Flow control is used to avoid data loss in transmission when one party is unavailable to receive data. When a DTE (such as a PC) wants to stop the data flow into it, it negates RTS. Read a negated “Request To Send” as “request NOT to send to me” (stop sending). When the PC is ready for more bytes, it asserts RTS and the flow of bytes to it can resume. Flow control signals are always sent in a direction opposite to the flow of bytes that is being controlled. DCE equipment (i.e., AVR) works the same way but sends the stop signal out the CTS pin (negated CTS: “(you are) NOT Cleared To Send”).