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Allen-Bradley ControlLogix

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Rockwell Automation's new ControlLogix architecture provides communications and system solutions.
Steve Zuponcic, Product Marketing Manager, Rockwell Automation.
Welcome to the future. Rockwell Automation's newest control platform - the Allen-Bradley ControlLogix architecture provides system level solutions aimed at integrating multiple control disciplines that have historically utilised separate hardware and software tools.
Today, co-ordinated drive control, motion control, process control and sequential control are each accomplished using disparate hardware and software products. The ControlLogix architecture, on the other hand, paves the road toward integration of these control disciplines in a seamless manner that gives the control designer a common software tool set and a common hardware platform. At first glance, the ControlLogix system may appear to be a newer version of the Allen-Bradley SLC product line. In actuality, this platform represents an entirely new high-end architecture that is designed to provide functionality beyond PLC-5 or Reliance AutoMax controllers in a more cost-effective way. 30625.gif (19776 bytes)

This article is intended to convey the core principles behind the ControlLogix architecture. Here, we will cover some of the basics behind this newest control system from Rockwell Automation including its fundamental producer-consumer communications foundation, its flexible memory architecture, its I/O capabilities and, finally, its performance characteristics.

Producer Consumer Model
One of the key features of the ControlLogix system is that communications functions are designed into every layer of the system starting with the backplane and on through to the I/O modules, the processors and, of course, the communications modules themselves. The ControlLogix backplane is a first cousin to the ControlNet network and utilises the same Producer Consumer Model that ControlNet does. Producer Consumer is the name we give to a communication model whereby network nodes or modules on the chassis, each produce data. Other nodes or modules can then consume that data on an as-needed basis without having to make discreet requests. This is vastly different from other models where, for example, a master must manage the communications task by asking each node or module if it has a message to send and arranging the transaction.


For processors, the use of the producer/consumer paradigm allows for the installation of multiple processors in the same chassis. In this way, the backplane itself acts as a high speed network which provides communications capabilities between all the modules on the backplane - as well as those that are extended out to other chassis' on ControlNet. For this reason, the capabilities, of the ControlLogix system are also extended well beyond those of the traditional programmable controller. Gateway functionality is a product of this communications architecture. Before the Logix5550 controller was released, the Ethemet, ControlNet and Data Highway Plus (DH+) modules, (along with chassis and power supplies,) were released to form the ControlLogix Gateway product.


The ControlLogix Gateway allows bridging and routing of messages between the various networks without the need for a PLC or other controller to be present in the system. This communications capability is possible because of the backplane's new model of acting as a network itself. Messages can now be routed from one 'link' to another (and in this model the backplane is another 'link' in the path) because of this very flexible and powerful communications model.


Multiprocessor support is also a spin off from the Producer/Consumer Model. Multiprocessing is supported in the chassis for any number of slot locations and for any combination of slots. Since the backplane acts like a high-speed network, each processor is like a node on that network; therefore, any number of processors can communicate to any other, regardless of the slot location of those processors. In this manner, a PLC and DCS processor for instance can co-exist in the same backplane or network, share the field I/0 and exchange data between each other. A software configuration tool, RSNetworx is used to assign processor ownership output to devices. A given processor operates on the premise that it seamlessly owns all input devices and user designated output devices on the network.
Finally, with the Producer-Consumer backplane, many of the boundaries and limitations inherent to older technologies disappear. Since each processor now acts like a node on a high-speed network, there is no single master or bus arbitrator for all communications and control on the backplane. For this reason, any failure of a single processor does not necessarily commit the control system to shutdown. If a given processor is not controlling a mission critical function, the rest of the control may indeed stay running while the processor in question is removed and replaced under power.

