Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using an RS-232 serial cable. The name stems from the historical use of the RS-232 cable to connect two teleprinter devices to modems in order to communicate with one another; null modem communication was possible by instead using RS-232 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 so it assumes 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.
Read more about Null Modem: Origin, Cables and Adapters, Pin Mapping, Applications, Types of Null Modem
Famous quotes containing the word null:
“A strong person makes the law and custom null before his own will.”
—Ralph Waldo Emerson (1803–1882)