This document provides an introduction to PLC programming and ladder logic. It discusses the most common programming languages for PLCs, with ladder logic being the dominant method as it was developed to mimic relay logic. Ladder logic uses graphic symbols of rungs and contacts to represent circuit diagrams. The document also briefly outlines other programming methods for PLCs such as sequential function charts, structured text, and function block diagrams.

1. Control System Training
PLC Programming Introduction

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Agenda
• PLC programming languages
• Ladder logic
• Ladder diagram programming
• PLC programming

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PLC PROGRAMMING
There are 5 programming languages available, but the commonly used
language is ladder diagram for PLC.

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LADDER LOGIC
Ladder logic is the main programming method used for PLC's. Ladder logic has
been developed to mimic relay logic. By selecting ladder logic as the main
programming method, the amount of retraining needed for engineers and
trades people was greatly reduced.
The first PLC was programmed with a technique that was based on relay logic
wiring schematics. This eliminated the need to teach the electricians,
technicians and engineers how to program - so this programming method has
stuck and it is the most common technique for programming in today's PLC.
INSTRUCTION LIST
There are other methods to program PLCs. One of the earliest techniques
involved mnemonic instructions. These instructions can be derived directly
from the ladder logic diagrams and entered into the PLC through a simple
programming terminal.

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PLC PROGRAMMING
SEQUENTIAL FUNCTION CHARTS (SFC)
SFC have been developed to accommodate the programming of more
advanced systems. These are similar to flowcharts, but much more powerful.
This method is much different from flowcharts because it does not have to
follow a single path through the flowchart.
STRUCTURED TEXT (ST)
Programming has been developed as a more modern programming language. It
is quite similar to languages such as BASIC and Pascal.
Structured Text (ST) is a high level textual language that is a Pascal like
language. It is very flexible and intuitive for writing control algorithms.
FUNCTION BLOCK DIAGRAM (FBD)
FBD is another graphical programming language. The main concept is the data
flow that start from inputs and passes in block(s) and generate the output.

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LADDER DIAGRAM PROGRAMMING
Introduction to Ladder Logic
Ladder logic uses graphic symbols similar to relay schematic circuit
diagrams.
Ladder diagram consists of two vertical lines representing the power rails.
Circuits are connected as horizontal lines between these two verticals.

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PLC PROGRAMMING
• Ladder diagram elements
• Power flows from left rail to right rail.
• The Inputs are placed on the left side of the rung and represents the
beginning of the circuit.
• The Outputs are placed on the right side of the rung and can not be
connected directly with the left rail.
• Contact can not be placed on the right of output.
• Each rung has to contain one output at least.
• The output address can be used only once in the program.
• The inputs can appear in more than one rung of a ladder.
The inputs and outputs are all identified by their addresses, the notation
used depends on the PLC manufacturer.

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PLC PROGRAMMING

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PLC PROGRAMMING

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LADDER DIAGRAM PROGRAMMING

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Topics to be covered in the next session:
• PLC Programming Questions

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Thank you!