Download Guidelines for Installation of Spyder BACnet Controllers

*** Technical Tips from TAC ***
5/13/10
Author: Paul Grinberg
C09-010
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Guidelines for Installation of
Spyder BACnet Controllers

Abstract—These instructions provide some guidelines for best
practices for installation of Spyder BACnet controllers in a
BACnet over MS/TP network. An overview of a typical MS/TP
network including wiring, biasing, termination, shielding, and
grounding including the typical pitfalls will be covered here.
S
I. INTRODUCTION
PYDER BACnet is a fully programmable controller in the
Spyder family of unitary controllers. It is a BTL certified
application specific controller (B-ASC) communicating over
Master-Slave Token Passing, or MS/TP, which is a
communication protocol residing on top of EIA-485 physical
layer.
As with all EIA-485 communication systems, adding new
devices to the bus can be both an art and a science. To ensure
robustness in communication, the user has to consider the
various installation options including wire selection, bus
length limitations, communication speed, termination, bus
biasing, cable shielding, as well as a number of non EIA-485
parameters that still affect the communication such as device
grounding, power cable selection, transformer selection, and
transformer loading to name a few. To complicate matters,
there is usually no “silver bullet” solution to address all
situations all the time; the user has to understand the trade-offs
of the various selections.
This application note will try to explain the importance of
each of the selection criteria necessary to create a robust EIA485 communication system. Even though the specifics will
focus on Spyder BACnet controllers, the concepts described
here can also be applied to other MS/TP devices.
II. COMMUNICATION CABLE
A. Cable Selection
The BACnet standard [1] contains a section which loosely
defines the MS/TP physical layer. Section 9.2.1 states that the
“MS/TP EIA-485 network shall use shielded, twisted-pair
cable with characteristic impedance between 100 and 130
ohms. Distributed capacitance between conductors shall be
less than 100pF per meter (30pF per foot). Distributed
capacitance between conductors and shield shall be less than
200pF per meter (60pF per foot). Foil or braided shields are
acceptable. The maximum recommended length for of an
MS/TP segment is 1200 meters (4000 feet) with AWG 18
(0.82mm2 conductor area) cable. The use of greater distances
and/or different wire gauges shall comply with the electrical
specifications of EIA-485.”
The primary reason for the last sentence of the standard is
that it is usually difficult to find a cable that meets the
specified needs. Honeywell has cables 3322 and 3251 which
meets all specifications. However, some installations may go
with different cables due to cost concerns, in which case it is
important to understand the cost/performance tradeoffs.
For short runs less than 100 feet, practically any 18-24
gauge twisted-pair wire may suffice. Even the shielding may
be omitted if the operational environment is expected to be
relatively noise free. For runs of 100 to several hundred feet,
significantly more care must be exercised. Shielding becomes
very important and with it comes the incurred expense of
selecting a cable with the distributed capacitance meeting the
specification. Otherwise, the rising and falling edges of the
data may have significant curvature instead of the expected
sharp edges. For longer than several hundred feet, the
characteristic impedance becomes crucial. Improper cable
selection will introduce reflections onto the cable, corrupting
the communication. If the installation requires cable runs
longer than 4000 feet, the cable must be broken up into
multiple sections with EIA-485 repeaters to ensure proper
operation. There should not be more than 3 repeaters on any
BACnet MS/TP bus.
B. MS/TP Topology
There are a number of ways to wire an MS/TP bus. Even
though it is possible to get each topology to work, it is very
difficult to properly install and subsequently troubleshoot
topologies other than a simple daisy chain.
The best connection scheme would have an MS/TP router
on one end of the bus, and a daisy chained series of MS/TP
devices (see Figure 1).
Fig. 1. Daisy-chained MS/TP connection topology.
Honeywell International Inc reserves all rights to the content of this
application note. No part of this application note may be reproduced without
the written consent of Honeywell.
C. Shield Connection
When using an MS/TP cable with a shield, it is important to
5/13/10
*** Technical Tips from TAC ***
Author: Paul Grinberg
drive the foil/braid to a low-impedance potential. Typically,
this is done by connecting the shield to ground by the network
router. It is important to note that the shield must not be
actively driven at multiple points along the cable. Doing so
will cause currents to flow in the shield making it ineffective
in keeping out the noise.
The Spyder BACnet controller provides a SHLD terminal
which is electrically isolated. This terminal is a convenient
way to connect the shield of the two MS/TP cable segments
comprising the daisy chained bus (see Figure 2).
Fig. 2. Spyder BACnet shield connection.
Since many non Honeywell devices do not have such a
terminal, it is important for the installer to still connect the
shield across such devices to ensure proper shielding
throughout the entire bus (see Figure 3).
Fig. 3. Example of propagating a shield across non Spyder BACnet devices.
D. Bus Termination
Even though the MS/TP communication rates are limited to
tens of kilobits per second, there is significant high frequency
content in the sharp rising and falling edges of every data bit.
