30 November 1998
c. Terminal unit control systems. Terminal unit control systems will use only electric or electronic
control devices. The foregoing requirement for standard instrumentation signals does not apply to
terminal unit control systems.
d. Standard controller. A single version of an electronic, self-tuning controller (generally known as a
single-loop digital controller (SLDC)) will be used as the standard controller for HVAC systems in all
applications except for terminal unit control-system applications (and when designing DDC systems).
This type of controller has a history of reliable use, and is available from multiple sources as a standard
product with the features described for its use in this manual. Using a standard controller will make
control systems easier to maintain. The standard controller will accept one analog signal as a process
variable input (PV) and one analog signal as a remote setpoint adjustment (CPA) input, and will produce
one analog output signal (OUT). The controller will fit in a standard-size panel cutout. A controller of
one manufacturer may be replaced by a controller of another manufacturer because several
manufacturers produce the same version of the controller.
e. BACnetTM communication protocol.
(1) In some cases it may be desirable and beneficial to connect different vendors DDC field
panels together to perform supervisory monitoring, management and control functions. The Building
Automation and Control Networking (BACnetTM) protocol provides a means to interconnect different
manufacturers control equipment.
(2) BACnetTM is a communication protocol specification. The development of this specification
was prompted by the desire of the building owners and operators for cost-effective inter-operability, i.e.,
the ability to integrate equipment from different vendors into a coherent automation and control system.
Work on the BACnetTM specification began in June 1987 and was completed/approved by ASHRAE
standards committee in June 1995 and approved by the American National Standards Institute in
December 1995. Although the specification is complete, work is not yet complete on a specification for
and a methodology for conformance testing of products claimed by vendors to be BACnetTM compatible.
(3) BACnetTM is intended as a standard communications protocol for HVAC&R. It is not directly
intended for other building services such as lighting, fire and security although it does not preclude
integrating these functions into a common system.
(4) BACnetTM is a mechanism that conveys information including, but not limited to:
Hardware binary I/O values
Hardware analog I/O values
Software binary and analog I/O values
Alarm and event information
(5) To use the BACnetTM standard the specifier should;
(a) Understand the structure of the protocol
- Conformance Classes
- Devices, Objects, Services