ENQ05H请求ACK06HPLC正确响应NAK15HPLC错误响应STX02H报文开始ETX03H报文结束帧格式：STXCMDDATA......DATAETXSUM(upper)SUM(lower)例子：STX,CMD,ADDRESS,BYTES,ETX,SUM02H,30H,31H,30H,46H,36H,30H,34H,03H,37H,34HSUM=CMD+......+ETX；30h+31h+30h+46h+36h+30h+34h+03h=74h;累加和过两位取低两位命令格式1、DEVICEREAD（读出软设备状态值）计算机向PLC发送：始命令首地址位数终和校验STXCMDGROUPADDRESSBYTESETXSUM例子：从D123开始读取4个字节数据02h30h31h,30h,46h,36h30h,34h03h37h,34h