There are many ways to communicate with you slave boards,as Erik stated ,it all depends on the cost and complexity (and the degree of reliability you seek)..
At the top end of the scale I would use CAN protocol with a mixed RF and rs485 transmission layer ..
And at the bottom end , use a 4 wire burglar alarm approach , with each zone in a loop and/or each device its own input to the master board..
The most intuitive for you would be the CAN approach , but it would also be the most costly.