RTS/CTS hardware flow

Request to send/clear to send (RTS/CTS) is sometimes called pacing or hardware handshaking instead of flow control.

The term hardware handshaking comes from the use of cabling and voltages as a method of data transmission control. Unlike XON/XOFF, which sends control characters in the data stream, RTS/CTS uses positive and negative voltages along dedicated pins or wires in the device cabling.

A positive voltage means data transmission is allowed while a negative voltage signifies that data transmission should be suspended.