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PLC的RS-422接口配接DB-25型连接器,而PC机一般用DB-9型连接器。硬件电路图如图1所示。
将RS-232的RS、CS短接,这样对计算机发送数据来说,PLC总是处于就绪状态。也就是说,计算机在任何时候都可以将数据送到PLC内。又由于DR、ER交叉连接,因此,对计算机接收数据来说,必须等待至PLC处于准备就绪状态。
2通信装置的软件描述
2.1 FX2系列PLC与计算机之间通信协议
FX2系列PLC与计算机之间的通信采用RS-232标准,其传输速率固定为9600bps,奇偶校验位采用偶校验。数据格式如图2所示。数据以帧为单位发送和接收。一个多字符帧由力所示的五部分组成,其中和校验值是将命令码ETX之间的的呢字符的ASCII码(十六进制数)相加,取得所得和的低二位数。STX和ETX分别表示该字符帧的起始标起和结束标志。
FX2系列与计算机之间的通信是以主机发出的初始命令,PLC对其作出响应的方式进行通信的。共有0、1、7、8四种命令,上位机实现对PLC的读写和强行置位。通过ENQ、ACK和NAK,上位机协调与PLC的通信应答。
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