> I wonder if there is a way i can convert the serial port of a 80C51
> into a serial port with CSMA/CD (Carrier Sense Multiple Access with
> Collision Detection). I
> don't mind using any external circuitry or 8051 port pins as long as
> i use the serial port of the 8051 and not a networking peripheral.
> Note that the serial
> comunication will not be using a baud speed of more than 19200. Plus
> i will be interfacing the serial port to a RS-485 or RS-422
> transceiver and not RS-232 due to
> the long distances between every node.

You can take a look at the Tiny Embedded Network. It uses CSMA/CD and is
designed to benefit of the presence of an on-board UART of
microprocessors. It can be implemented in an 8051, PIC, etc with or
without an UART.

You will find some more information on TEN on my homepage at
http://members.tripod.com/~mdileo/

Ciao
   Marco