1. 用于程序处理和交互式操作. 2. 数据传输采用ASCII码的形式. 3. 首先由计算机发送指令. 4. 由PLC对指令自动进行相应响应.(1. for program processing and interactive operation. 2. data transmission is used in the form of ASCII code. 3. first, the instruction is sent by the computer. 4. automatic