Hi Stephan,

Sorry I took so long, but I hadn't visited the forum for a long time
I see there's alot of other solutions to your problem up there too, so read on if this one still seems useful:

Take one free port on each uC, say P1 on the master and P2 on the slave. And Erik was right..You will also need 2 pins (Transmit and Receive) free on each uC for the handshake. I would suggest you use an interrupt pin (p3.2 or p3.1) as receiver pin, so that data can be collected as soon as it is recieved.

Communcation Master
uC uC
__________ Data lines ________
| |
P2.0 |<------------------>|P1.0
P2.1 |<------------------>|P1.1
P2.2 |<------------------>|P1.2
| . | P1
P2 | . |(Data)
Data) | . |
| . |
P2.7 |<------------------>|P1.7
| |
| |
| |
| |
| |
P3.3 |------------------->|P3.2
(Txmit)| |(Rec
| |
| |
P3.2 |<-------------------|P3.1
(Rec) | |(Txmit)
__________| |_______


The working would be:

Communication uC:
=================
Serial Interrupt-
Check if 8 bits have been accumulated,
mov P2,Data
setb p3.1
wait for acknowledge on P3.2
clear p3.1
reti

External Interrupt(p3.2)-
mov Data,P2
strobe p3.1 to acknowledge
Put Data into buffer for Serial communication
Transmit
reti

Master uC:
==========
External Interrupt(p3.2)-
mov Data,P1
strobe p3.1 to acknowledge
Use data or Set internal flag to use data
reti

Transmit-
mov P1,Data
setb p3.1
wait for acknowledge on p3.2
reti


The handshaking part needs to be refined, but that is the basic working.
I have also mailed you this stuff

Kunal