Hi Michael,

if you only want to connect some 8051 to your PC you can also use CAN transseiver. The CAN bus works also differential over 2 lines, but with a dominant and a recessive level. So you need no switching of the data direction. The only disadvantage, the bit rate must be a little lower for the same cable length as for RS-485.

E.g. on using the PCA82C251 (Philips) both lines can withstand upto +/-40V and data transfer was still working on ground voltage differences up to +/-5V.

Naturally on using the standard UART there is no arbitration and error correction included.
Only using of a CAN controller (SJA1000) give also these advantages.

Means on using of CAN transseivers only the CAN voltage levels used, not the CAN protocol.

The main difference of CAN and RS-485 voltage levels is only in the transmitter part, which avoid fighting of 2 active transmitters against another. So no direction signal was needed.


Peter