The following is from the Philips 80C51 family hardware description:

"Serial Data Buffer
The Serial Buffer is actually two separate registers, a transmit buffer and a receive buffer. When data is moved to SBUF, it goes to the transmit buffer and is held for serial transmission. (Moving a byte to SBUF is what initiates the transmission.) When data is moved from SBUF, it comes from the receive buffer."

So, as long as your software can keep up, it'll be OK.

Use the 'Search' button to search for "erik malund" and you will find the URL for the Philips 80C51 Family documents in most of his posts! ;-)

(sorry, Erik)