Three Hidden Demons in your Network
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Three Hidden Demons in your Network To get the full benefit from this paper, please read “DeviceNet Physical Layer, an Insider’s View” first. Demons … What are demons anyway … most people don't want to believe they exist, and those that believe in them fear them. They fear them, not because they understand the danger they may or may not represent, but because their lack of knowledge paralyzes them with the fear that they may have to deal with one some day. Let's take a deep breath and boldly name the demons lurking in your network: Bus Errors, Marginal Media, and the granddaddy of all demons, Noise (or electrical interference). The demons are lurking in your network, and the only way they can harm you is if you ignore them and simply hope they don’t bother you. The goal of this paper is to arm you with the information you need to exorcise the demons and assure long-term reliable operation of your DeviceNet networks. This paper addresses the network physical layer only and avoids upper layer topics such as messaging details and object models. Bus Errors Bus errors are the ultimate manifestation of a physical layer problem. It is quite common for a network to experience some bus errors and continue to operate correctly, which means that there are a significant number of networks with borderline physical layer problems that remain undiagnosed. There are a number of different causes for bus errors and, while some error sources cannot be completely avoided, it is reasonable to expect the vast majority of networks to operate error free. You can’t check for bus errors by looking at the indicator lights on the scanner or slave devices, you have to use a diagnostic tool. Any non-zero level of bus errors deserves investigation. The absence of bus errors is not an indication that your network is healthy. CAN Error Detection The CAN specification requires all nodes to check all messages for errors, regardless of the intended recipient. If any node detects a problem with a transmission it transmits an error frame, which corrupts the remainder of the original message and causes all other nodes (including the transmitting node) to detect the error also. The implications of this scheme are significant. One faulty node can cause errors during other node’s transmissions, even if the faulty node is not actively communicating. A localized error source (such as a cable problem) can cause errors in the transmissions of all nodes in the network. Analyzing Error Rates Examining individual node error rates can be a powerful diagnostic tool, but failing to properly analyze the results can lead to misdirected efforts. The rate of errors on a single node has extremely limited value by itself. The real diagnostic value is in comparing the ratio of errors to successful messages (Error Rate / Message Rate) of all nodes in the network. A localized problem will usually show up as a disproportionate error rate on the affected node(s). Caution: if the affected node(s) are not actively communicating then errors will only show up on the nodes that are actively communicating! To contact us: www.Woodhead.com 1 All trademarks contained herein are the property of their respective owners. © 2005 Woodhead Industries, Inc.
Three Hidden Demons in your Network
Marginal Media
Media problems can range from invisible, causing no bus errors, to severe where communication is not
possible at all. Marginal media conditions often result in no bus errors at all, but can grow in severity
and ultimately cause serious trouble. The connectors and interface circuitry inside each device can also
be considered part of the media and should not be overlooked when tracking down the source of
improper signals. Common media installation problems such as short and opens almost always affect
operation to the point where they are noticed. Other problems are not so obvious.
Problem Symptoms How to diagnose
Corroded Zero or more bus errors, Invalid CAN signal or differential voltages, with or
Connections possibly intermittent. without the presence of errors, are an indication of poor
Deteriorating or invalid connections, which can be caused by a number of
CAN signal voltages. factors, including connector corrosion.
Corroded connections become intermittent and
ultimately fail completely. Corrosion almost always
decays signal quality before errors occur.
Early detection can be achieved by recording a
“baseline” of dominant and recessive voltages for each
signal and differential voltages at installation time and
periodically checking for deterioration.
Loose or Zero or more bus errors, Invalid CAN signal or differential voltages, with or
Improperly possibly intermittent. without the presence of errors, are an indication of poor
Installed Deteriorating or invalid connections, which can be caused by a number of
Connectors CAN signal voltages. factors, including loose or improperly installed
connectors.
Loose connections can be intermittent or simply poor
electrical connections. Intermittent connections
operating perfectly one minute than completely failing
the next, and often do not result in improper signal
voltages. Poor connections cause various levels of
signal distortion, which may or may not consistently
cause bus errors. Relying solely on error rates or signal
measurements is not adequate to detect both variations
of loose connections.
