Null modem emulator is a communication method to connect two DTEs (computer, terminal, and printer). The original RS-232 standard only defined the connection of DTEs with DCEs like modems. With a null modem connection 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.
Null modems are commonly used for file transfer between computers, or remote operation. Under the Microsoft Windows operating system, the direct cable connection can be used over a null modem connection. The later versions of MS-DOS were shipped with the Interlink program. Both pieces of software allow the mapping of a hard disk on one computer as a network drive on the other computer. No Ethernet hardware (such as a network interface card or a modem) is required.
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