Raising BACnet® to the Next Level

Raising BACnet
to the Next Level
Connecting BACnet Devices
to an IP Infrastructure
George Thomas
Contemporary Controls
©2009. Contemporary Control Systems, Inc.
Few people understood what the
Internet Protocol (IP) was before
the Internet became wildly popular.
IP is the heart of the Internet, and
for Building Automation Systems
(BAS), IP is becoming increasingly
important. That was not the case
in the last decade of BACnet
development in which several data
link technologies were included —
including Ethernet. But Ethernet is
not IP, although it works well with
IP networks. The BACnet
community recognized the need
for a convenient method of
attaching to IP networks in a
meaningful way with the release of
BACnet/IP, but many legacy BACnet
devices exist and non-IP BACnet
devices continue to be produced
everyday. This paper reveals how
all BACnet devices can effectively
share a common IP network —
quickly converging with both areas
that attach to IP networks. It is
sharing a common IP network. This
clear that the backbone of choice
IP network is the quickest way to
is IP.
gain access to the Internet which is
the world’s wide-area-network
(WAN). All modern communication
networks are IP-based even if
Building Automation and Control
communication is restricted to
Network (BACnet) protocol was
Local-Area-Networks (LANs).
introduced in the mid-90s using
Modern buildings are designed and
four of the seven layers of the
built with structured wiring in
Open Systems Interconnection (OSI)
mind—with integrated telephone
model as shown in Figure 1. At the
and data wiring that can operate
lowest level of the model is the
at Gigabit Ethernet speeds. Why
physical layer which is concerned
run proprietary fieldbus networks
with sending symbols representing
when structured cabling is already
binary data across a medium. The
in place? Even non-BACnet
highest level is the application
building automation systems are
layer which defines the meaning of
connecting to IP — including access
the data as it is sent between
control systems, security systems,
stations. This is where BACnet
life-safety systems, and lighting
objects and properties are defined.
systems. LON-based systems are
After surveying what BAS vendors
using the IP networks as a tunnel,
were providing at the time, the
taking advantage of “free” wiring.
BACnet committee of the American
thereby enhancing the power and
When we think of IP networks we
convenience of BACnet.
think of Ethernet with speeds of
10, 100 or 1000 Mbps running over
Why the Internet
BACnet and Its Flavors
CAT 5, CAT 5e or CAT 6 twisted-pair
cabling. These same speeds can be
achieved over multimode or single-
Why are BACnet Plugfest
mode fiber optic cabling as well.
participants encouraged to attach
Granted, the higher speed
to an IP network when testing for
connections are wired, but there
interoperability? Why at trade
are plenty of wireless technologies
Society of Heating, Refrigerating,
and Air-Conditioning Engineers
(ASHRAE) decided to support five
data links—Ethernet, ARCNET,
Master-slave/Token-Passing (MS/TP),
Point-to-Point protocol (PTP), and
LonTalk. A data link defines how
stations are addressed and how
data in the form of frames are sent
between stations within a Local-
shows are IP networks used for
interoperability demonstrations? It
is because connecting to an
Ethernet backbone running the IP
protocol is convenient. Attempting
to do the same with a BACnet
MS/TP bus network is not as
convenient. IP is important for
reasons other than convenience.
Building automation systems and
information technology (IT) are
Figure 1. The BACnet four-layer model supports several data links including Ethernet.
Area-Network (LAN). A set of
bit worldwide unique value given
link control to BACnet’s network
similar stations within one data link
to every Ethernet controller chip
layer. Like ARCNET, MS/TP uses a
constitute a network. Stations with
and not the 32-bit IPv4 address we
token-passing protocol, but it is
incompatible data links cannot
would expect with an IP/Ethernet
implemented in software using a
communicate to one another at
device. Of all the BACnet data
serial port on a microcontroller.
this level. Since interoperability
links, Ethernet provides the
MS/TP relies upon a very common
between the various data links was
greatest speed.
physical layer called 2-wire EIA-485.
