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Discover the inner workings of PLC logic
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and scan cycles and unlock a deeper
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understanding of industrial automation.
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We delve into the world of programmable
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logic controllers, exploring the
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intricacies of scan cycles and how they
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Learn how PLC's process inputs, execute
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programs, and update outputs in a
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continuous cycle, and gain insights into
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the programming and configuration of
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Whether you're an experienced engineer
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or just starting out in the field, this
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video will provide a comprehensive
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overview of PLC logic in scan cycles,
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helping you to improve your skills and
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knowledge in industrial automation. When
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a PLC scans a rung with multiple series
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contacts, it stops evaluating the rung
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as soon as it finds a false condition.
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This means that if you have a series of
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five contacts and the first one is
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already false, the PLC will not even
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check the other four. Therefore, by
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placing the contacts that are most
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likely to be false at the beginning of
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the rung, you can save valuable micros
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secondsonds of scan time on every single
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scan, which adds up to a surprising
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amount of efficiency over time. At the
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heart of PLC programming is the
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execution of the PLC logic. The running
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of the PLC controller is also known as
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the scan. Scan and scan time are
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essential to understand an automation.
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Scan cycles run cyclically. This means
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that they repeat continuously at a very
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high speed which allows for real-time
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control of your automation project.
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There are many common misconceptions
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about what happens during PLC scan
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cycles. The PLC sees all inputs and
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updates all outputs simultaneously.
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Scan time is always constant or all
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inputs and outputs are updated in the
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same order as they appear in the
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program. PLC scan cycles are critical to
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understand for responsiveness and
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timing, troubleshooting, and program
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optimization. The concept of a scan
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cycle is central to how PLC logic
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operates. A PLC doesn't process its
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logic all at once. It does so in a
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continuous loop or scan. Each scan cycle
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consists of three primary steps.
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One, read inputs. The PLC checks the
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status of all its input devices. It
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first reads all inputs and stores their
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status in an input image table. If an
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input changes during this time, the
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input image table will not update until
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the next scan of the input.
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Two, execute logic. It runs the
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userdefined program from the beginning
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to the end using the input image table
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data it just gathered. If we look at
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ladder logic, the logic is solved left
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to right, top to bottom. The output
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image table is updated as the logic is
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solved. These changes are available for
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the next rung of ladder logic to use. As
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mentioned, when solving the logic, if
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any of the series inputs result in being
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negative, then the output is immediately
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updated in the output image table. This
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execution creates the fastest possible
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time to update and so the scan is not
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Three, update outputs. It writes the new
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status to all the output devices based
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on the output image table results of the
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This entire process repeats continuously
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and at very high speeds which allows for
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real-time control of the automation
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system. The PLC reads all inputs before
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executing the program logic and then
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updates all outputs after the logic is
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complete. The order of instructions
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within the program itself can affect the
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internal logic, but the physical IO is
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handled in a set sequence during each
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scan. The PLC scan cycle is a sequential
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process. It first reads all inputs and
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stores their status in an input image
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table. It then executes the program
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logic using the data from this table,
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not the live inputs. Finally, it updates
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the physical outputs based on the
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results of the program execution. This
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means there is a slight delay between a
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change in a physical input and the
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corresponding change in a physical
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Understanding this concept of PLC scan
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is critical for you to enhance system
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responsiveness, increase reliability and
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predictability and improve
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troubleshooting of your system.
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Let me know your thoughts and comments
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below. There are many different PLC
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manufacturers with their own hardware
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and software. All programmable logic
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controllers share similar basic
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features. To learn how to approach
4:29
learning about basic PLC's, click here.
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Click here to learn more about how PLC
4:36
simulators can be used to help your