Memory
Controller memory is another area where the ControlLogix system offers considerable flexibility, utilising technologies that outpace other controllers in its class. In adding memory to the Logix5550 controller, there are no fixed areas of memory allocated for specific types of data or for I/O. There is no artificial limit on the number of timers, counters or instructions. Memory is contiguous within any given controller and is utilised 'from top to bottom' as the user develops the application. In the ControlLogix chassis, memory is added specifically to each Logix5550 controller and tags are passed between processors like I/O within the system. This allocation of memory on a processor by processor basis is one of the reasons why processors can 'live' in any slot location in a chassis and with any number of processors in a chassis.

I/O points
As stated earlier, the ControlLogix system does not impose artificial boundaries on the number of I/O points that a control system can have. When I/O points are configured or when application code is created, memory is used contiguously. Therefore one of the limiting elements for the number of I/O points in the system is, of course, memory. If more memory is needed, the user can add more memory via a memory expansion card or add another processor to the chassis to increase the total number of I/O points in the system. A second factor that may bind the total number of I/O points for a given processor involves the concept of connections. The ControlLogix system uses a connection to establish a communication link between two devices. These devices may be controllers, communication modules, I/O modules, produced and consumed tags or messages.

Connections are quite different from individual I/O points. Each Logix5550 controller, for example, is capable of making 250 connections. In one configuration, a single connection may be made to an entire chassis. Considering the fact that 17 slot chassis are available, and assuming 32 point modules in our calculations, a single processor may actually support up to 128 000 digital I/O points; 4000 analog I/O points can be supported using similar calculations for analog I/O. Although these numbers we large in themselves they double as a second processor is added to a rack - and they triple with the third, etc. Although these numbers may seem extreme when viewed in this manner, the point remains that the ControlLogix architecture does not impose an artificial limit on the total number of I/O points that can be supported for any given application.

Performance
The ControlLogix system provides significant increases in performance over traditional control systems in a variety of ways. From a raw performance perspective, the Logix5550 controller outperforms other programmable controllers in its class, including the PLC-5 controller. In one specific example, a PLC-5/30 application that was converted to Logix5550 controller ran seven times faster on the Logix5550 controller than in the already fast PLC-5 controller. While this is an optimised case, the ControlLogix system will run three to four times the speed of a PLC-5 controller for typical ladder applications and around twice the speed for those process type applications that use more floating point math. More important than comparing instruction execution speed against instruction execution speed, however, is to understand that the ControlLogix system is optimised for performance in ways that are not even comparable to the typical programmable controller model. By supporting multiprocessing and multitasking, the ControlLogix system allows for efficient performance and utilisation of the processing power that is designed into the controller.

The Logix5550 controller supports a true, pre-emptive, priority structured, multitasking operating system. Simply put, this means that the user is capable of modularising his software design into functional components that make sense for the application. And in addition to this rnodularisation capability, these modules (called tasks) can be scheduled to ran at intervals that make sense for the time constant of the application. Furthermore, priorities are assigned to each task so that higher priority asks can pre-empt, lower priority tasks for access to the processor, when necessary. This operating model allows the user to utilise as much of the processing power of the system as necessary for individual functions-but not more than that. In this way, performance is optimised on an application level because those functions that need to execute very fast, can, while those that can afford to ran more slowly are not taxing the processor any more than necessary.

Multiprocessing is also an architectural benefit for system performance. When more processing power is needed, another processor can be easily added to the chassis with the software being distributed over these processors. In this way, system performance is achieved while maintaining a consistent software architecture. Finally, when considering system performance, the ControlNet network provides guaranteed performance not available in previous architectures. The ControlNet layer uses a system NUT (network update time) that is configurable by the user. During system configuration, scheduled and unscheduled data is set up by the user for each node on the network. During any NUT, the scheduled data is guaranteed to be sent out onto the network for other nodes to consume. In this manner, determinism is achieved giving the user guaranteed performance as defined by the network update time.

Rockwell Automation is the parent company for the Allen-Bradley, Sprecher + Schuh, Reliance Electric and Rockwell Software product brands and operates 11 offices in sub-Saharan Africa.
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