Master / Slave. The 1284p is the master, all other modules are slaves and only respond when addressed by the master.
I was hoping to keep it to half-duplex to minimize the number of lines necessary. However, I keep having trouble with getting anything intelligible across the divide.
As for other implementations, I wonder if the 74AHC125 chip from NXP might make sense. The openenergymonitor crew built a network around it that could transmit significant distances while maintaining simple TTL signalling. Not as robust perhaps as RS485 with its differential signalling, but unlikely a problem given the above parameters, I hope. Plus, a lot easier to troubleshoot since any FTDI cable and understand what is being sent.