说明：  西门子S7-200自由口通讯资料,我们将介绍如何用S7-200连接GSM调制解调器发送短消息，并通过这个例子来介绍应用S7-200自由口编程的一般原则时及应注意的问题。
(Siemens S7-200 free oral communications, we will introduce how to use S7-200 to connect GSM modem to send text messages, and through this example to introduce the application of S7-200 free of the general principles of programming I am and should pay attention to.)