BAS communication standards - Intelligent Buildings and Building Automation - page 74

EIB-ObIS (European Installation Bus Object Interface Specification) and
LonWorks. They are all object-oriented protocols or have an extension of
object-oriented technology. In this section, the issues related to protocol
standards are elaborated by addressing mainly BACnet and comparing
BACnet with other protocols.
5.2.1 Background of BACnet
BACnet is a data communication protocol for BA and control networks. A
data communication protocol is a set of rules governing the exchange of data
over a computer network that covers everything from what kind of cable
to use to how to form a particular request or command in a standard way.
What makes BACnet special is that the rules relate specifically to the needs
of BA equipment; that is, they cover things like how to ask for the value of a
temperature, define a fan operating schedule or send a pump status alarm.
BACnet has been developed under the auspices of the American Society of
Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). It is an
American national standard, a European standard, an ISO global standard
and the national standard in more than 30 countries. The protocol is sup-
ported and maintained by ASHRAE Standing Standard Project Committee
135. It is the only open protocol that was designed originally for BA from
the ground up, and it is an open protocol that supports high-end functions
such as scheduling, alarming and trending.
To achieve interoperability across a wide spectrum of equipment, the
BACnet specification consists of three major parts. The first part describes a
method for representing any type of BA equipment in a standard way. The
second part defines messages that can be sent across a computer network
to monitor and control such equipment. And the third part defines a set of
acceptable LAN architectures that can be used to convey BACnet communi-
cations. Let's look at each of these components of the BACnet specification
in more detail.
5.2.2 Representing BAS equipment in a standard way - BACnet
objects
BACnet provides a standard way of representing the functions of any device,
such as analogue and binary inputs and outputs, schedules, control loops,
and alarms, by defining collections of related information called 'objects',
each of which has a set of 'properties' that further characterize it. Each ana-
logue input, for instance, is represented by a BACnet 'analogue input object'
which has a set of standard properties such as present value, sensor type,
location, alarm limits and so on. Some of these properties are required while
others are optional. One of the object's most important properties is its identi-
fier, a numerical name that allows BACnet to unambiguously access it. Once
devices have common 'appearances' on the network in terms of their objects