单片机（stc89c52）通过RS485与PLC通信，采用modbus协议，利用单片机闲置的一个管脚置高或低，来判断出现单片机程序执行的情况。硬件连接比较简单，单片机通过485的A/B端连接到RS485/RS422，RS422连接但PLC。这个就是.c程序不是工程文件，下载后自己建立个keilC工程就可以了。-SCM (stc89c52) via RS485 communication with the PLC, using modbus protocol, using SCM idle a pin high or low, to determine the circumstances microcontroller program execution occurs. Relatively simple hardware connection, the microcontroller 485 A/B terminal is connected to the RS485/RS422 through, RS422 connection but PLC. This is not a project file .c program he established a keilC project after downloading it.