Need a solution to talk on your industrial network?
Let’s look at ControlNet.
ControlNet is an open industrial network protocol
and is managed by formerly “Open DeviceNet Vendors Association” or ODVA.
ControlNet is based on a token-passing bus control network
and we will talk more about how this part works as we move along.
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ControlNet utilizes the Common Industrial Protocol or CIP
for the upper layers of the Open Systems Interconnection or OSI model
that has seven layers:
physical, data link, network, transport, session, presentation, and application.
The designers of ControlNet adapted the CIP layers of transport,
network, data link and physical to fit the needs of the ControlNet network.
ControlNet was designed to provide reliable, high-speed control
and I/O data transfer
by using programming that sets the logic to specific timing over the network.
It also provides for critical messaging
that does not rely on timing to be executed
without interfering with the transport of control and I/O data.
The types of devices that the ControlNet network talks to
are Programmable Logic Controllers, I/O chassis,
HMIs, personal computers, drives, and robots.
It is typically used for redundant applications
and/or applications that work best with scheduled communications.
ControlNet has a data transfer rate
of 5 Mbps and is in the mid-range of the three popular networks
with EtherNet/IP ranging from 10 Mbps to 1 Gbps
and DeviceNet ranging from 125 to 500 Kbps.
Now let’s move on to the physical media of ControlNet.
ControlNet cables are RG-6 coaxial cable types with BNC connectors.
BNC connectors can either be a twist-on and locking type
or a more rugged design
that is a screw threaded type for more harsh environments.
You can choose cable types such as “flexible”, “direct burial”,
or “plenum” rated based on the application
and environment that your network will be used in.
There are some applications for ControlNet
that require longer distances and may use optical fiber in the cabling.
The maximum cable distance for ControlNet cables
is 1000 meters unless a repeater is used.
There can be a maximum of 5 repeaters (10 for redundant networks).
ControlNet can support a maximum number of 99 nodes on the network
and the repeaters do not use node numbers,
therefore they don’t count as a part of this maximum number.
When you are designing the network,
the topology options for ControlNet are trunkline-dropline, star or tree.
It is time to continue with the other physical media
of the ControlNet protocol such as Taps,
terminating resistors, segments, Repeaters, and Bridges.
Taps are devices that allow you to connect nodes to the trunk via a drop cable.
There are several types of taps, “T” or “Y”,
straight and right angle that lets you have flexibility
in configuring your ControlNet network.
A 75-ohms Terminating resistor is required at each end of the trunk cable.
Segments are collections of trunk cable
and taps that have terminating resistors at each end.
The length of each segment is dependent on how many devices are attached to the trunk.
Repeaters can be used to lengthen the ControlNet networks
by connecting them in between the segments.
Bridge devices are communication connections between networks
that only forward packets of network information but not any actual I/O.
Now that you have seen the basics of ControlNet,
let’s have a look to the other layers and get into the specifics of how it works.
As I stated earlier, it is based on a “token-passing” bus control network
which implements a logical “token ring” through a coaxial cable.
In ControlNet, the nodes are assigned a MAC ID address
from 1 to 99 and each node also knows the address of its predecessor and successor.
Each node has a scheduled time which is determined by a “token”.
While the node has possession of the token,
it transmits data frames until it reaches the end of its transfer
or the token reaches its time limit.
Then a new token is created
and is passed on to the next logical successor.
There may be nodes that do not have any data to transfer
and, in this case, it just passes the token onto the next logical successor.
The process is then repeated until all the nodes have cycled through the logical circle
and then it begins again.
In ControlNet, only one node can transmit at a time
which will prevent any collisions of data from happening.
If a node stops transmitting and does not pass the token,
the ControlNet protocol has provisions written into it
that will regenerate the token and send it to the next logical successor.
The data link layer is where the timing for ControlNet is done
through a NUT or Network Update Time
that is based on a fixed and repetitive time cycle.
The NUT can be set to a duration from 2 to 100 milliseconds
and have 3 major parts;
“scheduled”, “unscheduled” and “guard band”.
In the network and transport layers,
ControlNet has two forms of messaging;
“unconnected” and “connected”.
Unconnected messaging is used when establishing connections
or for low priority messages that do not happen frequently.
Connected messaging uses settings in each node
such as frequent explicit messages or real-time I/O data.
ControlNet also uses RPI or requested packet intervals
to determine how often you want I/O data to be transmitted during the scheduled NUT.
To sum it up, the NUT tells you how often
the entire logical ring or network is updated
and the RPI tells you how often the devices
ask to transmit on the ControlNet network.
Some of the advantages of a ControlNet network are high speed,
it can be strictly scheduled and have deterministic data transfer,
built-in support for fully redundant cables and very stable and reliable.
The disadvantages are the cost of hardware can be higher
and troubleshooting can be more difficult than other protocols.
In my experience, if ControlNet is setup properly
it can be a very stable and reliable and requires very little maintenance.
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