DeviceNet? Not familiar with it?
Well, let’s take a journey through the automation world of DeviceNet
DeviceNet is an application-level protocol
used in the automation environment
It is a communication tool that allows you to logically talk between a PLC
(Programmable Logic Controller)
and many control devices, such as motors,
conveyors, flowmeters, level sensors, etc.
Instead of the PLC talking directly to discrete I/O modules,
it talks via a DeviceNet scanner.
It was originally developed by Allen-Bradley
which is a Rockwell Automation brand,
and they decided to share this new technology with others
and make it an open network.
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DeviceNet is now managed
by Open DeviceNet Vendors Association
or ODVA
an organization that develops standards
and allows third party vendors to utilize the network protocol
DeviceNet follows the Open Systems Interconnection
or OSI model
that uses seven layers
which are physical, data link
network, transport
session, presentation, and application
It's based on the Common Industrial Protocol
or CIP
and uses the three upper layers of CIP
starting at Session,
while the bottom 4 layers
have been adapted to the DeviceNet application
The physical layer
consists of a combination of cables,
nodes, taps and termination resistors
in a trunkline-dropline topology
We will discuss
these physical components in just a bit
For the data link layer,
DeviceNet uses the Controller Area Network
or CAN standard
that handles all the messaging
between controllers and devices
The network and transport layers of DeviceNet
establish a connection with the device
by using connection IDs for the nodes,
consisting of the MAC ID of a device
and a Message ID
The valid range of node addresses
is 0 to 63
giving you a total of 64 possible connections
The advantage of the connection ID
is that it enables DeviceNet
to identify duplicate addresses
by looking at the MAC ID
and signaling to the user that it needs to be fixed
Now that we have looked at the structure
of the DeviceNet application layers,
let’s look at the part of DeviceNet
that we can put our hands on
The genius behind DeviceNet
is that it joined the power
and the signal into one cable,
saving money and
reducing the need for multiple cables
which utilize more space
There are 5 types of cable
with three that are round,
and two that are flat
They are thick Round,
thin Round,
Class 1 Round,
KwikLink Flat
and KwikLink Lite Flat
Which one you choose is determined
by the distances and
physical limitations of your application
You can use the Round (thick or thin) cable
for either trunk or droplines,
the flat is used for trunk lines
and the Class 1 Drop cable is used for drops
They all use twisted pairs of wire
One pair for the 24V DC power
and one pair for the signal
There is also a shield wire
used in the grounding process
The cable that you
choose to fit your application
will be based mainly on distance
because there are specific limits to the lengths
that will allow maximum data rates
These distances are measured by two variables
trunkline distance, and total dropline length
DeviceNet data rates are 125, 250,
or 500 Kilobits per second
The longer the length needed
will result in a slower data rate
and visa versa
Thick Round cable has a range of
maximum length of 1,640 feet
at 125 Kilobits per second
to a maximum length of 328 feet
at 500 Kilobits per second
“Thin” Round cable has the same maximum length of 328 feet
for all three data rates of 125,
250 and 500 Kilobits per second.
The “KwikLink” Flat cable
ranges from 1378 feet at 125 Kilobits per second
to 246 feet at 500 Kilobits per second.
The “KwikLink Lite” Flat cable ranges from 1148 feet
at 125 Kilobits per second to 180 feet at 500 Kilobits per second.
Since there are no predetermined cord lengths
you can put connections to the Flat cable
anywhere on the line, making this choice great for device placement.
The trunkline requires a 121 Ohms, 1 per cent,
0.25 Watts or larger terminating resistor at each end of the trunk
and directly connected across the signal wires (blue and white).
The terminating resistors reduce electrical noise and without them,
in their correct place, the DeviceNet will not work properly.
The drop lines connect the devices to the trunkline
and the data rate you choose determines the total drop line length allowed.
The biggest restriction for a dropline
is that the maximum cable distance from any device to the trunkline is 20 feet.
The maximum total drop length for each data rate
is 512 feet at 125 Kilobits per second,
256 feet at 250 Kilobits per second
and 128 feet at 500 Kilobits per second.
Once again showing that, at higher data rates
you get shorter distances your network can reach,
and lower data rates get you farther reaching capabilities.
You can connect the trunkline to the devices
using multiple types of taps and connectors.
Devices can be attached to these taps and connectors
either directly to the trunkline
or by branching or by daisy chaining them together.
This choice will impact the total drop length calculation
because the direct connections are considered zero drops
while branching or daisy chaining adds to that calculation.
These connections were specifically designed
to allow devices to be replaced without disrupting the network.
Now that we have the physical network constructed,
let’s talk about the software side
that is an interface to configure our DeviceNet network.
Allen Bradley developed RSNetworx for DeviceNet
to be able to map and assign addresses to all the devices in the network.
The RSNetworx for DeviceNet uses either a graphical
or spreadsheet layout to map out the network
and then the configuration is set up in all the devices
and then downloaded to the DeviceNet scanner.
The DeviceNet scanner is a hardware piece that resides in the PLC chassis
and talks to the PLC through the backplane of the chassis.
DeviceNet uses Electronic Data Sheets (EDS)
that are simple text files that contain all the information
to identify a device and assist in commissioning them onto the network.
The advantages of DeviceNet are low cost,
widespread acceptance, high reliability,
efficient use of network bandwidth and power available on the network.
The disadvantages of DeviceNet are limited bandwidth,
limited message size, and maximum cable length.
It is stated over and over in many documents
that 90 per cent to 95 per cent of all DeviceNet problems are one of two things;
a cabling issue or you don’t have the correct EDS file registered in RSNetworx for DeviceNet.
This makes DeviceNet a great network when looking at the overall picture of that.
Do you think it could work for your application? Maybe so.
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