I'd like to knowe concept of xmodem . Do you have Protocal,software or
    circuit ?

XMODEM the file download protocol?  See here:

    http://www.geocities.com/westfw/comp.htm

(Some ways down the page) ("westfw" is me, by the way.)

This has ancient (1985) documentation and a couple even more ancient
implementations (1982 and earlier.)

BillW

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