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Chapter 4 PLC

The document discusses two methods for implementing GRAFCET (Grafcet): 1) using combinatorial logic with logic gates or 2) using wired logic with flip-flops. It then provides examples of implementing a sample linear GRAFCET using each method. The document also discusses programming instruction lists, which are composed of operators and operands, as defined in the IEC 61131 standard. It notes that instruction lists can be used to convert ladder diagrams and GRAFCET models into a programming format.

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Jules Kouam
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119 views7 pages

Chapter 4 PLC

The document discusses two methods for implementing GRAFCET (Grafcet): 1) using combinatorial logic with logic gates or 2) using wired logic with flip-flops. It then provides examples of implementing a sample linear GRAFCET using each method. The document also discusses programming instruction lists, which are composed of operators and operands, as defined in the IEC 61131 standard. It notes that instruction lists can be used to convert ladder diagrams and GRAFCET models into a programming format.

Uploaded by

Jules Kouam
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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CHAPTER 4: IMPLEMENTATION OF GRAFCET

I. IMPLEMENTATION OF GRAFCET BY WIRING LOGIC


The materialization of a GRAFCET can be done in two ways:
 Combinatorial logic (logic gates)
 Wired logic based on flip-flops: it is simple and suitable for small fixed systems;
1. Combinatorial logic (logic gates)
One-step equation:
One-step wiring is done using 4 logic gates

A GRAFCET step is then symbolized in the form of a phase:

Example: case of a linear-sequence GRAFCET


Each step of the GRAFCET will be wired like the phase module described above. We then
creates an electrical sequencer based on logic gates
In order to take into account the start and stop modes of the GRAFCET, the module of a step
must integrate initialization and hard emergency stop as indicated by the logic equations of
the steps of the previous GRAFCET: (equations of the steps)

2. Implementation of a GRAFCET by wiring RS flip-flop

The basic element in this logic is the RS latch. The Reset action have priority (S=R=1 then
Q=0), the output equation Q is expressed as Q=(Q+ S) R

To materialize a GRAFCET, an SR flip-flop is associated with each step. In a GRAFCET, a


step:
 Is activated (Set action) by the condition (previous Step AND true receptivity);
 Remains activated even if the condition (previous Step AND true receptivity)
becomes false;
 Is disabled (Reset action) if the next step is activated.
 The action condition of a step is wired on the SET pin of the flip-flop
 The deactivion condition of a step is wired on the RESET pin of the flip-flop

1. Wiring the initial step

AC X i= X i−1 T i−1+ Init

DC X i= X i+1 . Init

2. Non initial
step

AC X i= X i−1 T i−1 . Init


DC X i= X i+1 + Init

Example

Table of activation and deactivation condition


GRAFCET wiring using RS flip-flop

INSTRUCTION LIST PROGRAMMING

Instruction list (IL) programming is defined as part of the IEC 61131 standard. It uses very
simple instructions similar to the original mnemonic programming languages developed for
PLCs. It is the most fundamental level of programming language - all other programming
languages can be converted to IL programs. Most programmers do not use IL programming
on a daily basis, unless they are using hand held programmers.

1. Instruction
An Instruction list (IL) is composed of a series of instructions. Each instruction begins on a
new line and consists of:

 an Operator ,
 if necessary with a Modifier and
 if necessary one or more Operands

Should several operands be used, they are separated by commas. It is possible for a label to
be in front of the instruction. This label is followed by a colon. A comment can follow the
instruction

2. Basic instruction

The following table gives a list representative of this language:


3. Conversion of the electric diagram into ladder
4. Conversion of ladder to instruction list

5. Conversion of the GRAFCET instruction list

Let consider the same GRAFCET used in ladder diagram the instruction list would be :

Out put

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