Mario, thanks for the response.
What is "GEP"?
Sorry... GEP in this context means 'Good Engineering Practices'.
As for what is supplied by the HC11, the Netduino (by means the MCU) is 5V tolerant so there's no problem.
As for what is supplied by the Netduino (3.3V), the "HC" voltage levels (68HC11) are able to recognize the logic high at 3V: again, no problem.
Sounds good. I was hoping something simple like this could be done.
Question: why don't you use the UART? It's much more convenient if you want exchange data between MCUs...
Originally, I thought I might have a dual master SPI system where both the nd+ and the hc11 shared some SPI slave devices. With both MCUs on SPI, it made sense to try to use it for communication between MCUs too. But, because there's no slave mode on nd+ it doesn't sound like such a good idea now.
Thanks again for the help.