a goal of the BACnet SPC 135
committee, the BACnet Network
Layer was introduced, completing
EIA-485 transceivers are relatively
inexpensive and typically found in
low-cost controllers. They can drive
Like Ethernet, ARCNET can be
long lines and can operate at the
classified as having both a data link
required upper limit data rate of
and physical layer. ARCNET’s data
78.6 kbaud. Two versions of EIA-
link and physical layer options,
485 transceivers are found in the
IEEE 802.3, now an ISO standard,
which include support for coaxial,
BAS industry — isolated and non-
refers to part 3 of the Institute of
twisted-pair and fiber optic cabling,
isolated. With isolated transceivers,
Electrical and Electronics Engineers’
are described in the ARCNET Trade
there is no galvanic connection
802 standard entitled Carrier sense
Association’s ATA 878.1-1999
between signal ground on the
multiple access with collision
standard. Data rates range from
transceiver and circuit common of
detection (CSMA/CD) access
156 kbps to 10 Mbps. Unlike
the BACnet device. The network
method and physical layer
Ethernet, ARCNET uses a
connection requires three wires
specifications. However, we
deterministic token-passing
and not two since the signal
typically call this 1500 page
protocol, thereby providing
common must be carried between
standard Ethernet. Ethernet, along
improved real-time performance.
isolated devices. With a non-
with a Logical Link Control (LLC) as
The ARCNET controller chip is
isolated transceiver, only two wires
defined in ISO 8802-2, forms a data
treated as a communications
are carried because there is only
link that can operate at speeds of
coprocessor, relieving the main
one common (circuit common of
10 Mbps, 100 Mbps and 1 Gbps.
processor from handling token-
the BACnet device is the same as
Ethernet’s physical layer includes
processing duties. One of the
the transceiver common). With
several copper and fiber options
ARCNET physical layers is the
non-isolated devices, more care is
such as 10BASE2, 10BASE5, 10BASE-
inexpensive EIA-485 standard which
required when interconnecting
FL, 100BASE-TX, 100BASE-FX, and
will support 32 stations in a bus
devices since the ground potential
1000BASE-T. Any of these options
configuration. Bus topology is less
must be the same at each device in
can be combined in one piece of
expensive than star topology since
the network.
equipment and each is compliant
no hub is required. However,
with BACnet/Ethernet. Ethernet
troubleshooting a bus problem is
provides for greater speed, star
more difficult than troubleshooting
topology, and transformer-
a star problem.
The Point-to-Point protocol is
the four-layer model.
isolated transceivers. However,
BACnet/Ethernet is not the same
as BACnet/IP Ethernet. With
mainly used to tie modems into
BACnet networks. A serial port on
a micro-controller is used for
BACnet/Ethernet, LAN addressing is
Master-slave/token-passing remains
communication and a simple EIA-
accomplished using the Ethernet’s
a popular BACnet data link with
232 interface is used between the
media access control (MAC)
baud rates from 9.6 kbaud to 76.8
serial port and modem.
address. The MAC address is the 48-
kbaud. It provides its own logical
This implementation is seldom
found and will not be discussed.
As the popularity of TCP/IP
exploded, the BACnet community
needed a strategy for using the
BACnet protocol in an IP world
without a major re-write of the
Figure 2. The role of infrastructure equipment depends upon the communications level.
standard. The result was BACnet/IP
(B/IP) which is described in Annex J
one IP address and is sent to all
cabling. All these devices operate
of the BACnet standard. The body
other IP addresses on the subnet.
on the symbols sent over the wires.
of the BACnet standard makes
exclusive use of MAC addresses for
all data links, including Ethernet.
