Texas Instruments | Critical Spacing of CAN Bus Connections (Rev. A) | Application notes | Texas Instruments Critical Spacing of CAN Bus Connections (Rev. A) Application notes

Texas Instruments Critical Spacing of CAN Bus Connections (Rev. A) Application notes
Application Report
SLLA279A – November 2008 – Revised January 2009
Critical Spacing of CAN Bus Connections
Steve Corrigan ............................................................................................... HPL - Industrial Interface
ABSTRACT
This application report presents guidelines for the spacing of CAN nodes along a bus
based on the lumped load capacitance of the system.
Errors such as "message priority inversion" in which high-priority messages receive
low-priority placement in a queue of messages waiting to be transmitted by a controller
are commonly found on buses with unevenly spaced node clusters.
CAN buses are constructed with many nodes that are often placed physically close
together. When these clumps of nodes are spaced a relatively long distance from other
nodes, random data errors can be generated that are not easily uncovered by a system
designer.
Contents
1
Minimum Distance Between Node Connections ............................................... 1
2
Conclusion ........................................................................................... 6
3
References .......................................................................................... 6
Appendix A
Mathematical Development of the Node Capacitance and Node Spacing Trade-off
............................................................................................... 7
List of Figures
1
2
3
4
5
6
7
8
A-1
1
Imbalanced CAN Bus Schematic Diagram ......................................................
Minimum CAN Device Spacing ...................................................................
Capacitive Load Example .........................................................................
Normal Data Transfer .............................................................................
Multiple Dominant-Bit Distortion ..................................................................
Multiple Dominant-Bit Reflected Wave ..........................................................
Multiple Dominant-Bit Negative Voltage Reduction ............................................
Half-Meter Bus Length Correction ................................................................
CAN Bus Schematic Diagram.....................................................................
2
2
3
3
4
4
5
6
7
Minimum Distance Between Node Connections
The ISO-11898-2:2003 CAN bus is a distributed parameter circuit whose electrical characteristics and
responses are primarily defined by the distributed inductance and capacitance (1) along the physical media.
The media is defined here as the interconnecting cable or conducting paths, connectors, terminators, and
CAN devices added along the bus.
The following analysis uncovers a trade-off between the amount of node capacitance that can be added
and the node spacing on a bus while maintaining signal integrity. For a good approximation, the
characteristic transmission line impedance seen into any cut point in an unloaded CAN bus is defined by
Z = √L/C, where L is the inductance per unit length and C is the capacitance per unit length. As
capacitance is added to the bus in the form of devices and their interconnection, the bus impedance is
lowered to Z'. When the bus impedance is lowered, an impedance mismatch occurs between unloaded
and loaded sections of the bus.
(1)
All capacitances are differential in this report. The differential is approximately one-half of the single-ended capacitance.
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Critical Spacing of CAN Bus Connections
1
Minimum Distance Between Node Connections
www.ti.com
d
t=0
S1
VS
120 W
ZO = 120 W
Load
Load
Load
Load
120 W
Figure 1. Imbalanced CAN Bus Schematic Diagram
A worst-case occurrence is during an arbitration when multiple dominant bits are simultaneously sent from
two or more nodes. In Figure 1, when S1 switches at time zero from a dominant state to a recessive state,
the CAN driver differential output voltage, VS, moves from a dominant state to steady-state 0-V recessive
differential signal on the bus. As this signal wave propagates down the line and arrives at the loaded
section of the bus, the mismatch in impedance reflects the voltage back towards the source.
See the Appendix for a mathematical derivation of this condition.
The minimum safe distance between nodes, d, is a function of the device lumped load capacitance CL,
and the cable's distributed capacitance per unit length, C,
where
d>
CL
0.98C
meters (if C is pF/m) or feet (if C is pF/ft).
Figure 2 displays this relationship graphically.
1.4
1.2
CL = 50 pF
Distance - m
1
CL = 40 pF
CL = 30 pF
0.8
CL = 20 pF
CL = 10 pF
0.6
0.4
0.2
0
40
50
60
70
80
Media Distributed Capacitance - pF/m
90
100
Figure 2. Minimum CAN Device Spacing
Load capacitance includes contributions from the CAN transceiver bus pins, connector contacts,
printed-circuit board traces, protection devices, and any other physical connections as long as the
distance from the bus to the transceiver is electrically short.
