I would like to setup a communication between an AT89C55WD and another mcu through the P0 port. The 89C55 will serve as the master. Ideally, I would like to issue commands over the P0 lines to the receiving mcu who will interpret the command and place data back on the P0 line. P0.0 ... P0.6 will serve as data lines and P0.7 will be used to signal when data is available from the slave mcu. This means the 89C55 will issue the command wait for P0.7 to toggle and then read the data from the other mcu on the P0 port.

I am unclear whether it is possible to setup something like that. Does the 89c55 need to tristate the lines in order to let the other microcontroller place data on the P0 lines. Can I somehow tristate the lines in firmware? Should I use pullup resistors? Any suggestions would be appreciated.

Oh, I almost forgot. I know that using the UART would be the preferred way of communicating but I am trying to support an old design and the board that has the 89c55 cannot change. The UART is already used for other purposes. The board the second mcu will be on can change and I can easily access the P0 on the 89c55 through a connector.

Thanks!

Marek