able to use hardware handshaking signals as a means of detection. Thus in PC and PLC communication error occurs, will you can't judge whether through hardware error occurs, or when the work rate is not the same as the PC and PLC, conflict will occur. These communication error will lead to the PLC control program does not work, so you must use the software to shake hands, to ensure the reliability of communication. Due to the communication between PC and PLC is coordinated, so in the PC and PLC controller communication program must also coordinate with each other, namely when sending data from one side of the other party must be in a state of receiving data. As shown in figure 7 - Figure 7-18, 19 is PC and PLC controller communication program flow respectively. Figure 7 - Figure 7-18 PC communication program flow chart 19 S7- The working process of the PLC communication program flow chart of the program: PC each send a word the first before sending handshake signals, PLC after receiving the handshake signals to be transmitted to the PC, PC only after receipt of the PLC sent handshake signal start sending a byte data. PLC received after the byte data will also be back to the PC, PC will be the original data with PLC send back data comparison, if the two are different, then communication error happened, PC sends the byte data; If both are the same, then the PLC received data is correct, the PC sends the next handshake signals, the PLC receives this handshake signal will first receive data in a designated storage area. The working process of the repeated until all the data transfer is complete. Using software after shaking hands, no matter how far is the speed difference between PC and PLC, the sender will never ahead of the receiver. Software faults of shaking hands is greatly reduced the speed of communication, because send each byte, back and forth on the transmission line will be transmitted twice, and will send to shake hands. But considering the reliability of the control and the control of the time required, sacrifice is worth a little speed, are also possible. Of PLC communication program is only a small part of the whole control program of PLC, PLC communication program can be written into the interrupt program, when PLC receives the PC sends data in the interrupt program to deal with the received data. Of PC communication program can use the languages such as VB, VC, also can be directly use Siemens special configuration software, such as STEP7, WinCC.
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