Chris Shepherd has an easy way to connect multiple NXT bricks if you don’t want to bother with bluetooth. Each light sensor also has a small LED, allowing it to send out or receive signals. The added benefit? It looks cool when it lights up. (Of course it’s not really limited to six.)
I have even set it up so that there are two “mailboxes.” I can send “Jobs” or “Messages.” I differentiate them by the time that the initial blink of the light stays on. If the initial blink is about 50ms long, it’s a “job.” If the initial blink is about 100 ms long, it is a “message.” Each of these types of messages is handled differently in the programs. Technically, there is a third type of mailbox too. If the sensor light stays on for 250ms, it is an emergency stop, and all the bricks interrupt what they are doing and immediate halt in place. Any brick can stop the entire process if there is a problem.
The type of data that is sent is binary numbers. I can send number from 1 to 63 for each Job or Message. For example, the master NXT sends out a job number 39, and each of the other five NXTs along with the master do the respective task assigned to the number 39, in this case play some music.