摘要 ......................................................................................................................................................................... 1关键词 ..................................................................................................................................................................... 1ABSTRACT. .......................................................................................................................................................... 1KEY WORDS ........................................................................................................................................................ 1引言 ......................................................................................................................................................................... 21 交通灯概述.......................................................................................................................................................... 31.1 课题背景 ........................................................................................................................................................... 31.2 研究目的和意义 ............................................................................................................................................... 31.3 本文的主要工作 ............................................................................................................................................... 42 可编程程序控制器（PLC ） .............................................................................................................................. 52.1 PLC 概述 ........................................................................................................................................................... 52.1.1 PLC 的发展历程 ...................................................................................................