If errors are present, analyzing the relative error rate of
all nodes on the network can help isolate a section of
cable or connectors to be checked. Suspect connectors
can be checked by applying mechanical force to the
suspect connector while monitoring the network error
rate and/or signal voltages.
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Problem Symptoms How to diagnose
Damaged Zero or more bus errors. Cable damage can occur as a result of stress
Cable (stretching and bending) encountered during
installation, and also occurs as a result of accidental or
unavoidable damage after installation. Some systems
that must flex the cable (i.e. robot arms & other moving
equipment) cause unavoidable stress damage to
cables over time.
Accidental damage typically causes immediate failure,
and while this is annoying it is unavoidable and does
not cause long-term reliability problems as the problem
is detected and repaired relatively quickly.
Stress damage results in the same types of symptoms
as poor connections and corrosion. Cable that has
been stressed no longer has the same impedance or
shielding characteristics, which affects signal distortion,
noise and signal attenuation. When cable suffers
sufficient damage to cause communication problems,
the dominant/recessive signal and differential levels will
change, and bus errors become more likely. Verifying
all system voltages after installation is a good way to
check for installation stress damage.
Early detection of unavoidable stress damage (i.e. in
flex applications) can be achieved by recording a
“baseline” of dominant/recessive and signal voltages at
installation time and periodically checking for
deterioration.
Excessive Intermittent or continuous If the dominant and recessive signal and differential
Cable Length bus errors. voltages, and common mode voltage are all valid, but
bus errors still occur, excessive cable length is a
possibility.
If reducing the network length by disconnecting a
section of cable (and/or a few nodes at the end of the
network) eliminates the errors then a thorough check of
cable length is warranted (don’t forget to add a
terminator if you disconnect part of the trunk)
Missing/Excess Zero or more bus errors. No termination is indicated by a non-zero recessive
Terminators Possibly invalid CAN voltage, but this symptom could also be an indication of
signal voltages. an open in one of the signal wires.
Excess or missing Excessive termination causes a low dominant
terminators are not differential voltage, but the same symptom can also
guaranteed to cause bus indicate an open or short in a signal wire. It often takes
errors. Incorrect more than one extra terminator to cause the dominant
termination is more likely differential to fall outside the acceptable range.
to cause problems at Excessive or missing termination can cause signal
higher baud rates. distortion, which is manifested as bus errors. Bus errors
caused by incorrect termination typically affect all
nodes equally (relative to their message traffic).
Checking termination is an important step in tracking
down the source of unexplained bus errors.
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All trademarks contained herein are the property of their respective owners. © 2005 Woodhead Industries, Inc.Three Hidden Demons in your Network
Noise
Noise is the demon that strikes fear in the hearts of many. Noise is poorly understood, partly due to the
complex nature of the physics involved, and partly due to the fact that systems installation guidelines are
designed to eliminate the need for users to deal with noise issues.
Discussions about noise range from those that are too technical for the average person to make use of,
to vague theoretical discussions that have no practical application. It is my intent to walk a fine line
providing sufficient detail to convey a general understanding of the nature of noise, while limiting
detailed information to that required for practical applications.
Noise Sources
Noise is caused by external influence. Three common methods of influence are electromagnetic,
magnetic and electrical. All three involve an alternating or transient electrical signal in one cable or
piece of equipment causing a similar signal in the network cable. The level of noise signal induced in the
network cable is affected by the strength of the noise source, the proximity of the network cable to the
noise source and cable characteristics.
EMI
Electromagnetic interference (EMI) occurs when a cable or device intentionally or unintentionally emits
electromagnetic radiation (e.g. radio waves). Intentional emitters include cell phones, personal
communicators, radio modems and wireless networks – basically anything that uses radio waves to
perform a function. Unintentional radiators include virtually any equipment that contains a
microprocessor, digital electronics or uses high frequency or high-energy signals in its operation. A
great deal of industrial systems incorporate high-energy electrical equipment. Even though the electrical
supply itself is not high frequency (60Hz AC or DC), the way it is used often results in unintentional
radiation of EMI (for example, arc welders, electric furnaces, drives & servos, contactors etc.).
