NEMA BS 31017-2023 PDF

ClimateTalk is an open standard that defines a set of messages and commands to enable interoperability, enhanced user interface, and machine to machine control independent of the physical layer connecting the devices. 

The messages and commands defined by ClimateTalk Information Model (CIM) are the presentation and application layers as defined by the OSI Model .  ClimateTalk Applications are fully defined at Layer 7 of the OSI model by a combination of a Device Specific Application Profile, the Generic Application Specification and the Command Reference. 

ClimateTalk messages can be carried over any physical medium following the OSI model. The ClimateTalk Presentation Layer defines how messages are executed over the various physical mediums in use.  

CT-485 and CT-LWP are wired serial physical and network layers designed to support the formation of ClimateTalk networks and transport ClimateTalk messages, but other OSI based protocols – including wireless transports - can be used as well.
 
CT-485 is a Physical, Data Link, and Networking set of specifications that define one of the physical media over which ClimateTalk messages are sent.  CT-485 is a variant of EIA/TIA-485  standards with provisions against incorrect wiring and grounding requirements that meet the needs of residential systems.

This document defines the Data Link Layer requirements for CT-485.  Corresponding to OSI Layer 2, the Data Link specification defines CRCs, bit level error correction, physical node addressing, subnet addressing, and data transmission rules. See Figure 1 - OSI Layers for a CT-485 Implementation for a diagram of relevant standards.