Tips for a successful ControlNet system
Users might want to know why they need to worry about ControlNet systems with the emergence of industrial Ethernet networks, which are becoming more common when installing a new network architecture. The reason is the majority of ControlLogix installations, prior to a few years ago, have all used ControlNet as the remote input/output (I/O) and peer-to-peer communication. Intrinsically safe and redundant media are other reasons that ControlNet were selected for control systems in the past and are still selected today. With ControlNet systems, there are two very important acronyms that users need to know when installing them:
- Network update time (NUT) is the repeatable base time at which the network can transmit data. It is typically set at 5.00 ms by default.
- Requested packet interval (RPI) value, which is determined by how often the user wants particular pieces of I/O or scheduled peer-to-peer data to be transmitted within the scheduled bandwidth of the network update interval.
The NUT is how often the entire network is updated and the RPI is how often each device requests to communicate on the ControlNet network.
When installing or modifying an existing ControlNet system, consider these tips to make the process easier.
First, always make the RPI a multiple of the NUT.
For example, if the user had a variable frequency drive (VFD) on the network and the user wanted it to communicate at 200 ms intervals that would be a multiple NUT which is 5 ms. Also, when there is a module that requires a connection that resides in a remote rack that has a daughter ControlNet bridge card, make sure to set the RPI for that card at a multiple of the daughter ControlNet bridge’s RPI. Typically, digital I/O is scheduled at the fastest RPI of the parent ControlNet bridge, but the analog I/O which is a connection in itself needs to be set at a realistic RPI that correlates with the update interval of the process. See the illustration below for a typical network layout.
Also, be sure to check the physical media when dealing with an existing network, especially if there are intermittent errors. Correct installation of the physical media is very important to reliability of that network.
For example, there was an existing network that was having intermittent failures, which caused partial plant shutdowns. When the connectors for the ControlNet media were examined, the users were able to pull the cable right out of the connector. Once it came apart, it was easy to see that the cable was improperly stripped to begin with.
Note the difference in the image below and see how the cable was prepared for installation. After the ControlNet media connectors were correctly installed, no more errors occurred on that network.
These are just a few tips, and there are undoubtedly more. Please share additional thoughts and experiences below.
This post was written by John Boyd. John is a technology leader at Maverick Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. Maverick delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization, and more.
Maverick Technologies is a CSIA member as of 8/16/2016.