The 3.3-V supplied CAN transceivers, such as the SN65HVD233 used in this example, have about 16 pF
of differential capacitance. Board traces add about 0.5 pF/cm to 0.8 pF/cm depending on their
construction. Connector and suppression device capacitance can vary widely and media distributed
capacitance ranges from about 35 pF/m for low-capacitance, shielded-twisted-pair cable to 70 pF/m for
backplanes.
2
Critical Spacing of CAN Bus Connections
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Minimum Distance Between Node Connections
www.ti.com
As a demonstration of this condition, Figure 3 displays 10 SN65HVD233 CAN transceivers connected to
the bus with 5 inches of 120-Ω, twisted-pair cable between each node. The last node of the group is
terminated with a 120-Ω termination resistor and the first node is connected through an additional 200 m
of Belden 3105A twisted-pair cable to another node and terminated.
200 m
120 W
Load
ZO = 120 W
5"
Load
1
Load
2
Load
3
Load
10
120 W
Figure 3. Capacitive Load Example
Figure 4 displays the receiving waveforms of the 250-kbps data being transmitted onto the bus from the
single-node load to the capacitive clump of nodes over the 200-m cable.
Figure 4. Normal Data Transfer
Figure 5 displays the same waveform when more than one node sends a dominant bit onto the bus during
an arbitration. Note the change in magnitude when more than one node is sending a dominant bit. The
propagation delay of 5 ns per meter for 200 m is 1000 ns, or 1 µs, and is clearly evident in each of the
waveforms.
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Critical Spacing of CAN Bus Connections
3
Minimum Distance Between Node Connections
www.ti.com
Figure 5. Multiple Dominant-Bit Distortion
The negatively charged waveform is reflected back and attenuates the waveform at the receiving clump of
nodes. Figure 6 and Figure 7 present a higher resolution of this reflection.
Figure 6. Multiple Dominant-Bit Reflected Wave
4
Critical Spacing of CAN Bus Connections
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Minimum Distance Between Node Connections
www.ti.com
Figure 7. Multiple Dominant-Bit Negative Voltage Reduction
Figure 7 exemplifies the possible arbitration bit-error problem since the waveform voltage due to the
negative reflection reduces the differential voltage magnitude of the signal below the 900-mV dominant-bit
threshold. The dominant threshold may or may not be reached for the last 1 µs of the bit, and can possibly
affect the sync seg of the bit. If the signaling rate is increased to 500 kbps, this reflection lasts for 50% of
the 2-µs waveform. Note that is a simple bus structure and any long drop lines with additional nodes
added to the bus increase the severity of the problem.
The lumped load capacitance, CL of each CAN transceiver, board trace, and Berg connector amounts to
approximately 20 pF per node in this example, and the distributed capacitance per unit length, C, is about
40 pF per meter. The calculations presented in Figure 2 for a CL of 20 pF and a C of 40 pF indicate that a
half-meter of cable added between each of the 10 clumped nodes in place of the 5-inch cable corrects the
problem.
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Critical Spacing of CAN Bus Connections
5
Conclusion
www.ti.com
Figure 8. Half-Meter Bus Length Correction
2
Conclusion
Clearly, the calculations prove to be correct. The reflected wave has almost completely disappeared and
the half-meter of twisted-pair cable rolls up neatly out of the way next to each node.
3
References
1. High-Speed Digital Design : A Handbook of Black Magic, Dr. Howard W. Johnson, ISBN
0-13-395724-1
2. Using CAN Arbitration for Electrical Layer Testing application report ( SLLA123)
3. ISO-11898-2:2003 CAN Specification
6
Critical Spacing of CAN Bus Connections
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
Appendix A
www.ti.com
Appendix A Mathematical Development of the Node Capacitance and Node Spacing Trade-off
The characteristic transmission line impedance that can be seen into any cut point in an unloaded CAN
bus is defined by Z = √L/C, where L is the inductance per unit length and C is the capacitance per unit
length. As capacitance is added to the bus, in the form of devices and their interconnection, the bus
impedance is lowered to Z'. When the bus impedance is lowered on a section of the bus, an impedance
mismatch occurs between unloaded and loaded sections of the bus.
d
t=0
S1
120 W
VS
ZO = 120 W
Load
Load
Load
Load
120 W
Figure A-1. CAN Bus Schematic Diagram
A worst-case occurrence is during a dominant-to-recessive transition during arbitration or an ACK bit.
