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PLC Programming Concepts Not Well Understood, Part 1
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PLC, Programma Logic Controller, programming
is a critical aspect of industrial automation
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yet many individuals struggle to grasp its fundamental concepts. This guide will
dive into the key principles of PLC programming that are often misunderstood
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shedding light on these concepts and providing
clarity for beginners and experienced professionals
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By addressing common misconceptions and exploring
the core principles of PLC programming, we
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will aim to demystify this essential aspect of industrial control systems.
Whether you are new to PLC programming or are looking to refine your understanding
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this will serve as a valuable resource to
deepen your knowledge and proficiency in this
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essential field. Sometimes I wish I had
this knowledge before my first PLC program
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Part 1 will discuss the PLC scan cycle and ladder logic.
These are some common areas where people often struggle
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Let's get started. Detailed information contained in
this video can be found at accautomation.ca, a link has
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been put in the description below. If you have not watched the
other videos yet, there will be links in the description below
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that will start you with video 1. There will be
links to the rest of the videos in the series as well
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PLC Concepts 1 – Scan Cycle Understanding how the PLC
program scans will help us program and troubleshoot the PLC
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The most straightforward scan cycle
of a PLC consists of four steps
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Read inputs, execute programs, diagnostics and communication, and
update outputs. Programmable logic controllers' PLCs use a cyclic scan
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This means that the PLC is not waiting for
an input condition to change from one state
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to the other. It constantly looks at the condition
of the input and makes a copy of that in memory
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The time it takes to complete
one scan is called scan time
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Typical scan times range from 10 milliseconds to 10 microseconds.
This translates from 0.01 to 0.0001 seconds per PLC scan
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Once the PLC inputs are read, the
program or programs are executed
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The program will read the memory area for
the inputs and set the memory areas for the
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outputs as it executes your instructions. Programs can be written in
several different programming languages depending on the controller used
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Standardization such as IEC has set 5
official PLC languages for programming
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They are Structured Text ST, Function Block
Diagram FBD, Sequential Function Chart SFC
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Function List IL, and Ladder Diagram LD. When the scan of your program
is complete, the PLC will then handle the communication diagnostics
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Internal diagnostics will ensure that the PLC
continues to scan the logic with no errors
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are detected. These are system errors such
as memory integrity and execution timing
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Communication is for the external devices such
as programming software, HMI, human-machine
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interface, etc. The final step of
the PLC scan is to update the outputs
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Output internal memory is then copied to the
physical outputs and the scan cycle begins again
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Every PLC controller has this essential operation. However, some will have
additional instructions to help you with your application if the input
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signal you are looking for is faster
than the scan time of the cycle
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This can be handled with PLC instructions to
update the physical I.O. within the program
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or a separate processor operating input and
output settings independently from the main
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PLC scan. This is usually referred to as a high-speed counter. Understanding
the timing and order of operations is critical within the PLC controller
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This will help you understand and correct
unexpected behavior in time-sensitive applications
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PLC Concepts 2 – Ladder Logic Ladder Logic, LD,
is the most popular programming language for PLCs
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It was written to mimic the mechanical relays
in a panel that the program controller replaced
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It has two vertical rails and a series
of horizontal rungs between them
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This looks like a flowchart or ladder. Contacts
or inputs are on the left side of the rung
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If the conditions are true or on, this creates
a flow turning on the outputs to the right
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side of the rung. Controllers usually execute the
ladder logic program from left to right, top to bottom
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The output of one rung is available for the next rung. The common
misconception about ladder logic programming is the sequential evaluation of
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rungs and how specific conditions influence program
flow. We know that the scan cycle of the PLC is cyclic
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Inputs are scanned and put into an internal memory location.
This means that the PLC program can manipulate this input table
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The second rung of our sample program will
set the input X0 as the output based on the
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X1 input. When we use the X0 as an input condition
on rung 3, you will see that this is based on
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input X1. At the beginning of the ladder
logic execution, X0 will turn on Y0
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X1 input will now control the value
of bit X0 in memory, in rung 2
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Rung 3 will control Y1 based on the input X0. You can see that basic
ladder logic concepts seem simple at first, but understanding the
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program execution is essential for troubleshooting and programming in this language.
If any of these topics are unclear, please ask for a deeper explanation or examples
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We will break them down further. If you find specific PLC programming
concepts challenging, let me know and I can help clarify them
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To learn how to use the free Do-more Designer
programming software we used in this video
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click here. To learn more about the actual
PLC and its development, click here
