9 五、 PLC 编程语言··········································································· 10 六、 S7-200 PLC 编程元件及指令系统介绍··········································· 12 6.1 S7-200 的控制程序······························································· 12 6.2 S7-200 指令系统·································································· 12 6.3 触点指令············································································ 13 6.4 输出指令与其它指令····························································· 14 6.5 定时器与计数器指令····························································· 15 6.6 计数器指令········································································· 16 七、十字路口交通信号灯系统 PLC 控制应用······································· 17 7.1 模型介绍及控制过程分析······················································· 17