They provide low latency but do
Infrastructure Equipment
not understand protocols or even
the data link under which they
But in the BACnet/IP world, IP
In order to understand how
operate. These are very simple
addresses are needed. For
infrastructures are built, you need
devices. In the case of Ethernet
BACnet/IP, Annex J defines an
to understand the role of
repeating hubs and Ethernet media
equivalent MAC address comprising
infrastructure equipment. Figure 2
converters, they reside within a
of a four-byte IP address followed
shows the four-layer
single Ethernet collision domain
by a two-byte UDP port number.
communications reference model
which limits the maximum network
The BACnet community registered
that BACnet uses — with
a range of 16 UDP port numbers as
infrastructure equipment
hexadecimal BAC0 through BACF.
associated at each level.
BACnet/IP incorporates the same
Ethernet operates on the CSMA/CD
principle. A station that wants to
initiate a transmission first waits to
four-layer ISO model as shown in
hear if the channel is clear. If it
Figure 1. Routers operate at the
As mentioned before, the physical
appears to be clear, it initiates the
network layer. Since routers were
layer is concerned with the
transmission while continuing to
already defined in “The Network
transmission and reception of
observe if the transmission was
Layer” (clause 6 of the BACnet
symbols representing binary data
successful as evinced by a lack of a
standard), Annex J makes reference
sent across the medium. A
collision from another transmitting
to this clause. Annex J introduces
repeater is used to simply link
station. If a collision is detected, all
the concept of the BACnet Virtual
together two cable segments to
parties that initiated a transmission
Link Layer (BVLL) which provides
make a longer run. A good
will back-off for a random time
an interface between Clause 6
example is a 2-port EIA-485
and then try again. In order for this
equipment and another
repeater used to extend MS/TP or
collision detection mechanism to
communications subsystem. BVLL
ARCNET cable lengths. An example
function, all stations must reside
messages can be either directed or
of a multi-port repeater is an
within one collision domain —
broadcast. A directed message is
Ethernet repeating hub. A media
defined as twice the time for a
exchanged between two IP
converter is used to change one
transmission to be sent between
addresses and no others. A
medium to another such as from
the two most distant nodes.
broadcast message originates from
copper cabling to fiber optic
This time constraint defines the
This is called store and forwarding.
network by restricting
collision domain which in turn
No forwarding occurs at the port
transmissions to only those ports
limits the geographic size of the
where the frame was received.
party to the transmission. This
network. This limit is called the
Therefore, a switch has much more
process is very efficient as long as
maximum network diameter. This
latency than a repeater because it
all transmissions are unicast or
is a potential issue when cascading
must wait until the complete frame
directed messages — where the
repeating hubs or using fiber optics
is received. Although an Ethernet
transmission is from one station
and, therefore, Ethernet repeating
repeating hub is part of the
and directed to another. With
hubs are seldom used except at
Ethernet collision domain, an
broadcast transmissions, all stations
BACnet Plugfests.
Ethernet switch terminates an
must hear the transmission so the
Because a single Ethernet collision
individual collision domain at each
transmission is flooded to all ports
domain is shared by all stations, it
of its ports. Therefore, cascading
on the switch. In this situation the
is possible to view all traffic at any
switches does not limit the physical
switch functions much like a
point in the network. This is very
Ethernet network like repeating
repeating hub. To allow for
useful when troubleshooting
hubs. Much greater distances can
topology changes, the database is
protocols since a network analyzer
be achieved with switches than
periodically cleared and the
can be attached anywhere within
with repeating hubs. A switch can
location of MAC addresses is re-
the network. This is called shared
also operate in full-duplex mode —
learned. This process is called
Ethernet. Besides geographic
assuming its link partner is full-
limitations, shared Ethernet
duplex compatible. Full-duplex can
Switches are commonly used in
restricts operation to a single data
effectively double the throughput.
