Specifications

50 Chapter 9 Pinout reference
EIA 232 Information
RS-232 flow control primer
Hardware flow control uses pins RTS and CTS to gate flow back and
forth between to connected serial devices. The Data Terminal Equipment
(DTE) device uses RTS to start and stop flow from the Data
Communications Equipment (DCE) device, and the DCE device uses CTS
to gate flow from the DTE device. This method is popular for higher-speed
connections, where flow control reaction time is more critical. It is also
popular where the data stream is such that embedded flow-control
characters cannot be tolerated by the protocol running on the link. The cost
of this method is running two more wires in the cable.
Software flow control uses special START (XON) and STOP (XOFF)
characters embedded in the data stream to gate flow. In other words, the
receiving device sends a STOP character, typically Control-S, to the
sending device to halt flow. It then later sends a START character, typically
Control-Q, to resume flow. This flow-control method is preferred for slower
links running a protocol that supports embedded flow-control characters,
and because it requires just three wires for the link to function:
Receive (RxD), Transmit (TxD), and Ground (GND).