Handbook of air conditioning and refrigeration / Shan K

5.42 CHAPTER FIVE
Application Layer
3. It collects data from different systems and offers greater flexibility for extended systems in
retrofit projects.
Most of the EMCS manufacturers agreed to fabricate BACnet-compliant products from the late 1990s.
There are many other data communication protocols. One was developed by Echelon Corporation,
called LonTalk Protocol, and it became a working system in the mid-1990s and was favored
by members of the LonMark Interoperability Association. LonTalk protocol is neither an opento-public
protocol nor a standard.
An independent consortium called the OPC foundation, formed as a nonprofit organization in
1996 in Boca Raton, Florida, has dedicated itself to provide interoperability with Microsoft technologies
to develop a global specification and multivendor interoperability in industries. In 1990,
OPC is leading 140 member companies including Honeywell , Johnson Controls, and Siemens.
Layered Structure. A data communication system often adopts a hierarchical layered structure so
that a complex problem is broken into smaller and more easily solved problems. BACnet is based
on a four-layer collapsed architecture that corresponds to application, network, data link, and physical
layers in an International Organization for Standardization (ISO) model. This is the result of
careful consideration of the characteristics and requirements of the building automation control
(including HVAC&R) together with a constraint that protocol overhead be as low as possible.
An application layer is the highest layer in BACnet. It serves to define the objects and services
(including control operations, information exchange, and control devices) in a building automation
control (BAC) system. It also provides communication services and data encoding schemes
required by applications to perform monitoring and control functions.
Object Types, Properties, and Devices. The BACnet defines a set of standard object types instead
of conventional points. Analog input, analog output, binary value, command, file, program, schedule,
etc., grouped in 18 types are standard object types, for every device in an EMCS must have a
device object. There are 123 properties that have been identified by BACnet which fully describe
the device, or object type, in the network. Certain properties are required to be specified whereas
others are optional. An object identifier specifies its object name, object type, etc., and optional
properties such as description and device type. In BACnet, a device is defined as any device, real or
virtual, that supports digital communication using the BACnet protocol.
Services. In BACnet, services are the operations by which one device acquires information from
another device, commands another device to do something, or announces that some event
happened. BACnet defines 32 services that can be grouped into five categories:
● Alarm and event services refer to changes in conditions detected by a device, such as acknowledgment
of an alarm and confirmed change of value notification.
● File access services are used to read and manipulate files that are kept in devices, such as only
one read or write operation at a time.
● Object access service provides the means to read, to write, and to modify properties, such as to
add one or more items to a property.
● Remote device management offers disparate operations such as to tell a device to stop accepting
messages.
● Virtual terminal services are used by an operator to establish a bidirectional connection with an
application program implemented in a remote device.
Services are classified as confirmed when a reply is usually expected with data and unconfirmed
when no reply is expected. In BACnet, a given device is not required to implement every service.
However, “read property” is required to be executed by all the devices.