说明：  STEP7梯形图编程手册第二版——S7-300和S7-400梯形逻辑（LAD）编程参考手册,本使用手册旨在提供指南，以使用梯形逻辑（LAD）编程语言生成用户程序。本手册中还包含一个参考章节，阐述了梯形逻辑语言元素的语法和功能。
(STEP7 Ladder Programming Manual Second Edition- S7-300 and S7-400 Ladder Logic (LAD) programming reference manual, this manual is intended to provide guidance to use the ladder logic (LAD) programming language that generates the user program. This manual also contains a reference section, describes the syntax of language elements of ladder logic and function.)