Honeywell describes their Smart Distributed System protocol (or SDS protocol) in this way:
“The Smart Distributed System is a bus system for intelligent sensors and actuators that streamlines the system installation process and empowers your inputs and outputs to operate at levels you never thought possible. Over a single 4-wire cable, Smart Distributed System can interface up to 126 individually addressable devices. These intelligent sensor and actuator devices do much, much more than just turn on and off.”
We will add more in-detail information here as soon as we come across a good description of SDS. You can find more information on the SDS homepage and the SDS protocol specifications can be downloaded from Honeywell.