Ethernet networks since their
rate and half-duplex. Although bi-
With switches, there is potentially
benefits far outweigh their
directional transmissions are
no restriction on data rates.
shortcomings. Their greatest
allowed, they are not allowed at
Different ports on the switch can
inconvenience is when attempting
the same time.
operate at different data rates.
to troubleshoot network protocols
There is another advantage to
— since network traffic does not
switches. A switch learns the
appear on all ports of a switch.
location of Ethernet stations by
One step up the reference model is
noting in its database the MAC
the data link layer which is the
address of each station initiating a
home for bridges. Bridges link two
transmission and noting at which
At the network level there are
or more of the same data links
port the transmission was received.
routers that link networks together
together with the best example
This process is called learning. This
to form an internetwork. Instead of
being an Ethernet switch. An
MAC address/port number
individual frames, routers operate
Ethernet switching hub is
combination remains in memory so
upon packets. The most famous
fundamentally different from an
that the switch will know to which
internetwork is the Internet and
Ethernet repeating hub. A
port it should direct transmissions
the most famous network layer
switching hub operates upon the
destined to a particular MAC
protocol is the Internet Protocol.
frames sent from Ethernet stations
address. When a switch receives a
The Internet is a public network,
or from other Ethernet switches.
message with a destination MAC
but it is not necessary to have
In the normal mode of operation, a
address recorded in its database, it
access to the Internet in order to
complete frame is received on one
will forward the message only from
construct an IP infrastructure.
port of the switch before it is sent
the port with the associated MAC
Using the same Internet Protocol
out to all other ports on the switch.
address. This reduces traffic on the
and the same IP infrastructure
Continued on page 6
Contemporary Controls’
Approach to the Challenge
of Connecting BAS
Equipment to an IP
Although it is easy to delegate
infrastructure equipment decisions
to the information technology (IT)
department, IT personnel are
frequently unfamiliar with building
automation equipment
requirements. Besides, not all
equipment is IP-compliant. For
example, most commercial IT
infrastructure equipment is mainspowered while BAS equipment is
24 VAC/VDC powered using Class 2
wiring methods. The sharing of a
single control transformer in a
control panel requires all powered
devices to have half-wave power
supplies. BAS equipment is usually
mounted directly to a sub-panel or
DIN-rail mounted in a control panel,
while commercial IT equipment is
either table-top mounted or rackmounted in wiring closets. Locating
Router provides connectivity
BAS equipment in wiring closets is
between a BACnet/IP Ethernet client
not convenient or practical.
residing on a laptop to BACnet
Contemporary Controls’ products
MS/TP devices located in the field.
address the required IT standards,
Power for the router is derived from
but are designed with building
the laptop’s USB port, but all
automation needs in mind. The
communication is via the laptop’s
result is a neat, professional-looking
Ethernet port. An optically-isolated
installation, worthy of the effort —
MS/TP port is capable of handling
while complying with the
baud rates up to 76.8 kbaud.
performance and regulatory
Configuration is via a web page.
requirements of the job. What
Modern commissioning tools are
follows are some example products.
Ethernet-based, so a portable
Ethernet device that allows
Portable BAS Router
Convenient for commissioning and
troubleshooting, the Portable BAS
attachment to a MS/TP network is
an indispensible device to keep in
the tool box.
DIN-rail BAS Router
For permanent installations, the
DIN-rail BAS Router provides a lowcost method of connecting isolated
islands of BACnet MS/TP devices to a
BACnet/IP Ethernet network. The
half-wave rectified low-voltage
power supply allows for the sharing
of power with other 24 VAC/VDC
control devices. The MS/TP port is
optically-isolated and is capable of
handling baud rates up to 76.8
kbaud. Built-in End-of-Line (EOL)
termination and bias can be
disabled for mid-span MS/TP
installations. The BAS Router
supports BACnet/IP Broadcast
Management Device (BBMD) and
Foreign Device (FD) registration —
troubleshooting. Unlike office-grade
Ethernet equipment not adaptable
to control panels, the BAS Switch
makes an Ethernet installation neat,
professional, and dependable.
