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Null modem
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{{Short description|Serial cable connecting two computers}} [[File:Null modem.jpg|300px|thumb|A null modem adapter]] '''Null modem''' is a communication method to directly connect two [[data terminal equipment|DTE]]s ([[computer]], [[computer terminal|terminal]], [[printer (computing)|printer]], etc.) using an [[RS-232]] [[serial cable]]. The name stems from the historical use of [[RS-232]] cables to connect two [[teleprinter]] devices or two [[modem]]s in order to communicate with one another; ''null modem'' communication refers to using a crossed-over RS-232 cable to connect the teleprinters directly to one another without the modems. It is also used to serially connect a computer to a printer, since both are [[data terminal equipment|DTE]], and is known as a ''Printer Cable''. The RS-232 standard is asymmetric as to the definitions of the two ends of the communications link, assuming that one end is a [[data terminal equipment|DTE]] and the other is a [[Data circuit-terminating equipment|DCE]], e.g. a [[modem]]. With a null modem connection the [[Transmission (telecommunications)|transmit]] and receive lines are crosslinked. Depending on the purpose, sometimes also one or more [[Handshake (computing)|handshake]] lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by the RS-232 standard.
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