RTS / CTS Flow Control is another flow control mechanism that is part of the RS232 standard. It makes use of two further pins on the RS232 connector, RTS (Request to Send) and CTS (Clear to Send). These two lines allow the receiver and the transmitter to alert each other to their state.
A transmitter raises its RTS line, which causes an interrupt on the receiver, I.e. – “Hey can I send some data”? If the receiver is in a position to receive the data it will assert its CTS line, I.e. – “Yes you can start sending”.
The raising and lowering of these lines allows device drivers which implement hardware flow control code to maintain a reliable data connection between transmitter and receiver. The advantage to this approach is that damaging data in transit does not affect the flow control mechanism (as in Xon/Xoff).