5-Port Plug-and-Play
Skorpion Switch
For control panels where DIN-rail
space is at a premium, the Skorpion
series of plug-and-play Ethernet
switches with widths as little as one
inch (26 mm) is the answer. Housed
in a rugged metal enclosure with a
metal DIN-rail clip, the Skorpion
series includes models with 5, 8 and
16 ports. Some models provide one
or two-ports of either single-mode
or multimode fiber optics with
either SC or ST connectors. No
configuration is necessary. All
copper ports automatically
configure their data rate and duplex
using the Auto-negotiation
protocol. Depending on the
thereby allowing the linking of
either 10 Mbps or 100 Mbps and at
BACnet devices over a sub-netted
either half- or full-duplex. Each port
IP network. Configuration is via a
will adapt to either a straight-
web page.
through or crossover cable by using
the Auto-MDIX protocol. There is no
5-Port Plug-and-Play BAS
need to stock crossover cables for
switch-to-switch cabling. The BAS
Switch is housed in a rugged metal
capability of the link partner,
communication is established at
either 10 Mbps or 100 Mbps and at
either half- or full-duplex. Each port
will adapt to either a straightthrough or crossover cable by using
the Auto-MDIX protocol. Fiber
ports are fixed at 100 Mbps fullduplex for the highest performance.
The BAS Switch is ideal for low-cost
enclosure and can be either panel or
10/100 Mbps Ethernet switching
DIN-rail mounted. A writable label
applications where equipment is
on the unit allows for documenting
mounted in shallow-depth control
the location of field devices
cabinets. No configuration is
connected to each port. The half-
necessary. Each port automatically
wave rectified low-voltage power
configures its data rate and duplex
supply allows for the sharing of
using the Auto-negotiation
power with other 24 VAC/VDC
The half-wave rectified low-voltage
protocol. Depending on the
control devices. LED port indicators
power supply allows for the sharing
capability of the link partner,
of data rate and activity are
of power with other 24 VAC/VDC
communication is established at
prominently located to assist in
control devices. LED indicators of
Multimode cable lengths cannot
exceed 2 km and single-mode
lengths cannot exceed 15 km
without a special order. Fiber optics
is ideal for inter-building cabling in
a campus environment.
data rate and activity assist in
traffic is restricted to other VLANs.
router. On the LAN side of the
troubleshooting. For control panels
This eliminates the need to pull
router is a four-port 10/100 Mbps
of nominal depth, the Skorpion
separate network cabling for the
unmanaged Ethernet switch that
series offers the widest range of
BAS system because the BAS traffic
supports both Auto-negotiation and
Ethernet connectivity solutions.
can be carried by the building’s
Auto-MDIX. On the WAN side is a
structured wiring system.
single 10/100 Mbps Ethernet port.
Not only does the Skorpion
8-Port Plug-and-Play
Managed Skorpion Switch
Cable Redundancy — Protocols
The next big step-up from a plug-
such as Spanning Tree Protocol (STP)
and-play switch is a fully-managed
and Rapid Spanning Tree Protocol
switch. The industry considers a
(RSTP) allow for Ethernet networks
managed switch as one that
to be cabled in either a ring or mesh
complies with the Simple Network
topology. Proprietary ring protocols
Management Protocol (SNMP) and
such as Contemporary Controls’
retains a Management Information
RapidRing allow for fast recovery in
Base (MIB) that provides detailed
a ring topology. Cabling redundancy
analysis of port status. The SNMP
adds another layer of robustness to
switch functions as an agent for a
an Ethernet-based BAS system.
IP/Ethernet Router transfer packets
between the LAN and WAN sides of
two networks, it provides a stateful
firewall to protect the LAN side
from unauthorized intrusions. The
router is configured through its web
page — where a host of features
allow secure access to Wide Area
Networks (WANs) such as the
PPPoE Protocol — External
manager functioning as a client on
Ethernet to DSL modems that
the network. A trap can be set by
Quality of Service (QoS) — The
the agent and reported to the client
priority of messages can be assigned
when an exception occurs.