Regulations exist for both intentional and unintentional radiators limiting the output levels to specific
levels in various frequency bands. All intentional and unintentional radiators are tested to ensure
compliance with appropriate levels. Equipment intended for residential use have tighter emission
requirements, commercial and industrial requirements are looser. Certain types of industrial equipment
that simply cannot be designed with low emissions have very lax emission requirements. Special care
should be taken to protect network cabling from known high emission sources.
Noise induced in a network cable as a result of EMI is usually quite high in frequency (100’s of kHz to
GHz).
Magnetic (Inductive) Coupling
Magnetic interference occurs when the alternating magnetic field around a cable or piece of equipment
induces an alternating current in a network cable. The effect is the same that occurs intentionally in a
transformer; an alternating current in one conductor causes an alternating magnetic field, which in turn,
induces a proportional alternating current in a second conductor. Magnetic coupling occurs at
frequencies from Hz to GHz, and is limited by distance – the network cable has to be located quite close,
and parallel to, the magnetic field to be significantly affected (i.e. two cables in the same wire tray).
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All trademarks contained herein are the property of their respective owners. © 2005 Woodhead Industries, Inc.Three Hidden Demons in your Network Capacitive Coupling Any two electrical conductors in close proximity form a capacitor. The electric fields related to the positive and negative charges in the two conductors interact and can permit high frequency electrical signals to pass between the conductors just as if they were electrically connected. The impedance (effective resistance) of this capacitive connection is related to the frequency of the signal, the total area of the two conductors that is in proximity and the distance between the conductors. Two cables that run in the same wire tray for a distance have a relatively large area in close proximity to each other and consequently have relatively high capacitive coupling. Noticeable noise induced in a network cable via capacitive coupling can cover a wide frequency range from kHz to GHz and usually resembles the original noise source. Examining the frequency and nature of the noise can often provide clues to the source of the interference. Electrostatic Discharge Electrostatic Discharge (ESD) is not a method of external influence; rather it is a signal source that can induce noise in a network cable by one of the three methods already described. ESD occurs more often than you might think. Any time two electrically conductive items are insulated from each other it is likely that they will have a difference in voltage. When the two items come into contact or close proximity a current flows between them until the voltage difference is neutralized. Even though the two parts involved in a discharge have a DC voltage difference, the discharge pulse itself is not purely DC and contains a broad spectrum of frequencies that can range from kHz to MHz. In some cases, when the difference in voltage is sufficient, the current flow can jump gaps and form a spark. Many discharges are not detectable by humans as the voltage difference is insufficient to cause a spark or be felt. Any equipment can build up electrical differences due to friction with the air or moving parts. In cases where non-conductive moving parts exist (especially static generating materials such as acrylic) static build-up and frequent discharge is likely. A low-level discharge (when there is no detectable spark) directly into a network cable can cause a transient of several volts. Depending on the nature of the discharge the result can be differential and/or common mode noise with the same effect on communications as noise from other sources. High-level discharges (usually with a detectable spark) directly into the network cable can cause serious network problems and even permanent damage to network components depending on the voltages involved. High-level discharges (usually with a detectable spark) result in very large current pulses. These discharge pulses are a noise source like any other and can induce noise in the network cable via electromagnetic and capacitive coupling depending on the strength of the discharge and proximity to the network cable. Noise induced as a result of ESD is usually random in nature and often takes the form of a relatively large, short duration, noise signal. To contact us: www.Woodhead.com 5 All trademarks contained herein are the property of their respective owners. © 2005 Woodhead Industries, Inc.
