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null modem
Null modem is a communication method to directly connect two DTEs (computer, terminal, printer, etc.) using an RS-232 serial cable. The name stems from the historical use of RS-232 cables to connect two teleprinter devices to two modems in order to communicate with one another; ''null modem'' communication was possible by using a crossed-over RS-232 cable instead to connect the teleprinters directly to one another.
The RS-232 standard is asymmetrical as to the definitions of the two ends of the communications link, assuming that one end is a DTE and the other is a DCE, e.g. a modem. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by a standard.
== Origins ==
Originally, the RS-232 standard was developed and used for teleprinter machines which could communicate with each other over phone lines. Each teleprinter would be physically connected to its modem via an RS-232 connection and the modems could call each other to establish a remote connection between the teleprinters. If a user wished to connect two teleprinters directly without modems (null modem) then they would crosslink the connections. The term ''null modem'' may also refer to the cable or adapter itself as well as the connection method.
Null modem cables were popular means of transferring data between the early personal computers in the 1980s and early 1990s.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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