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采用Bland C编写主机与PLC的通信程序。首先必须对COM1口进行初始化,波特率为9600bps,奇偶校验位采用偶校验,七位有效数据。通信的初始化检测过程如图4所示。当计算机接收到来自PLC的应答字符ACK后,就可以进入数据通信了。通信的时序图如图5所示。
计算机可对PLC内各软设备进行读、写和强制ON/OFF操作。除开PLC的计时器和计数器的设定值采用常数时,以及文件寄存器内的数据,FX2系列PLC的所有开关量输入、输出以及各软件设备对计算机都是透明的。其操作时的多字符帧的格式如图3所示。但不同的操作在“多个字符”项内有所不同。例如,计算机对PLC的软设备Y20~Y37进行读操作,查装置地址表为00A2,读取2字节数据。其状态如图6(a)所示,主机发出的命令格式如图6(b)所示,PLC返回的信息如图6(c)所示。
如传送的命令有错误,PLC返回NAK信号,本次操作失败,重新进行。
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