when switches support QoS,
Although this functionality is
allowing more important messages
seldom used in a BAS environment,
to be sent ahead of those with a
the installation of managed
lesser priority. A priority field within
switches may be mandated by IT
a VLAN tag is appended to
departments. The Managed
messages sent between switches. By
Dynamic Host Configuration
Protocol (DHCP) — On the LAN
Skorpion series is a cost-effective
reading the priority field, the switch
side, a DHCP server can provide
solution in meeting the needs of IT
understands what messages take
private IP addresses to attached
department requirements.
precedence. Each port on a
stations. Address assignments can
Because of the sophisticated
managed switch can be configured
also be fixed with a one-to-one
Ethernet controller chips used in
for a particular level of priority so
mapping of MAC Ethernet addresses
managed switches, several useful
that the simple connection of a
to IP addresses. On the WAN side,
features are available that have
cable to a port sets the level instead
a DHCP client can request a public
nothing to do with the SNMP
of VLAN tags.
IP address or the IP address can be
directly supported — allowing easy
access to the Internet through an
Internet Service Provider (ISP).
Virtual Local Area Network
(VLAN) — VLANs allow for the
support the PPPoE protocol are
Skorpion IP/Ethernet
In order to interconnect two IP
Network Address Translation
(NAT) — A one-to-one mapping of
separating of network traffic while
networks, an IP router is required.
LAN-side client IP-addresses can be
sharing the same network
The Skorpion IP/Ethernet router
made to WAN IP-addresses —
infrastructure. This is ideal for BAS
combines the functionality of an
allowing stations on a private
where all BACnet traffic is relegated
Ethernet switch and that of an IP
network access to the Internet.
to an assigned VLAN while other IT
Port Address Translation (PAT)
from attached devices or from the
— If public IP-addresses are limited,
BAS Remote’s internal I/O can be
then PAT can be used. With PAT,
mapped to BACnet objects or
one public IP-address is shared by a
simply presented as Modbus TCP.
number of private IP-address on the
The BAS Remote is ideal for
LAN side by using individual port
accessing non-BACnet compliant
number assignments.
devices from an IP infrastructure
when no convenient fieldbus wiring
BAS Remote
exists. A half-wave 24 VAC/VDC
power supply allows for the sharing
The BAS Remote is a utility device
of power with other half-wave
that can provide additional
devices. The internal 24 VDC power
input/output capability, routing and
supply of the BAS Remote is made
gateway functionality in a Euro-style
available for powering 4–20 mA
DIN-rail mounted enclosure. There
transmitters and other pilot devices.
are several models.
By using Ethernet, you can take
advantage of some of the recent
advances in Ethernet technology.
One advance is Power over Ethernet
(PoE) in which a single cable
provides both power and data to
attached devices. This would be
ideal for attaching field devices to a
network where power is not
present or the power levels are not
conducive for easy attachment.
BAS Remote PoE
A BAS Remote Master is
BAS Remote Expansion
BAS Remote Master
One-Cable Solution
Up to three BAS Remote Expansion
configured for powering over
Ethernet. The BAS Remote PoE has
the same features as the BAS
The BAS Remote Master is
modules can be connected to a
BACnet/IP compliant device
single BAS Remote Master using a
conforming to the Application
proprietary serial port—increasing
Specific Controller (B-ASC) profile. It
the universal input/output count to
is a remote I/O device that allows
24 points and the relay output
sensors and actuators to be accessed
count to 8. A convenient 24 VDC
from an IP network. It also functions
power connection is provided for
as a Modbus serial router to a
powering transmitters. Like the BAS
Modbus TCP device. Finally, it is a
Remote Master, the expansion
gateway between Modbus serial
modules are powered from a 24
Skorpion PoE Injector
and BACnet/IP.
VAC/VDC supply.
PoE power can be derived from a
A 10/100 Mbps auto-negotiating
Ethernet port connects to the IP
infrastructure. Its IP address can be
static or dynamically assigned
through a DHCP server. Six points of
universal input/output points can be
configured through a web server.
