ROCKWELL DeviceNet, DeviceNet Troubleshooting
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DeviceNet will not function correctly if design rules are not followed.
Even a Network previously thought to be functioning correctly may begin
to exhibit abnormal or anomalous operation due to incorrect system design.
The following are tips to help locate and correct these abnormalities.
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For specific DeviceNet system installation information.
Please refer the "DeviceNet Planning and Installation Manual".
Manuals can be ordered via the Internet at http://www.theautomationbookstore.com/
Verify that all devices on the network have been certified by ODVA and
carry the DeviceNet Conformance check on their name plate.
Termination Resistors:
A termination resistor equal to 121 Ohms 1% , 1/4W must be attached at each end of the
Trunk cable. The resistors must be connected across the Blue & White wires of the
DeviceNet cable.
Resistor connection can be verified by disconnecting DeviceNet power and measuring
the resistance across the Can_H & Can_L lines (Blue & White Wire). This can be
measured with an Ohm meter. The reading should be approximately 60 Ohms.
Very Important: The DeviceNet network will not operate correctly without terminating
resistors. Termination resistors can be ordered from your local Allen-Bradley Distributor
using the part number 1485A-C2.
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Network Grounding:
The DeviceNet cable must by grounded at only one location. This should be done closest
to the center of the network. Connect the network Shield and Drain wire to an earth
ground using #8 AWG wire up to a maximum 3m ( 10Ft ) in length.Also connect the Vconductor ( Black Wire) of the network trunk cable and the DC ground of the power
supply to this ground connection.
Power Supply Discussion:
DeviceNet requires 24VDC. Use a power supply rated 24VDC (+/- 1% ). Make sure the
power supply has it's own current limit protection. Provide fuse protection for each
segment of the cable system. DeviceNet requires a power supply to have a rise time of
less then 250mS to within 5% of its rated output voltage. The power supply must be sized
correctly to provide each device with its required power.
1. The thin wire trunk line can only handle 3 amps and the thick wire trunk line can
physically handle 8 amps. However, in North America the current is limited to 4 amps.
Multiple power supplies can be installed on a DeviceNet network, but no section of cable
should have more current flowing than the appropriate rating. An important note is that
when putting multiple power supplies on a network; break the Red V+ wire between the
power supplies. This effectively isolates the power supplies from each other.
2. Common mode voltage can sometimes be an issue on DeviceNet networks which have
trunk lines that are extra long and/or have devices on them, drawing large currents at
longer distances. If the voltage on the Black V- wire ever gets more than 4.65 volts
different from one point of the network to another, then communication problems could
occur. On an existing network, if the voltage between the Red V+ wire and the Black Vwire ever gets below 15 volts, then common mode voltage could be adversely affecting
network communications. Adding an additional power supply or moving an existing
power supply closer to the heavier current loads will normally cure common mode
voltage problems.
Important:
the DeviceNet network only. When multiple power supplies are required, verify that the
V+ connection is broken between the supplies. Please refer to the " DeviceNet Planning
and Installation Manual " for more specific installation procedures.
It is recommended that the DeviceNet power supply should be used to power
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Verify Network Voltages
It needs to be understood that DeviceNet is actually a three wire Differential Voltage
network communication is accomplished by switching the CAN-H (White wire) and
CAN-L (Blue wire) signals relative to the V- line ( Black Wire ). The CAN-H swings
between 2.5 VDC (Recessive State) and 4.0 VDC (Dominant State) while the CAN-L
swings between 1.5 VDC (Dominant State) and 2.5 VDC (Recessive State)
Without a network master connected to the DeviceNet, the CAN-H and CAN-L lines
should read between 2.5 VDC and 3.0 VDC relative to V- and the voltages should be
identical. (Recessive State). Measure these voltages right at the SDN scanner. Use a
voltmeter in DC mode.
With a network master connected and polling the network,
will be around +3.2 VDC. The CAN-L to V- voltage will be around 2.4 VDC. The
reason these values appear a little different than the ranges shown on the scope trace, is
that the signals are switching, which slightly affects the DC value being read by the
VOM.
If Can-H to V- and Can_L to V- are too low; less than 2.5 V dc and 2.0 V dc respectively
the issue is probably a bad transceiver or bad wiring. To find a bad transceiver remove
one node at a time measuring Can-H and Can-L to V- each time a device is removed.
To check a transceiver (rough test) with everything removed from a device use an ohm
meter to measure resistance between V+ and Can-H V+ and Can-L V- and Can-H V- and
Can-L. These impedances should all be greater than 1 M ohm.
the CAN-H to V- voltage
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Examine the System Design of the Installation
Walk the network if possible to determine the actual layout. ( Make a sketch of the network )
Check number of nodes
Check cumulative drop length
Check individual drop lengths
Check branched drop length
Check total trunk length, including long drop near the ends
Check the termination location and measure the terminators
Check the power supply cable length and gauge
Check for one, and only one, earth ground of the V- and shield
Break the combination shield/V- connection to frame ground and verify >1.0 Mohm
to frame ground
Check for one and only one V- to shield connection
Break the shield/V- connection at the power supply and verify >1.0 Mohm shield to
V- with 24VDC off
Check for shorts of CAN- and/or CAN+ to shield or V- verify with an OHM meter
Check the length and gauge of the earth ground connection
Check total power load and its distribution points
Check the Power
Check trunk and drop current limits
Check type (size and length) of cable bringing power into the trunk
Measure the 24V supply at the middle and ends of the network
Consider spot checking the power for noise with an oscilloscope
It is recommended but not required that the 24 VDC Network Power for DeviceNet
should be used for the DeviceNet network only to aid in eliminating noise issues.
Check the Wiring
Check lead dress in junction boxes
Check that connectors are screwed together tightly
Check that glands are screwed tightly
Check for foreign material (electrical tape, RTV, etc.) in glands
Check that nodes are not touching extremely hot or cold surfaces
Check that cables are kept a few inches away from power wiring
Check that cables are not draped on electric motors, relays, contactors or solenoids
Check that cables are not constrained so as to place excessive tension on connectors
Wiggle connectors to provoke intermittent failures
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Check the Scanner Configuration
Verify the scanlist
Baud Rate
Node Address
Series/Revision of the 1747/1771-SDN scanner
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Refer to Knowbase for documents on factory defaulting DeviceNet Scanners
Check the Nodes
Cycle power to the 24V supply, this will reset the scanner to initialize the network
Examine the scanner display codes to identify problem nodes.
( Reference the SDN manual for a description of these codes )
At problem nodes
Blinking GREEN means the node is not being allocated by the scanner
Check that the node is in the scan list
Check that the scanner is not bus off
Check if connection is timing out
Blinking RED means no communication
Check for missing power on all nodes
Check if all other nodes are disconnected
Check node baud rate ( Bad baud rate does not always cause buss off )
Check scanner, if a code 91 is displayed then the communications connection with the node
has timed out.
Recycle 24V supply and then reset scanner.
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