When S1 switches at time zero from a dominant state to a recessive steady state, the CAN driver
differential output voltage, VS, can move from a 3-V signal to a 0-V recessive state. As this signal wave
propagates down the line and arrives at the loaded section of the bus, the mismatch in impedance reflects
voltage back towards the source.
As the input signal wave arrives at this mismatch, an attenuation (or amplification) of the signal occurs.
The signal voltage at an impedance mismatch is VL1 = VL0 + VJ1 + VR1 , where VL0 is the initial differential
voltage, VJ1 is the input signal differential voltage transition, and VR1 is the reflected differential voltage.
CL
d>
0.98C and is the coefficient of
The voltage reflected back from the mismatch is VR1 = pL × VJ1 where,
reflection commonly used in transmission line analysis. The voltage equation can now be written as
VL1 = VL0 + VJ1 +pL × VJ1.
Assuming the bus is terminated at both ends with the nominal media impedance, a CAN driver creates a
high-to-low differential voltage change from the standard maximum of 3 V to 0 V, or a VJ1 of -3 V. The
signal voltage at the load, VL1, must go below the receiver recessive bit input voltage threshold of 0.5 V. In
equation form,
0.5 > 3 + ( - 3) + ρL ×( - 3)
ρL >
0.5
= - 0.167
-3
(A-1)
Now, solving for Z',
Z ¢ - Z0
ρL =
> - 0.167
Z ¢ + Z0
Z ¢ - Z0 > - 0.167(Z ¢ + Z 0 )
Z ¢ (1 + 0.167) >Z0 (1 - 0.167)
Z ¢ > 0.71Z 0
(A-2)
If the loaded bus impedance is no less than 0.71Z0, the minimum threshold level must be achieved on the
incident wave under all allowed cases.
What bus configuration rules must be used to keep the loaded bus impedance above 0.71Z0?
In the derivation of the minimum loaded-bus impedance, treat the addition of devices and their
capacitance in a distributed model. As such, the loaded-bus impedance can be approximated by
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
7
Appendix A
www.ti.com
Z' = √L/(C +C'), where C' is the added capacitance per unit length. If the distributed inductance and
capacitance of the media are known, Z' can be calculated directly. Unfortunately, these are not commonly
specified by manufacturers. They generally do specify the characteristic impedance Z0 and the
capacitance per unit length, C. With these, one can solve for L, from the relationship Z0 = √L/C, as L =
Z02C. Substituting into the equation for Z' and simplifying,
Z 02 C
Z¢ =
= Z0
(C+C ¢)
C
C+C'
(A-3)
C' is the distributed device capacitance, CL, divided by the distance, d, between devices or C' = CL/d.
Substituting this into the equation and solving for d,
Z¢ = Z0
æ Z' ö
ç Z ÷
è
0ø
C æç
è
Z0
d=
C
C+ CL
d
2
ö
Z' ÷ø
C
=
C+
2
= C+
CL
d
CL
d
CL
2
ææZ
ö
ö
C ç ç 0 Z' ÷ - 1÷
çè
÷
ø
è
ø
(A-4)
Now, substituting the minimum Z' of 0.71Z0 gives,
CL
d>
2
æ Z
ö - 1÷ö
C ç çæ 0
÷
çè
÷
0.71Z 0 ø
è
ø meters (if C is pF/m) or feet (if C is pF/ft)
d>
8
CL
0.98C
Critical Spacing of CAN Bus Connections
(A-5)
SLLA279A – November 2008 – Revised January 2009
Submit Documentation Feedback
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products
Amplifiers
Data Converters
DLP® Products
DSP
Clocks and Timers
Interface
Logic
Power Mgmt
Microcontrollers
RFID
RF/IF and ZigBee® Solutions
amplifier.ti.com
dataconverter.ti.com
www.dlp.com
dsp.ti.com
www.ti.com/clocks
interface.ti.com
logic.ti.com
power.ti.com
microcontroller.ti.com
www.ti-rfid.com
www.ti.com/lprf
Applications
Audio
Automotive
Broadband
Digital Control
Medical
Military
Optical Networking
Security
Telephony
Video & Imaging
Wireless
www.ti.com/audio
www.ti.com/automotive
www.ti.com/broadband
www.ti.com/digitalcontrol
www.ti.com/medical
www.ti.com/military
www.ti.com/opticalnetwork
www.ti.com/security
www.ti.com/telephony
www.ti.com/video
www.ti.com/wireless
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2009, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising