The Arduino Slave is designed to be simple to make and very low cost. Just an ATmega328 microcontroller, a 16MHz crystal and a reset circuit. In addition a 74AHC125 tristate buffer is used to control access to the serial network and ack as a buffer and line driver. One digital output line is needed to control the tristate buffer - leaving all of the remaining I/O lines for the Slave application.
The primary function of the slaves is for remote distributed monitoring and control tasks. Slaves are used to monitor changes in physical quantities - such as temperature and then perform some related control task. If the role of the slave is merely for monitoring a changing quantity - such as gas meter pulse counting, then the Slave acts as a small part of a multi-channel datalogger.
The Slaves are intended to run semi-autonomously. For example if a Slave is running a room temperature monitoring and control application, it is expected to do this without regular updates from the hub. It will continue to maintain the temperature set point and control the radiator valve accordingly until it receives an update to the set point form the Hub.
RAM resources on the '328 microcontroller are limited to 2K, of which about 1.5K will be allocated as a buffer to allow the slave to record up to 1 hour of data, and then feed this forward to the hub in the form of a CSV file sent across the network. Central to this store and feedforward mechanism working is the implementation of the OpenLog opensource datalogger on the Hub. A slave could take temperature readings every 6 seconds for an hour, package them up into a CSV file and then output them to the network in about 5 seconds at 9600 baud - in the time between taking consecutive temperature readings.
The datastreams will be organised as channels. A channel is a series of sampled readings taken from a time variant physical variable - for example "living room temperature", "electricity instantaneous power", "solar panel power output". A channel can be named, logged and then passed up to the Hub or laptop for subsequent further processing, graphing or display.