在自由口模式下,通信协议由用户自己的梯形图程序控制。用户可以使用梯形图程序调用各种子程序,来进行接收中断、发送中断、发送指令(XMT)、接收指令(RCV)等通信控制操作。研究了在自由端口模式下PC与S7-200型PLC之间的通信,并采用Visual Basic编程来实现上位机与下位机之间的通信。 In Freeport mode, the communication protocol is controlled by the user’s own ladder program. Users can use the ladder program to call various subroutines to receive interrupts, send interrupts, send commands (XMTs), receive commands (RCVs), and other communications control operations. The communication between PC and S7-200 PLC in free port mode was studied and the communication between upper computer and lower computer was realized by Visual Basic programming.