This is for applications where the two functions never are required simultaneously. Use two 232 converters (such as MAX232) two AND gates and two OR gates. http://www-us.semiconductors.philips.com/acrobat/v...WARE_1.pdf pp 12-15 will show you that an inactive '51 serial line is high. You will use the OR for Txd and the AND for Rxd.
If you need simultaneous operation, look at http://www-us.semiconductors.philips.com/acrobat/a.../AN446.pdf

Have fun,
Erik