When high frequency content propagates along a wire and
encounters an impedance mismatch, some of that energy is
reflected back, creating interference (see [2] for affects of
reflections). If the MS/TP cable is sufficiently long (typically
greater than several hundred feet) and the baud rate is
sufficiently high, an unterminated cable may create
communication issues. It is up to the installer to ensure that
the selected cable is properly terminated at the end of the bus.
Fig. 4. MS/TP bus termination. R1 may be internal to the router.
As seen in Figure 4, correct termination requires the
presence of a resistor on both ends of the bus. Sometimes, the
BACnet router may contain an internal termination resistor as
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is demonstrated by R1. Both R1 and R2 must have the same
value and be ±1% ¼ Watt or better. Both resistors also have to
match the characteristic impedance of the MS/TP cable. As a
rule of thumb, the termination resistors will be somewhere in
the 110-130 Ohm range. However, the exact value will
depend on the number and the type of devices present on the
MS/TP network. For exact calculations see the “Network
Biasing” section below.
The Spyder BACnet transceiver is slew rate limited, which
reduces the burden of the installer to perfectly match the
cable. However, non Honeywell devices may not be as
tolerant. Therefore, proper termination, especially on cable
runs longer than several hundred feet, is a must. Spyder
BACnet controllers do not have any internal termination so
R2 must be installed externally. By default JACE based global
controllers do not have internal termination and also require
an externally placed R1.
E. Network Biasing
EIA-485 specifies that when a device is not transmitting
data, its transceiver must be in a high impedance mode. Since
the MS/TP protocol does not guarantee that at least one device
on the bus will always be transmitting, it is possible to have a
bus that is not driven by any device. Without active drivers,
the voltage on the bus may drift anywhere including
potentially dangerous or ambiguous logic level voltages. To
prevent such a condition, network biasing resistors (sometime
also known as failsafe bias resistors [3], or simply bias
resistors, or biasing) must be added to the EIA-485 bus.
Fig. 5. Example EIA-485 bus with various network biasing options and
termination resistors
As seen in Figure 5, there is lots of variability in resistances
used for biasing. If multiple devices provide bias resistors,
those resistances appear in parallel increasing the affect of the
bias. If too much bias is applied, drivers may not be able to
effectively control the voltage on the bus, corrupting the
communication. In general, bias resistors may be applied
anywhere along the bus. Typically, this is done only in one
place at either end of the communication cable. Even though
there are certain benefits to applying biasing at both ends of
the cable [4], and even at any device along the communication
bus, Honeywell does not recommend this practice.
The exact value of bias resistors needed for an EIA-485 bus
depends on several factors including the characteristic
impedance of the bus, the number of devices on the bus, the
current load (known as unit load) each presents, the need to
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provide at least 200mV of bias when no device is driving the
bus, and the need for any one driver to generate at least 1.5V
on the bus when active.
All biasing calculations are based on the worst case
scenario of devices on a segment. Since devices may use
different hardware, for the sake of standardization, this
maximum is specified in terms of 32 unit loads. Spyder
BACnet controllers use a ¼ unit load EIA-485 driver. This
translates to a theoretical total of no more than 128 devices on
any given EIA-485 segment. However, the actual maximum is
likely much smaller. The primarily limiting factor is available
bandwidth. Even simple token passing and additional MS/TP
overhead such as Poll-For-Master can use up substantial
amount of bandwidth when communicating with many
devices. On top of that, all BACnet communication adds to
the bandwidth burden. Currently, the recommended maximum
of Spyder BACnet controllers on a single EIA-485 segment is
30, although somewhat larger installations may be possible,
depending on usage.
III. POWER CONNECTIONS
A. Transformer loading
The Spyder BACnet controller is powered from 24 VAC.
Typically, this is provided by a line voltage to 24 VAC
transformer. It is important that the transformer is sized
correctly to never exceed the maximum UL Class 2 rating of
100 VA.
Transformer loading comes from 3 different sources: the
idle load of the controllers which is 5VA per unloaded
controller, the load of all configured outputs, and the resistive
losses due to connecting the transformer to the controller. The
latter can be easily calculated from
(2  d   ) 
C09-010
 
P 2
VRMS
where d is the distance between the transformer and the
farthest controller, ρ is the resistance of the wire per foot
which can be obtained from Table I, P is the total power draw
from the transformer, and VRMS is the transformer voltage.
TABLE I
COPPER RESISTANCE OF WIRE
AWG
Diameter (in)
Resistance (Ω/foot)
Gauge
12
0.0808
1.588e-3
14
0.0641
2.525e-3
16
0.0508
4.016e-3
18
0.0403
6.385e-3
20
0.0320
1.015e-2
22
0.0253
1.614e-2
24
0.0201
2.567e-2
Resistance is for solid copper wire at 68oF, computed based on 100%
IACS conductivity of 58.0 MS/m. See [5] for more details
It is typically a good idea to not load the transformer to the
full 100%. Doing so may saturate the core and start
introducing errors in both I/O and communication. A good
rule of thumb is to keep the loading below 80%.
B. Power connection
It is important that one side of the transformer secondary be
designated the 24VAC side. This way, the transformer can be
used to power multiple controllers as long as that wire is
connected to the 24VAC terminal of every controller.
