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PLC, logic, programming, discrete, control, process control, motion control, Ethernet
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A classic problem with automation products is the fact that they are exceedingly com-
plex stemming from the inclusion of so many features, and the diversity of applications
they are expected to control. The Rockwell Automation Logix family is a good indica-
tion of the way that automation suppliers are trying to balance
7KH /RJL[ (QJLQH LV WKH XQGHUOLQJ complexity with usability. Rockwell Automation’s Logix Engine is
ILUPZDUH XVHG IRU 5RFNZHOO the underlying software/firmware/hardware architecture used for
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ControlLogix, FlexLogix, CompactLogix, DriveLogix, and SoftLogix
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controllers, and combines a feature-rich product offering with a fa-
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miliar look-and-feel.
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Product complexity is a double-edged sword. On one hand, suppliers need to include a
maximum amount of features in order to meet the expanding requirements of today’s
automation users. Offering too many features, however, makes a product look very
complex. This was the problem faced by the Rockwell Automation Logix engine, the
common software/firmware/hardware architecture underlying their ControlLogix,
FlexLogix, CompactLogix, DriveLogix and SoftLogix controller products. On the sur-
face, each of these products appears to be a controller programmed by either relay
ladder logic or function block diagramming. The details are revealed by the program-
ming package, RSLogix 5000. On the face of it, RSLogix 5000 is a PLC programming
package, but further inspection reveals the existence of features and capabilities not
found in similar packages. Rockwell Automation chose to keep this programming tool
friendly and familiar to PLC users while enabling all of the power of object based pro-
gramming “under the hood”.
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Logix is a common architecture and software package underlying all of the newest con-
trollers from Rockwell Automation. The design of the Logix engine is such that it can
run in any of their newest controllers, be implemented on a variety of microprocessors,
and supports a variety of operating systems. User application logic implemented on any
Logix controller can be ported without change to any other Logix controller.
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While Relay Ladder Logic was the first programming language for Logix, an extensive
list of function blocks is available as well.. These same function blocks are used with
Function Block Diagram Programming (FBD), the other IEC 61131-3 standard language
that is also offered. Rockwell has gone one step further however, by including true
process and drive loop -specific function blocks in their version of FBD.
Included in the CIP (Control and Information Protocol) application layer common to
ControlNet, DeviceNet, and EtherNet/IP, is a broad set of function blocks designed for
common functions in discrete control. The CIP I/O function blocks come from the
Electronic Data Sheets (EDS) defined for DeviceNet which include common types of sig-
nal processing and functionality needed for discrete sensors and actuators. They
provide such functionality as contact debounce, time delay, counting, pulse width detec-
tion, pulse width modulation, and many similar I/O functions, which take time and
effort to program, and may sometimes be forgotten. EDSs are defined in full object nota-
tion and while they may be called “function blocks”, they are actually true automation
objects. The Logix engine also provides many other function blocks for control and
computation.
Programming for Logix, even with Relay Ladder Logic, is unlike programming on most
other systems since much of the logic is performed inside these function blocks. Ladders
become easier to read and debug – even at the electrician level. FBD programming with
the Logix function blocks makes the logic even clearer.
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One of the new features of RSLogix 5000 is the ability to assign each I/O point, variable,
or control relay with a tag name that is used to refer to that point in all logic elements.
Tags can be assigned an alias in order to assign it to physical I/O devices. The tag name
substitutes completely for the specific I/O address and makes programs independent of
I/O assignment or location. The tag name is assigned at the time
/RJL[ )HDWXUH 7UDGLWLRQDO 3/ the I/O point, control relay, or other variable is defined, and re-
7DJ QDPHV +DUGZDUH DGGUHVVHV mains not in the RSLogix programming software, but is assigned
3URJUDPPLQJ /DGGHU ORJLF RU VWHS to the appropriate controller where the logic is executed. This is a
EDVHG RQ IXQFWLRQ ORJLF completely new idea for Rockwell Automation products. When
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I/O points or other variables are accessed from the Logix engine,
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3. they can be accessed either by tag name or as the traditional bit in
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a register for high efficiency. For most applications this form of
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tag name implements a “single tag” requirement allowing HMI
/RJL[ 'LIIHUHQFHV software to access data values without depending on the export of
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the tag structure from the programming software. Previously, the single tag concept was
only available between HMI suppliers and the controls supplier by cooperative agree-
ment, but now the single tag is available to all HMI suppliers directly from Logix.
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Logix devices support the form of data communications called producer/consumer,
which eliminates management of data across a communications link. Tagged I/O points
or other variables can send values (producer) using a multicast message on a regular
time schedule or when the value changes and with periodic updates when no change
occurs. Consumers listen to the produced data and do not need to poll for new values.
Producer/consumer protocol is optimized for discrete automation, while other suppli-
ers, but not Rockwell Automation, use a similar publish/subscribe
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protocol for the more data-intensive process control environment.
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Producer/Consumer always produces, while publish/subscribe
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HOLPLQDWHV PDQDJHPHQW RI GDWD only transmits when there are active subscribers. Both methods al-
DFURVV D FRPPXQLFDWLRQV OLQN low data access openly across the network from controller to
controller.
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Since the common architecture of all members of the Logix family is the same, pro-
gramming for process control uses the same tools as for discrete control. Application of
the Logix engine to batch process control can be accomplished using the programming
form most natural to the type of control required. Discrete interlocking is easily pro-
grammed with Relay Ladder Logic while continuous feedback loop control is most easily
programmed with FBD. Process control function blocks are also in the Logix object li-
brary to handle the types of scalar data and computations needed for regulatory
feedback and feedforward loop control. Promised for the beginning of 2002 is the avail-
ability of Sequential Function Chart (SFC) programming to allow the structured
multitasking form of programming needed for batch process control.
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Rockwell Automation’s Logix family is a good indication of the way controllers are
changing along the path toward a future object-base. Do not look at it as a dressed-up
PLC with a lot of features, but as the beginning of a new wave of controllers. This is an
excellent beginning embodying these elements of the controller of the future:
• Tag-based variables
• Support of multiple IEC 61131-3 programming languages portable across several
sizes of controllers
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• Integration of discrete control with process, motion and drives control
• Supports many communications architectures without requiring software changes.
For further information, contact your account manager or the author at rcaro@arcweb.com.
Recommended circulation: All MAS clients.
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