Standard 231: A Control Description Language for Building Environmental Control Sequences

Purpose

The purpose of this standard is to define a declarative graphical programming language for building environmental control sequences that are both human- and machine-readable designed for specification, implementation through machine-to-machine translation, documentation, and simulation.

Scope

This standard applies to building automation systems controlling environmental systems such as mechanical systems, active facades, and lighting.

About ASHRAE S231

ANSI/ASHRAE Standard 231 defines and documents Control Description Language (CDL), a standard interchange format for the control logic to be used in control systems. This standard is complementary and tightly connected to other work being developed in ASHRAE controls standards and guidelines. CDL defines the syntax for implementing the logic and documentation of a control logic. CDL is used for the control logic design as well as for control logic simulation. Commercially available control system tools used for control logic programming shall be able to import and export logic using either CDL or its JSON representation, called Control eXchange Format (CXF). CDL consists of the following elements:

1. Elementary blocks
2. Connectors through which these blocks receive input values and make accessible output values
3. Permissible data types
4. Syntax to specify:

1. How to instantiate blocks and assign values to parameters
2. How to connect inputs of blocks to outputs of other blocks
3. How to document blocks
4. How to add annotations
5. How a group of blocks and connectors can be used to define a composite block
6. How to support new (extension) blocks that are not part of this standard

5. A model of computation that describes when blocks are executed and when outputs are assigned to inputs.

Sequence from ASHRAE Guideline 36 represented in CDL

Source code

• A digital representation of CDL is being developed and maintained by Lawrence Berkeley National Laboratory (LBNL) in the Modelica Buildings Library GitHub repository: https://github.com/lbl-srg/modelica-buildings/tree/master/Buildings/Controls/OBC/CDL 
• The normative definitions of elementary blocks in Standard 231 are based on the LBNL Modelica Buildings Library version 12.1.0 GitHub repository:: https://github.com/lbl-srg/modelica-buildings/archive/refs/tags/v12.1.0.zip 

SSPC 231 is being formed to  work on maintaining and updating Standard 231.

Informative Note: Reference CDL implementations of ASHRAE Guideline 36 sequences can be found at: https://github.com/lbl-srg/modelica-buildings/tree/master/Buildings/Controls/OBC/ASHRAE/G36 

Informative Note: Additional information on usage of CDL can be found at obc.lbl.gov.