BAS communication standards - Intelligent Buildings and Building Automation - page 75

and properties, it is then possible to define messages that can manipulate this
information in a standard way. BACnet defines 18 standard object-types at
the beginning. As the standard has evolved, object types have been extended
from the original 18 to 25 currently. These 25 object types are referred to as
standard objects. A BACnet device does not need to support all object types,
but if an object type is supported, it must comply with the standard object
model for that object type. Each object type has a list of required properties
and optional properties. Optional properties can be included at the manu-
facturer's discretion. These standard objects are listed in Table 5.1.
The purpose of most BACnet objects is clear from the name of the object
but some require explanation and are described as follows:
￿ Calendar represents a list of dates that have special meaning when sched-
uling the operation of mechanical equipment. A list of holidays would be
one example.
￿ Command represents a multi-action command procedure, such as a
sequenced start-up of several devices.
￿ Device contains general information about a particular device, such as
vendor name, model name, location, protocol version supported, object-
types supported, etc.
￿ Event Enrolment provides one way to define alarms or other types of
events and to indicate who should be notified when they occur. Some
objects (Analogue Input, Analogue Output, Analogue Value, Binary
Input, Binary Output, Binary Value and Loop) contain optional proper-
ties to support intrinsic event-reporting capability and do not need to use
Event Enrolment objects.
￿ Group provides a shorthand way to read several values in one request.
For example, it may be used to simultaneously update several fields on
an operator graphic display.
￿ Loop can be used to represent any feedback control loop, which is some
combination of proportional, integral or derivative control.
￿ Notification Class provides a way to manage the distribution of alarm or
event notifications that are to be sent to multiple devices.
￿ Accumulator object type defines a standardized object whose properties
represent the externally visible characteristics of a device that indicates
measurements made by counting pulses. It maintains precise measurement
of input count values, accumulated over time.
￿ Life Safety Zone object type defines a standardized object whose prop-
erties represent the externally visible characteristics associated with an
arbitrary group of BACnet Life Safety Point and Life Safety Zone objects
in fire, life safety and security applications.
￿ Life Safety Point object type defines a standardized object whose proper-
ties represent the externally visible characteristics associated with initiating
and indicating devices in fire, life safety and security applications.