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DeviceNet products - 3 / 68 Pages

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DeviceNet™
DeviceNet supports data rates of 125k, 250k and 500k baud. All nodes on the network must be communicating at the same specified rate.
Data rates are governed by the length of the bus line as shown in the following table:
The cycle time of a DeviceNet network is affected by several factors including:
• The amount of data being produced and consumed by each of the nodes
• The number of nodes being scanned
• Network communications rate
• The type of messaging being used (change of state, polled, strobe, cyclic)
• Explicit messaging going on
• The scan time of the PLC/PC control program
The DeviceNet protocol supports 64 nodes which can be addressed from 0 to 63. Software node commissioning can be done as well as through hardware
via dip or rotary switches. When these switches are used the device must be switched to the programming mode (usually by setting a dip switch setting)
such that the switch dictates the address. EDS (Electronic Data Sheets) are specifically formatted ASCII files which contain detailed device information.
Information found in the EDS includes information on how to configure a device, I/O characteristics, device description and the device configurable
parameters among other things.
Duplicate MAC IDs or node addresses are not allowed in DeviceNet networks. If the same address is set on multiple nodes one node will take control of
the address and the others will go into a “failure” status.
COMMUNICATION RATE
ADDRESSING
TRANSMISSION SPEED BUSLINE SINGLE DROP LINE DROP LINE NODES
(BAUD RATE) (MAX. LENGTH) (MAX. LENGTH) (OVERALL LENGTH) (MAX. NUMBER)
125K BAUD 1640 FEET (500 METERS) 20 FEET (6 METERS) 512 FEET (156 METERS) 64
250K BAUD 820 FEET (250 METERS) 20 FEET (6 METERS) 256 FEET (78 METERS) 64
500K BAUD 328 FEET (100 METERS) 20 FEET (6 METERS) 128 FEET (39 METERS) 64
MAXIMUM RATINGS (THICK CABLE AS TRUNK, THIN CABLE AS DROP)
DEVICENET – SYSTEM OVERVIEW
DeviceNet is designed to be a low-cost, real-time, device level bus architecture
that connects sensors and actuators. Wiring and installation of automation
devices is greatly simplified while the diagnostic information about the
various I/O of the system is greatly enhanced (the sensor is ON, the lens is dirty,
it has not been cleaned for 90 days). It defines a standard device software
model that permits multi-vendor interoperability and interchangeability of
like devices.
DeviceNet supports a variety of control architectures, giving the control
designer the flexibility to choose either:
• Highly distributed control (with “smart” devices and no identifiable
central controller)
• Semi-distributed control (with multiple PLCs, PC-based controllers and/or
smart controllers), or
• Centralized control (single PC or PLC-based control)
DeviceNet is a CAN (Controller Area Network) based communication
protocol that transmits a maximum of 8 bytes as a single message. Its primary
usage is with digital I/O and non time-critical analog control. DeviceNet
supports on-line node insertion and removal.
Broadcast Messaging
DeviceNet provides a good selection of communication mechanisms that
fulfill a variety of needs. They may be individually implemented or used all
together in the same system. DeviceNet uses a CSMA bus arbitration
method where the highest priority message is always assured of being
transmitted. Only the dominant node will finish the transmission and win
the arbitration.
Strobe/Poll mechanisms – In this type of communication one message
containing output data is sent by the master to each slave unit; each slave
unit transmits a polled response. This is ideally suited to control systems
where many I/O points change rapidly. However this makes inefficient use
of the network bandwidth due to the frequent need to check for changed
inputs.
Cyclic mechanism – The master and slave transmit unsolicited cyclic
messages at a configured rate. This requires a greater effort to configure
but generally provides better system performance while using bandwidth
more efficiently. This requires knowing the minimum update rate for the
individual I/O devices.
Change of state – This technique is used when the message is sent
unsolicited at a given rate when the data changes. While this maximizes
bandwidth, much more effort is required in the system design and testing.
Greater system knowledge is required in this communication mechanism.
Explicit messaging – This can be set up for devices where limits, boundaries
and thresholds need to be reported back immediately (but required
very infrequently). These are only triggered when the control logic triggers
them and therefore have negligible bandwidth impact.
For product & technical information visit www.woodhead.com/eCatalog

DeviceNet products
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    14Discrete I/O ModulesDeviceNet™• 8 inputs• 4 inputs / 4 outputsCompact 30mm I/O ModulesBradControl™ IP67 Compact 30mm I/O modules...
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