When using one transformer to power multiple controllers,
it is important to reconsider the power load of all devices and
their outputs when sizing the transformer (see “Transformer
loading” ). When daisy chaining the power connection, it is
also important to account for the IR drops between the
controllers.
C. Grounding
Making a proper ground connection is one of the most
important steps of an installation. Since Spyder BACnet uses a
non-isolated MS/TP connection scheme which requires a
common ground voltage on every device in the BACnet
network, grounding has to be considered for every device.
This not only includes the Spyder controllers, but also
transformers, routers, and other BACnet devices.
In order to guarantee proper BACnet communication, the
voltage on the COM terminal of every device must be noise
free and within 200mV on every device. The best way to
ensure this is to tie the second leg of the transformer
secondary to a known good ground.
Occasionally, the local ground may not be the best available
source of ground. One such example is in the proximity of a
Variable Frequency Drive (VFD). In such cases, ground noise
may get injected into the communication signals, introducing
errors. In such cases, the installer has to find a better, possibly
more distant and clean source of ground. If such installation is
impossible, then the MS/TP connection has to be converted
into a 3-wire isolated connection.
When powering multiple devices from a single transformer,
it is important to consider the IR drops on the COM
connection. In some installations, it may be possible to daisy
chain the devices without violating the specifications.
However, installers may have to run a separate COM wire to a
known good earth connection if the IR drops become
significant.
It is also important to understand that Spyder BACnet
devices have an EGND terminal in addition to the COM
terminal. These two terminals have a pre-installed jumper on
the terminal block which guarantees proper operation of the
controller.
5/13/10
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Author: Paul Grinberg
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IV. TROUBLESHOOTING TECHNIQUES
Installing a robust MS/TP network can be a bit of both art
and science. Therefore, it is quite possible to have
installations that perform poorly or are completely
nonfunctional. Such installations require troubleshooting.
Here, we shall discuss the basics of troubleshooting.
Most problems stem from one of two errors: improper
Fig. 9. This scope trace demonstrates (1) significant noise. Grounding issue.
Fig. 6. Example of EIA-485 bus as viewed three different ways. Channel 1 is the
Data + with respect to COM, channel 2 is the Data - with respect to COM, and
channel M is a math channel representing Channel 2 – Channel 1. All
oscilloscope traces depicted here are reproduced with explicit permission of
Saia-Burgess.
respect to COM and BAC + with respect to BAC - (this is a
differential connection requiring either a differential probe or
an oscilloscope with math capabilities). An example of typical
signal levels can be seen in Figure 6. In such a scope capture,
the important features include:
 CH1 idle voltage is more positive than CH2
 CH1 and CH2 move in opposite directions
 Neither CH1 nor CH2 go above +12V
 Neither CH1 nor CH2 go below -7V
 Neither CH1 nor CH2 show significant noise
 Math channel shows idle voltage greater than 200mV
 Math channel shows peak to peak voltage greater than
1.5V
 Math channel has a rising edge when transitioning from
idle to beginning of message
 Math channel does not show significant noise
Fig. 7. This scope trace demonstrates (1) differential voltage of less than 200mV
when idle, and (2) falling edge at beginning of message. Improper biasing.
grounding and improper communication connections. One
way to troubleshoot both of these issues is by using an
oscilloscope. A lot of information can be learned by
looking at the BAC + with respect to COM, BAC - with
Fig. 8. This scope trace demonstrates (1) differential voltage is always flat, and
(2) CH1 and CH2 are not in opposite direction. BAC+ and BAC – are reversed.
*** Technical Tips from TAC ***
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Author: Paul Grinberg
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


Do not overload the transformer
Use the best available local ground
Start with the highest baud rate MS/TP communication
and back down if necessary
REFERENCES
[1]
[2]
[3]
[4]
[5]
Fig. 11. This scope trace demonstrates (1) curves on the edges. MS/TP cable is
too long or not terminated properly.
Figures 7 through 11 show examples of bad installations.
Fig. 10. This scope trace demonstrates (2) curves on the edges. MS/TP cable has
too much capacitance.
V. CONCLUSIONS
Installing a robust BACnet system can be complicated even
for experienced installers. There are many possibilities for
improper installation that may result in an impaired or nonfunctional system.
This Application Note attempts to document and explain the
common pitfalls of installations along with some
troubleshooting techniques. Many of these problems can be
avoided from the start if proper installation techniques are
used. In summary, the rules of thumb to ensure good
installation include:
 Use a proper cable for MS/TP connections
 Terminate, bias, and shield the MS/TP bus properly
 Do not daisy chain the power connections
ANSI/ASHRAE 135-2004 BACnet – A Data Communication Protocol for
Building Automation and Control Networks, 2004, American Society of
Heating, Refrigerating and Air-Conditioning Engineers, Inc.
http://www.ultracad.com/simulations.htm, Transmission line simulator,
UltraCAD Design Inc.
http://en.wikipedia.org/wiki/Fail-safe
http://www.ccontrols.com/pdf/Extv9n2.pdf
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wi
re_sizes
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