Specifications

103Environmental Automation
since its outputs are used for several
other things.) Responding to the BAS,
the VAV unit simply needs to generate a
xed temperature at its output plenum
and is programmed to always set it to the
maximum allowed room temperature.
It also must keep the plenum pressure
at a xed level. As the room thermostat
sees its set point moved up by a
message from the BAS, it responds by
requesting warm air from the TU. The
TU receives the command to open the
damper only a small amount since the
temperature-error message is only a
couple of degrees. The damper motor on
the TU opens the damper slightly, which
allows some warm air into the room;
the VAV unit responds by increasing
its fan speed slightly to maintain the
plenum pressure. It then increments
the heating a small amount to maintain
its output temperature. As the room
reaches the set temperature quickly,
the thermostat is satised and the TU is
instructed to close. This makes the VAV
unit slow down its fan, and less energy
is needed to keep the plenum warm. As
the thermostat is incremented again, the
process repeats producing small steps
of temperature increase in the room. By
slowly increasing the room temperature
this way there is no overshoot and
no massive amounts of heat needed
to bring the room up to temperature
quickly. Doing so is an overall lower
energy method to warm the room,
although it takes longer, but by starting
the process early this is not a problem.
This example shows that even though
the VAV unit has its controller built in,
its job is not to bring the room up to
the nal temperature. Its job is simply
to keep the plenum temperature and
pressure at a constant level. Similarly, the
TU damper controller responds only to
dierences in temperature between the
set point and the actual temperature,
with no knowledge needed of the actual
nal temperature requested. And the
thermostat itself is remotely controlled
by the BAS instead of a person having to
enter the room and make the repeated
adjustments. There are certainly dierent
ways to divide up an automation of
processes like this, but this example
demonstrates distributed control. Each
controller has its limited, well-dened
job that contributes to the overall control
system, which can be quite complex
and running sophisticated algorithms.
But it is made up of many simple pieces
with the result being, in this case, energy
savings, predictable room temperature,
and minimal human interaction
needed to achieve these results.
Communications/
Networking (Wired/
Wireless)
BACnet and LonTalk
BACnet and LonTalk are two competing
networking standards for building
automation. They are used extensively
for HVAC control, lighting, access
control, re-detection applications,
and more. BACnet is under continuous
maintenance by the American Society
of Heating, Refrigerating and Air-
Conditioning Engineers (ASHRAE),
Standing Standard Project Committee
135. The BACnet protocol denes a
number of data link and physical layers,
including ARCNET, Ethernet, BACnet/
IP, point-to-point over RS-232, Master-
Slave/Token-Passing (MS/TP) over
RS-485, and Echelon’s LonTalk, which is a
rival protocol to BACnet. BACnet/IP and
“Virtual LANs” allow for TCP/IP, ATM, etc.
BACnet creates the concept of an “object”
and communication rules that make
equipment from various vendors all
communicate following the same rules.
Any BACnet device is simply a collection
of objects that represent the functions
actually present in the real device.
The network structure is a client/
server model. Messages are
“services” that are carried out by
the server on behalf of the client.
LonWorks and LonTalk
LonWorks
®
is a networking platform
created by Echelon Corp. for control
applications in buildings. LonTalk is
the communications protocol. The
protocol is also one of several data
link/physical layers of the BACnet
ASHRAE/ANSI standard for building
automation. ISO and IEC have granted
the communications protocol, twisted-
pair signaling technology, powerline
signaling technology, and Internet
protocol (IP) compatibility standard
numbers ISO/IEC 14908-1, -2, -3, and -4.
Optical Fiber
Many new buildings install optical
ber to ease high-speed networking
capability. Because a single ber can
carry much more data than electrical
cables, they are small, and glass is much
cheaper than copper, optical ber is an
economical choice. Fiber is also immune
to electrical interference and voltage
surges, and there is no crosstalk between
signals in dierent cables. They form
a natural electrical isolation barrier
since they do not conduct electricity.
Additionally, optical signals can be used
in explosive environments without
added danger because they cannot
cause any sparks. Of course, power is
still needed for the optical transceiver.
A variety of modules provide transitions
between electrical systems and the ber
links. Maxim oers several products
for these modules. For details, go to:
www.maxim-ic.com/optical-module.
Powerline Communication
Where it is dicult to run networking
cable, there are now a variety
of powerline communications
technologies that provide robust data
communications at high data rates
over existing power lines. These new
technologies overcome the problems
of data loss due to noisy power lines
and transformers blocking the signals.
Power over Ethernet
PoE technology enables power through
switch/router in either midspan or
endpoint systems, collectively named
power-sourcing equipment (PSE), to
deliver power over the data cable.
The power-receiving system is called
a powered device (PD). The PSE must
detect and classify the PD successfully
before powering it. Once the PD is
powered, the PSE keeps monitoring for
the PD disconnection and must power
down the cable in approximately 350ms