Points can be configured for contact
closure inputs, pulse inputs, 0–10
VDC or 0–20 mA analog inputs, or
0–10 VDC or 0–20 mA analog
outputs. There are two contact
closure outputs. An EIA-485 2-wire
interface provides access to Modbus
serial devices. Modbus registers
Remote Master without the need to
provide 24 VAC/VDC power. Field
devices can then be powered using
the BAS Remote’s 24 VDC
convenience connection. This is a
slick way of providing power in the
field when none exists.
PoE switch or from a PoE injector.
BAS Remote MS/TP
The BAS Remote MS/TP is BACnet
MS/TP compliant device conforming
to the Application Specific
Controller (B-ASC) profile. It has the
same universal input/output relay
output capability as the BAS Remote
Master but only connects to the
MS/TP network. Powering the
device and the presence of a 24 VDC
convenience connection is the same
as the BAS Remote Master.
The Skorpion PoE Injector is
classified as a mid-span device with
two RJ-45 Ethernet ports. The first
port connects to the Ethernet
network and the second provides
both power and data to an
attached device such as the BAS
Remote PoE.
Ethernet operation is transparent
and wiring cost is lowered by
running only an Ethernet cable to
the attached device. Power for
the injector is derived from a
24 VAC/VDC source.
equipment, an intranet can be
messages, or messages constructed
created to achieve the highest level
in an unexpected manner, are
of security with the convenience of
dropped with no indication given
using a structured wiring system.
to the sender.
An intranet is a private network
A BACnet router is not necessarily
that uses the Internet Protocol.
an IP router. A BACnet router
Best practice calls for assigning
understands the BACnet Network
private IP addresses to all devices to
Layer protocol and not the Internet
ensure no conflict with public
Protocol unless it is supporting
addresses. Private addresses are
BACnet/IP. A BACnet router
restricted to the LAN side of the
attaches two or more BACnet data
router while public addresses are
links together to form one BACnet
on the WAN side. If there is no
internetwork. The physical layers
intention to connect to the
could be quite different on each
Internet, then there is no need to
side of a BACnet router because of
supply a router for this purpose.
the different data links BACnet
But if the intranet is so large or
supports. With an IP router,
complex that sub-netting the
Ethernet is typically present on
networks is desired, IP routers are
each side of the router.
still needed.
Think of an IP router as one device
An Infrastructure Example
In the side bar article, a typical
building automation system is to
share the same IP infrastructure as
other information technology
equipment. Perhaps the
specification calls for existing
structured wiring to be used to
reduce installation expense while
allowing the complete system to be
monitored from any point on the
IP network. For remote access, the
Internet is to be used.
At the top is a single BACnet/IP
compliant building controller (BC),
having no point I/O of its own, is
attached to Ethernet and
supervises the complete system. It
can be called a building controller,
application engine, or IP network
controller. A BACnet/IP operator
two halves vary but typical terms
The terms gateway and router are
workstation (OWS) provides an
are LAN side – WAN side, Private
sometimes intermixed. In the strict
overview of the system. A laptop
Side – Public Side, Trusted Side –
sense, gateways operate on
computer functions as a
Not Trusted Side, and simply LAN1
messages sent between two
commission tool for a BACnet
– LAN2. The LAN – WAN
different application layers. Since
MS/TP device.
terminology may be easier to
the application layers are
At the bottom are found individual
understand when talking about IP
incompatible, custom software
sensors and actuators that are not
routers connected to the Internet,
must be developed in the gateway
part of any network — although
while the LAN1 – LAN2 references
so that meaningful data can be
sometimes proprietary sensor buses
are best for sub-netting a larger
exchanged between the two
are used. However, for sake of
devices. The best example is the
discussion we will say that sensors
BACnet application layer and the
and actuators connect on a one-to-
Modbus application layer. Because
one basis to Direct Digital
there is no application layer
Controllers (DDC) or Remote
commonality, a gateway must
Input/Output (RIO) devices. A DDC
provide an interface. Converting
is an intelligent device that can
Modbus serial to Modbus TCP is
execute a control algorithm, while
different. The application layers are
an RIO is an intelligent device that
compatible, so it would appear a
can concentrate input/output
router is in order. However, the
points and forward the data to a
term gateway could be loosely
DDC over a serial bus or network.
applied to this situation as well.
