STX 02H 报文开始
ETX 03H 报文结束
帧格式
STX CMD DATA ...... DATA ETX SUM(upper) SUM(lower)
例子
STX ,CMD ,ADDRESS, BYTES, ETX, SUM
02H, 30H, 31H,30H,46H,36H, 30H,34H, 03H, 37H,34H
SUM=CMD+......+ETX
30h+31h+30h+46h+36h+30h+34h+03h=74h;
累加和过两位取低两位


1 DEVICE READ 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
STX CMD GROUP ADDRESS BYTES ETX SUM
例子从D123 开始读取4 个字节数据
02h 30h 31h,30h,46h,36h 30h,34h 03h 37h,34h