Null modem
（重定向自Null modem cable）
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.