unit delay 与memory有很多的相似之处，这给我们的建模和仿真带来了很多的困扰，确实搞清楚这两者的异同便是当务之急：

1、在这两者各自的帮助文件里有这样两句话

 Note（unit delay） ：   The Unit Delay block accepts continuous signals. When it has a  continuous sample time,  the block is equivalent to the Simulink Memory block.
他们的区别应该就在于采样时间上

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Treat as a unit delay when linearizing with discrete sample time

  Select this check box to linearize the Memory block to a unit delay when the Memory block is driven by a signal with a discrete sample time.