PLC Programming Languages

PLC
Programming Languages
Liju G Chacko

Liju G Chacko

PLC Programming Languages

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Table of contents
1

Introduction

2

PLC programming languages
Instruction List Programming
Structured Text Programming
Functional Block Programming
Ladder Logic Programming
Sequential Function Chart

Liju G Chacko


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Introduction

Introduction
PLC(Programmable logic controller) is a multiple
inputs and output digital computer used for
automation of electromechanical processes.
These are the processes normally a PLC will carry
out during one cycle.
Step 1-: Self diagnosis.
Step 2-: Read inputs and store values.
Step 3-: Process logics/instructions.
Step 4-: Write outputs
(Above four steps are repeated again and again)
PLCs are programmed using application a software on personal
computers. From computer, binary or pseudocode will be
downloaded to PLC through Ethernet, RS-232, RS-485 or
RS-422 cable.
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PLC Programming languages mentioned in IEC
61131-3
Sequential Function Charts
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Instruction List(IL) Programming
Allen Bradley supports IL programming only on the
Micrologix 1000,
IL is similar to mnemonic programming languages
IL make use of an important concept – STACK
IL instructions(Allen Bradley version)
LD A- Store A to stack
LD A- Put A back to memory
ORB - ’OR’ the last two variables in stack
ANB - ’AND’ the last two variables in stack

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Similar to high level language, such as Basic or C
ExampleAverage of 5 numbers
F8:10 := 0;
WHILE (N7:0 < 5) DO
F8:10 := F8:10 + F8:[N7:0];
N7:0 := N7:0 + 1;
END_WHILE;

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Structured Text(ST) Programming
Structures in ST Language
Structures

Explanation
IF-THEN-ELSIF-ELSE-END IF normal if-then structure
CASE-value:-ELSE-END CASE a case switching function
FOR-TO-BY-DO-END FOR
for-next loop
WHILE-DO-END WHILE
while loop
TAN(A), COS(A), A**B,,. . .
Basic math functions

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It is a graphical dataﬂow programming method

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It is a graphical programming language evolved from
electrical relay circuit

Each program statement is a line called the rung
with inputs to the left and the output to the
right.Execution starts from the top rung, down to
the last rung.
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Types of contact inputs

Examine if close

Examine if open

Positive transition sense

Negative transition sense

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Types of relay outputs

Output energized

Negative output energized

Output latch/set

Output unlatch/unset

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Function Blocks

On Delay Timer

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Oﬀ Delay Timer


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SEQUENTIAL FUNCTION CHART
Sequential Function Charts (SFCs) are a graphical
technique for writing concurrent control programs
SFC is based on GRAFCET (GRAphe Fonctionnel
de Commande Etapses/Transitions) [itself based on
binary petri nets]
Main components of SFC are:
Steps with associated actions
Transitions with associated logic conditions
Directed links between steps and transitions

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Sequencial Function Chart
SFC structure

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WORKING

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Macrostep(subroutines)

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Concurrent control ﬂow (two or more loops running in
parallel)

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Action Block

Actions can be written in Ladder Logic, Instruction language or
structured text language. Action block is not a part of a Step. Step
just activates or deactivates the action block depending on the
qualiﬁer, as long the step itself is active
eg;- Action 1

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Action Qualiﬁers
Non-store(N)
Action is executed as long as the step is active

Set(S)
The action starts execution as soon as the step is active,
continues till R qualiﬁer is executed .
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Action Qualifiers(Continue...)
Overriding(R)
The action started by S, SD, DS, and SL qualifiers are
aborted by R qualifier.

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Action Qualiﬁers(Continue...)
QUALIFIER
-NoneN
R
S
L
D
P
SD
DS
SL
P1
P0
Liju G Chacko

EXPLANATION
Non-stored(null qualiﬁer)
Non-stored
Overriding reset
Set(stored)
time limited
timer delayed
pulse
stored and time delayed
delay and stored
stored andtime limited
Pulse(rising edge)
pulse(falling edge)


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Important rules for SFC evolution[IEC 61131-3]
1
Evolution of active states of steps shall take place
along the directed links.(i.e, transition from one step
to another should take place along the connected
lines only )
2
The clearing time of transitions(transition condition)
should be as short as possible. Clear the transition
condition as soon as the next step is activated.

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References

[1] IEC 61131
[2] Automated Manufacturing Systems PLCs, by Hugh
Jack
[3] http://en.wikipedia.org/wiki/
Programmable logic controller

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