plc programming languages

PLCProgramming Languages

Liju G Chacko

Liju G Chacko PLC Programming Languages 1 / 25

Table of contents

1 Introduction

2 PLC programming languagesInstruction List ProgrammingStructured Text ProgrammingFunctional Block ProgrammingLadder Logic ProgrammingSequential Function Chart


Introduction

Introduction

PLC(Programmable logic controller) is a multipleinputs and output digital computer used forautomation of electromechanical processes.These are the processes normally a PLC will carryout 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 personalcomputers. From computer, binary or pseudocode will bedownloaded to PLC through Ethernet, RS-232, RS-485 orRS-422 cable.Liju G Chacko PLC Programming Languages 3 / 25

PLC programming languages

PLC programming languages

PLC Programming languages mentioned in IEC

61131-3

Instruction List Programming

Structured Text Programming

Functional Block Programming

Ladder Logic Programming

Sequential Function Charts


PLC programming languages Instruction List Programming

Instruction List(IL) Programming

Allen Bradley supports IL programming only on theMicrologix 1000,

IL is similar to mnemonic programming languages

IL make use of an important concept – STACKIL instructions(Allen Bradley version)

LD A- Store A to stackLD A- Put A back to memoryORB - ’OR’ the last two variables in stackANB - ’AND’ the last two variables in stack


PLC programming languages Instruction List Programming

Instruction List Programming


PLC programming languages Structured Text Programming

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 ExplanationIF-THEN-ELSIF-ELSE-END IF normal if-then structureCASE-value:-ELSE-END CASE a case switching functionFOR-TO-BY-DO-END FOR for-next loopWHILE-DO-END WHILE while loopTAN(A), COS(A), A**B,,. . . Basic math functions


PLC programming languages Functional Block Programming

Functional Block Programming

It is a graphical dataflow programming method


PLC programming languages Ladder Logic Programming


It is a graphical programming language evolved fromelectrical relay circuit

Each program statement is a line called the rungwith inputs to the left and the output to theright.Execution starts from the top rung, down tothe last rung.Liju G Chacko PLC Programming Languages 10 / 25



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 graphicaltechnique for writing concurrent control programs

SFC is based on GRAFCET (GRAphe Fonctionnelde Commande Etapses/Transitions) [itself based onbinary petri nets]Main components of SFC are:

Steps with associated actionsTransitions with associated logic conditionsDirected links between steps and transitions



Sequencial Function Chart

SFC structure

Figure : SFC structureLiju G Chacko PLC Programming Languages 15 / 25



WORKING




Macrostep(subroutines)




Concurrent control flow (two or more loops running inparallel)

Figure : Unlocking two doors with the same keypadLiju G Chacko PLC Programming Languages 18 / 25



Action Block

Actions can be written in Ladder Logic, Instruction language orstructured text language. Action block is not a part of a Step. Stepjust activates or deactivates the action block depending on thequalifier, as long the step itself is activeeg;- Action 1




Action QualifiersNon-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 areaborted by R qualifier.




Action Qualifiers(Continue...)

QUALIFIER EXPLANATION-None- Non-stored(null qualifier)

N Non-storedR Overriding resetS Set(stored)L time limitedD timer delayedP pulse

SD stored and time delayedDS delay and storedSL stored andtime limitedP1 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 stepto another should take place along the connectedlines only )

2 The clearing time of transitions(transition condition)should be as short as possible. Clear the transitioncondition as soon as the next step is activated.



References

[1] IEC 61131

[2] Automated Manufacturing Systems PLCs, by HughJack

[3] http://en.wikipedia.org/wiki/Programmable logic controller


THANK YOU

plc programming languages

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