Strictly speaking, the DCE should check the RTS before sending a new character (and so should DTE check CTS). As a '51's UART asserts RI in the middle of startbit, you have a half bit's time to set RTS.

However, this behaviour is not always guaranteed and it is a common practice to have a buffered receiver in the DTE, with a threshold (when setting RTS) a couple of bytes lower than the buffer size. The smarter modems have adjustable behaviour in his respect. And be aware, some modems assume that the DTE is a PC with abundant sources, and they simply ignore RTS (or have no RTS at all).

Jan Waclawek