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同步通信:采用统一的时钟信号
读数据
T1上升沿主设备给出从设备地址
T2上升沿给出读命令
T3上升沿读数据
T4上升沿撤销
写数据
T1上升沿主设备给出从设备地址
T1下降沿给出数据
T2上升沿给写命令
T4上升沿撤销
异步通信分为三类:
不互锁:下降沿不受控制,主设备以为请求成功了
半互锁:应答被接收到才撤销请求,从设备以为自己的应答被接收到了
全互锁:主设备的请求被应答才撤销,从设备的回答必须在主设备请求撤销后(证明收到了应答)才撤销
本来对于同步来说
读数据
T1上升沿主设备给出从设备地址
T2上升沿给出读命令
T3上升沿读数据 现在 变成了wait,wait结束才读
T4上升沿撤销
分离式通信 特点
从模块也要申请占用总线了(主从互换)
采用同步通信(子周期1,2),不等对方回答
各模块准备数据是不占用总线
硬盘的读取
寻找扇区好慢
cpu发指令让读写头沿着硬盘径向移动然后去干别的事
硬盘的控制器自己控制去找指定柱面
cpu再发命令,磁头不动盘片转到指定扇区