Three Hidden Demons in your Network
Susceptibility Testing
Standards exist for testing the susceptibility of industrial control equipment to EMI and other types of
interference, including electrostatic discharge. Each test standard has a number of levels, or classes, of
equipment operation: A) Equipment tolerates test conditions with no change in operation B) Equipment
fails under test conditions, but automatically recovers C) Equipment fails under test conditions and
requires manual intervention (i.e. reset) to resume normal operation and D) Equipment in non-functional
after exposure to test conditions. The product manufacturer determines which level of operation is
appropriate. The presence of a compliance mark (CE, for example) on a product is no guarantee that the
product will operate fault-free in a factory environment. Only examination of the test standards the
product was tested to will tell you what the product can be expected to tolerate.
Noise & DeviceNet
Noise is a difficult thing to deal with and, in the case of DeviceNet, there are two different types of noise
effects to consider: differential and common mode. To understand the difference you need to know how
data is transmitted on DeviceNet (review “DeviceNet Physical Layer, an Insider’s View”, Common
Mode Voltage and Grounding & Shielding).
DeviceNet uses differential transmission; data is transmitted as opposite signals on two wires. The
receiver subtracts the signal on one wire from the other to extract the data signal. This technique has
several advantages, including good noise immunity.
Common Mode Noise
When the two signal conductors are twisted together (or placed in close proximity as in flat media) they
each tend to pick up the same noise signal relative to ground. This is called common mode noise.
A differential receiver cancels common mode noise when it subtracts one signal from the other like this:
Received Signal = (SIGNAL_A + Common Mode Noise) – (SIGNAL_B + Common Mode Noise)
= SIGNAL_A – SIGNAL_B + Common Mode Noise – Common Mode Noise
= SIGNAL_A – SIGNAL_B (Common Mode Noise is cancelled)
Now for the bad news: The receiver cancels common mode noise, but only if both signal voltages are
within the range the receiver is designed to handle (common mode range). If the total of the nominal
signal voltage range, DC common mode voltage and common mode noise exceed the receiver’s
common mode range, it may incorrectly interpret the bus level resulting in bit errors.
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Differential Noise
Depending on the proximity and type of noise source, as well as characteristics of the cable, there is a
certain amount of difference in the noise signal picked up by the wires relative to each other. This is
called differential noise.
Since differential noise looks just like the data signal (as a voltage difference between the data wires),
the receiver cannot cancel it:
Received Signal = SIG_A – SIG_B + Differential Noise
Differential noise can cause bit errors if has sufficient magnitude to change the differential signal
voltage from Dominant to Recessive or vice-versa.
How Much Common Mode Noise is OK?
Since common mode noise is cancelled by the receiver, it has no negative effect until the total of the
nominal signal voltage range, DC common mode voltage and common mode noise exceed the receiver’s
common mode range. With signal voltages outside the receiver’s common mode range, it is not
guaranteed to properly decode differential signals correctly.
In the case of DeviceNet, there is no specified common mode noise limit, but by making a few
inferences we can arrive at a rule of thumb.
The nominal signal range for DeviceNet is 0.5V to 4.5V (ISO11898, CAN dominant signal levels)
The maximum DC common mode voltage is 5V (DeviceNet specification)
Transceivers must work with a normal signal range from –5V to 10V (DeviceNet specification)
Few commercially available CAN transceivers meet this requirement!
Commercially available CAN transceivers (that meet DeviceNet’s signal requirements) have a
common mode range of –7V to 12V
By adding the signal range (0.5V to 4.5V) to the maximum DC common mode voltage (5V) we arrive at
the nominal signal voltage range including common mode voltage effects (-4.5V to 9.5V). Since many
devices (but not all) include a shottky diode in the DC common connection we should add a typical 0.5V
positive offset to the maximum voltage. The result is a nominal signal range of –4.5V to 10V.
By comparing the nominal signal range (-4.5V to 10V) and the transceiver common mode range (-7V to
12V) we can determine the maximum p-p common mode signal that can be tolerated without violating
the receiver common mode range (4Vp-p, assuming a symmetrical noise signal).
Due to the nature of noise, it is extremely difficult to capture and measure the maximum amplitude. It is
wise to assume that the peak noise level is somewhat higher than measured. For this reason 3Vp-p max
is a safe practical limit for common mode noise.