RIO devices do not implement
with two halves. The names of the
IP routers can be much more
complex than an Ethernet switch,
but they serve a much different
purpose. IP routers block
broadcasts from propagating from
one side to the other. They restrict
blocks of IP addresses to one side
or the other. They can provide a
firewall function by allowing only
those messages through the router
that are intended. Unintended
control. The DDCs and RIOs in this
example connect to either an
“measures” the state of the binary
MS/TP, Modbus serial, or BACnet/IP
input. Binary output devices could
be motor starters, interposing
This BAS example identifies
relays, pilot lights, and solenoid
equipment residing in several
valves. The driving device must
have sufficient capability to power
the actuator. For analog output
* Sensors and Actuators
devices, either a 0–10 V or 0–20 mA
command signal is usually required.
* Modbus Serial
* BACnet MS/TP
* BACnet/IP Ethernet
* Internet
Sensors are used to measure
parameters within the process,
while actuators manipulate the
process. These active or passive,
analog or binary, input/output
devices reside at the lowest level of
the control hierarchy. Analog input
devices are used to measure
temperature, humidity, air quality,
or velocity. An analog output
device can be a motor speed
control, a value positioner, or a
BACnet/IP devices can directly
attach to the building’s IP
infrastructure. The most convenient
method is to use Ethernet. In this
example there is one building
controller and one workstation
attached to all other devices
through an Ethernet copper
connection. That does not
Modbus Serial
Besides BACnet and LON, Modbuscompatible equipment is frequently
used for building automation—
Sensors and Actuators
BACnet/IP Ethernet
especially for energy measurement
or for motor drives. The Modbus
physical layer is typically 2-wire EIA485. While a Modbus TCP device
would allow for a direct connection
to an IP/Ethernet network, Modbus
serial devices (because of the
preclude the use of fiber optics as
shown in the example. BACnet
MS/TP devices are connected to
routers which are attached to
switches. Ethernet switches are not
protocol-aware and only facilitate
connections to the IP infrastructure.
They do not block broadcasts and
the complete IP network can be
treated as a single BACnet
incompatible application layers)
will require either a gateway to
BACnet/IP or a Modbus Serial to
Modbus TCP router. Either way,
Modbus serial devices can be
viewed from an IP network.
In order to gain remote access to or
from the Internet, an IP Router is
needed. For security, the IP router
also functions as a stateful firewall.
On the LAN side of the router is
damper positioner. Passive analog
sensors, such as thermistors that
measure temperature, require low-
The popularity of MS/TP ensures
level excitation from the receiving
that there will be plenty of devices
device to accomplish the
that require attachment to an IP
measurement. Calibration curves
network. Unitary controllers and
are built into the receiving
programmable thermostats bused
equipment. More accurate
over an EIA-485 physical layer are
measurements call for an analog
typically found at this level.
transmitter capable of outputting
Attachment to an IP infrastructure
either a 0–10 V or 0–20 mA signal
is accomplished using a BACnet
to a receiving device. Binary input
MS/TP to BACnet/IP router. This
devices can be passive pushbuttons,
routing capability can be found in
limit switches, or level switches.
either a building controller that
The receiving equipment provides
supports routing or in a stand-
the sensing current that
alone router.
the BAS which is treated as an
intranet. On the WAN side is the
Contemporary Control
Systems, Inc
2431 Curtiss Street
Downers Grove, IL 60515
Contemporary Controls Ltd
Sovereign Court Two
University of Warwick
Science Park
Sir William Lyons Road
Coventry CV4 7EZ
United Kingdom
Contemporary Controls
(Suzhou) Co. Ltd
11 Huoju Road
Science & Technology Park
New District, Suzhou
PR China 215009
Contemporary Controls GmbH
Fuggerstraße 1 B
04158 Leipzig