The term null modem may also refer to the cable or adapter itself as well as the connection method. If a user wished to connect two teleprinters directly without modems (null modem) then they would crosslink the connections. 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. Originally, the RS-232 standard was developed and used for teleprinter machines which could communicate with each other over phone lines. Several wiring layouts are in use because the null modem connection is not covered by the RS-232 standard. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. With a null modem connection the transmit and receive lines are crosslinked. The RS-232 standard is asymmetric 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. It is also used to serially connect a computer to a printer, since both are DTE, and is known as a Printer Cable. ![]() ![]() The name stems from the historical use of RS-232 cables to connect two teleprinter devices or two modems 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. Null modem is a communication method to directly connect two DTEs ( computer, terminal, printer, etc.) using an RS-232 serial cable.
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