plclanguages-131108224019-phpapp02 (2).pptx

PLC
Programming Languages
Liju G Chacko
Liju G Chacko PLC Programming Languages 1 / 25

Table of contents
1 Introduction
2 PLC programming languages
Instruction List Programming
Structured Text Programming
Functional Block Programming
Ladder Logic Programming
Sequential Function Chart

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.

PLC programming languages
PLC programming languages
PLC Programming languages mentioned in IEC
61131-3
Sequential Function Charts

PLC programming languages Instruction List Programming
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

PLC programming languages Instruction List Programming

PLC programming languages Structured Text Programming
Similar to high level language, such as Basic or C
Example-
Average 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;

PLC programming languages Structured Text Programming
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

PLC programming languages Functional Block Programming
It is a graphical dataflow programming method

PLC programming languages Ladder Logic Programming
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.

Types of contact inputs
Examine if close Examine if open
Positive transition sense Negative transition sense

Types of relay outputs
Output energized Negative output energized
Output latch/set Output unlatch/unset

Function Blocks
On Delay Timer Off Delay Timer

PLC programming languages Sequential Function Chart
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

Sequencial Function Chart
SFC structure

WORKING

Macrostep(subroutines)

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

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

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 .

Action Qualifiers(Continue...)
Overriding(R)
The action started by S, SD, DS, and SL qualifiers are
aborted by R qualifier.

Action Qualifiers(Continue...)
QUALIFIER EXPLANATION
-None- Non-stored(null qualifier)
N Non-stored
R Overriding reset
S Set(stored)
L time limited
D timer delayed
P pulse
SD stored and time delayed
DS delay and stored
SL stored andtime limited
P1 Pulse(rising edge)
P0 pulse(falling edge)

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 )
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.
2

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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