Differences in the amount of DC common mode voltage in a system (managed by end-user), or the
transceivers common mode range (controlled by product developer) affect the total common mode noise
that can be tolerated before bit errors become likely.
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How Much Differential Noise is OK?
Since differential noise looks just like the data signal (as a voltage difference between the data wires),
the receiver cannot cancel it. Differential noise can cause bit errors if has sufficient magnitude to change
the differential signal voltage from Dominant to Recessive or vice-versa.
The receiver dominant differential threshold is 0.9V (ISO11898, ECU dominant signal levels)
The minimum dominant differential on the bus is 1.2V (ISO11898, bus dominant signal levels)
The receiver recessive differential threshold is 0.5V (ISO11898, ECU recessive signal levels)
The maximum recessive differential on the bus is 12mV (ISO11898, bus recessive signal levels)
By subtracting the dominant receiver threshold (0.9V) from the bus minimum dominant differential
(1.2V) we determine than a 0.3V margin exists. Therefore, differential noise in excess of 0.6Vp-p
(assuming a symmetrical noise signal) can cause incorrect interpretation of a dominant bus state.
By subtracting the bus maximum recessive differential (12mV) from the recessive receiver threshold
(0.5V) we determine than a 0.498V margin exists. Therefore, differential noise in excess of 0.996Vp-p
(assuming a symmetrical noise signal) can cause incorrect interpretation of a recessive bus state.
The different noise margins for dominant and recessive bus states recognizes the fact that there is a
lower load impedance between CANH and CANL when a node is transmitting a dominant bit (the
driving transceiver acts as an additional bus load as far as the differential noise is concerned) and
consequently lower induced noise for a given noise source.
Measuring Common Mode Noise
An oscilloscope can be used to check noise levels, but be careful: Improper connection of the scope can
result in high noise readings that don’t reflect the actual noise in the system.
Use a scope with a minimum of 100MHz bandwidth
Connect two scope probes to the two CAN signals (Channel A – white, Channel B – blue)
Connect the ground lead of each probe to DC common with as short a ground lead as possible (1-2
inches). Grounding both probes is essential, as is using the same type of probe and length of
ground on both probes. Ground leads longer than 3 inches are likely to affect the accuracy of noise
measurements.
It is difficult to directly measure the noise signal, as the data signal is also present. The data signal
frequency on DeviceNet is ½ of the baud rate or less (62.5kHz, 125kHz or 250kHz), typical noise signals
have a much wider frequency range. If the noise signal is greater than 3Vp-p (or a single spike is more
than1.5V over the normal signal level) bit errors are more likely to occur on networks with maximum
DC common mode voltage. Networks with lower DC CMV have proportionally higher common mode
noise margins.
It is often easier to set the scope trigger at 11V and -6V (just inside the maximum signal range for
typical CAN chips) and check for any spikes over those levels. Since the scope is referenced to DC
common, it is necessary to perform this test at each end of the network, and at the power supply in order
to check the full network common range of the network. If you see unfavorable voltages, there is
probably either a DC common mode voltage or common mode noise problem. In either case the network
cannot be relied upon for long-term error-free operation.
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The easiest option is to use a tool that measures the common mode voltage of the network, including
common mode noise. This type of tool can check the total common mode voltage (DC common mode
voltage and common mode noise) of the system by connecting at only one place in the network, saving
eliminating the time-consuming effort of testing in multiple locations.
Measuring Differential Noise
Differential noise can be measured using the same scope connections as common mode noise. Configure
the scope to display the difference between the two signals (Channel A – Channel B, some scopes
perform this directly, others require you to invert one channel then display the sum of the two).
The difference between the two signals is the differential voltage, which includes the data signal and
differential noise. The data signal has the same frequency characteristics described above, but has an
amplitude equal to the sum of the individual signal amplitude. You can expect some data bits to have a
much higher amplitude during the start of a frame and at the very last dominant bit of a message, this is
caused by multiple nodes transmitting dominant bits simultaneously.
