DeviceNet Overview
Device Bus Network
Conventional I/O System vs. DeviceNet Network
DeviceNet Overview
DeviceNet is feature-rich, yet cost effective.
DeviceNet is a CAN based Layer 7 protocol originally developed by Allen-Bradley. Operation of the
DeviceNet is based on an object-oriented communications model.
DeviceNet is maintained by the Open DeviceNet Vendor Association
(ODVA).
DeviceNet is designed to connect simple devices from multiple vendors that comply with the DeviceNet
network standards. DeviceNet device profile standards provide interchangeability between device
manufacturers.
Each DeviceNet segment can connect up to 64 devices. It is a four-wire system delivering 8 amps at
24VDC, sufficient for field devices such as solenoid valves. The four wires carry signal and power
typically on a single cable. Multiple power supplies can be used for redundancy and additional power
requirements.
DeviceNet uses a trunk (bus) line with drop cables connecting devices. The trunkline requires 121
ohm terminating resistors at each end of the trunk.
DeviceNet supports Master/Slave, Peer-to-Peer, and Multi-Master network models. Data can be
transferred on a cyclic or change of state basis using a Producer/Consumer paradigm that conserves
network bandwidth. DeviceNet is very commonly used for communications from host systems to
motor control centers and variable speed drives.
DeviceNet Network Highlights
Type of Network
Device Bus
Physical Media
Two Shielded twisted pairs in one
shielded thick, thin or flat cable
(one pair for signal, one pair for power)
Primary usage
Motor Control Centers, Variable Speed Drives, Remote I/O applications
Network Topology
Bus with drops
Power and Communications on same cable
24VDC power on power bus (multiple supplies may be used for additional
power or as backup). A separate 24VDC power supply for communication
bus is recommended.
Maximum Devices
62 devices per segment
Device Power Supply
CONVENTIONAL I/O SYSTEM
Advantages
- Technology is already understood
- Lower device cost
- Independent wiring from devices to the control system means
wiring problems with one device don’t affect other field devices
Drawbacks
- Higher installed cost
- Point-to-point wiring is expensive
- Many wiring connections:
- are labor intensive to install
- create many points of failure
- increase complexity when troubleshooting
- require large amounts of cabinet or rack space for installation
of terminal blocks
- create time-consuming initial checkout and startup
- Expansion requires duplicating the entire wiring scheme
for each additional point
DeviceNet is most commonly used when
device populations are primarily discrete
but have some analog, and when motor
control centers and variable frequency
drives are present.
DEVICENET NETWORK
Advantages
- Excellent support for motor control centers, variable frequency drives,
and conventional I/O
- Moderate device cost adder
- Lower installed cost
- I/O modules allow for conventional analog and discrete device
integration
- Relatively fast transmission speeds:
- Transmission Speed and cable lengths:
- 125kb @ 420m
- 250kb @ 200m
- 500kb @ 100m
- Power and Signal on same cable
- Up to 64 addressable nodes
- Wide variety of topologies available, including Trunk, Line, Drop
- Duplicate node address detection
- Supports some device diagnostics
Drawbacks
- Slaves can only be owned by one master
- Does not support Intrinsically Safe installations
24VDC on power bus
TopWorx Comments on DeviceNet
Maximum Distance
(using Thick cable)
Maximum Distance with repeaters 6,000 meters
125Kbps
- 500m (1640 ft)
- 6m (20 ft) individual drop cable length
- 156m (512 ft) cumulative drop cable length
250Kbps
500K bps
- 250m (820 ft)
- 6m (20 ft) individual drop cable length
- 78m (256 ft) cumulative drop cable length
- 100m (328 ft)
- 6m (20 ft) individual drop cable length
- 39m (128 ft) cumulative drop cable length
* Thin cable may be used as trunk. Maximum distance is 100 meters,
regardless of data rate
Communication Methods
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Master/slave, multiple master, peer-to-peer,
change of state or cyclic
(uses Producer/Consumer Paradigm)
Wiring Types
Thick Cable
Thin Cable
Mid Cable
(ODVA Type II cable), generally used
for trunk cable
(ODVA Type I cable), commonly used
for drop cables
(ODVA Type III cable), used when more
flexible drop cable is needed
Blue/White conductors for communications
Red/Black conductors for power
Grounding aspects
Ground only the power supply closest to the middle of the network
Strengths
DeviceNet is capable. DeviceNet delivers a solid combination of
cost-effective simplicity with a bit of added functionality. It is designed
to handle discrete devices but can support analog signals and some
diagnostics as well.
DeviceNet is robust. DeviceNet supplies 8 amps of power, offers
acceptable cable run lengths, and can control up to 64 devices
per segment.
Limitations
Hazardous Areas
Since DeviceNet is an 8 amp bus, it cannot be intrinsically safe.
TopWorx has created a variety of solutions for installing DeviceNet in
Class I, Div 1 (Zone 1) and Class I, Div 2 (Zone 2) hazardous
environments.
Cost When Simplicity is Needed
If customers have only discrete devices and need no added
functionality, then some other protocols are less expensive.
When to Use DeviceNet
Generally speaking, TopWorx recommends DeviceNet when:
- device populations are primarily discrete and secondarily analog
- end users desire some diagnostic capability for predictive
environments
- plants are not intrinsically safe
Terminators
121 ohm terminator at each trunk line end
Web Site
www.odva.org
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