The key indicators to look for are: A) a recessive state where the differential noise takes the differential
signal over 0.5V and B) a dominant state where the differential noise forces the differential signal below
0.9V. If you see either of these conditions, there is sufficient differential noise to cause bit errors – you
may not actually see bit errors at this point, but the conditions are ripe for problems. Ideally there should
be significantly lower levels (very close to zero) of differential noise on a network.
The easiest way to check differential noise levels is to use a tool that independently measures the
dominant and recessive differential signal levels and records minimum and maximum levels. By
comparing “normal” signal levels with minimum and maximum levels, the level of noise can be
determined.
Reducing Noise
Noise control should be considered before a system is installed rather than assuming it can be ignored
until a problem exists. The following rules can be used to design minimum-noise systems, or to reduce
noise in systems that have not been installed with low noise in mind.
Follow recommended shielding & grounding practices for each network, do not mix shielded and
unshielded network segments
Use only recommended cable types and avoid substitutions of cables that “seem” to be similar.
Simply comparing the number of conductors and wire gauge is not enough; even the insulation
and shield construction has an effect. This is especially important to minimize differential noise.
Keep maximum distance between network cables & other cables in a system, use separate wire
trays and conduit if possible.
Avoid placing network cables near high current cables where the power is turned on and off
rapidly (i.e. motor drives & servos) or the current flow changes rapidly (i.e. welders). Independent
wire trays with separation distances measured in feet is a wise choice.
Where network cables must cross other cables, cross them at 90º to minimize capacitive, and
inductive coupling. A cable that crosses other than at 90º is, in effect, parallel for a short distance
and subject to higher induced noise.
Shielded cable is effective in reducing common mode noise levels when properly connected and
grounded. Improper grounding or failing to connect the shield of all cable segments can eliminate
the benefit of the shield, and in some cases can actually increase noise levels in the cable.
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Improving Common Mode Noise Tolerance
DeviceNet’s common mode noise limit is a function of the level of DC common mode voltage (DC
CMV) in the power system (voltage drop in the network common conductor). The sum of DC common
mode voltage and common mode noise must be less than ±6.5V (±7V is the actual transceiver limit).
With the maximum 5V DC CMV only ±1.5V (3Vp-p) remains for common mode noise before bit errors
can be expected. This is sufficient for most systems, but in some cases high levels of common mode
noise are unavoidable.
Every volt reduction in DC CMV results in a ±1V (2Vp-p) increase in the common mode noise budget.
In systems where unavoidable common mode noise exists, and all noise reduction steps have been taken,
the common mode noise tolerance can be increased by moving the power supply, or adding another
power supply to reduce the system DC common mode voltage.
Real example:
A robot system consists of a control panel, robot arm with numerous servomotors, and a
DeviceNet network connecting the controller to end-of-arm tooling I/O devices.
The network cable and servo power wires must all pass through the inside of the arm; maintaining
distance between the cables in the arm is not possible.
From the base of the robot to the control panel, the network and power cables follow different
paths to minimize noise coupling.
Tests indicate that 3.3Vp-p of common mode noise is induced in the network cable when the
servos are energized. Since noise measurements are often affected by the test equipment itself it is
prudent to assume that the actual noise under all conditions is somewhat higher than measured.
This level of noise is getting close to the point where it may cause communication errors in
systems with 5V DC CMV.
Further testing determines that the DC common mode voltage of the system is 0.3V, confirming
the predicted CMV from the design phase.
Since the DC CMV is 4.5V lower than the maximum permitted, the acceptable noise margin for
this system can be increased by the same amount from 3Vp-p to 7.5Vp-p.
The measured noise (3.3Vp-p) is much less than the calculated noise margin (7.5Vp-p), which
indicates that reliable operation is assured. The reliability of this determination is entirely
dependent on accurately measuring the maximum noise levels.
To confirm the calculations, a tool that measures system common mode voltage is connected to
the robot for a period of time. The total common mode voltage (including DC and noise) is
measured at 3.65V confirming the calculated results.
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