PLC AND SCADA 2ND MODULE

PLC and SCADA Sep 2, 2022

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The operation of an electrical system is achieved by a number of combination and sequential operations. The pre-determined sequence of operations may be called the logic combinational and the system operation. When the operational sequence is carried out with the help of relays, it is called relay-based logic whereas if it is carried out with a PLC, it is called PLC logic

Input and output Contact Program Symbols

A ladder logic diagram consists of one or more horizontal lines. Each horizontal line is called a rung. A rung is one program statement as shown in fig

Power always flows from left bus rail to the right bus rail. Suppose in a DC Circuit the left bus rail represents the positive terminal and the right bus rail represents the negative terminal.

A program statement consists of a condition or condition along with some form of action. Inputs are the conditions and the actions or the output is the result of these conditions.

Each rung of the PLC Ladder can be looked at as a problem the processor have to solve

The program symbol for a PLC input will look like a normal open or normal closed contact used in typical electrical diagrams. These symbols are shown in Table below

Numbering System of Inputs and Outputs

The real world switches are connected to the input terminals and the output relay and contacts are connected to the output terminals of the PLC.

The input and output terminals are grouped together and placed in modules. Each of the manufacturers produces a variety of input and output modules. Since the programmable controller may have hundreds of input or output modules, it is important to have some kind of numbering system to identify each module and all its input and output terminals.

The generalized identification numbering system of input and output terminals of a PLC consists of the following parameters:

Input/Output – Module number – Rack number – Slot number – Word number – Terminal number

Following this generalized numbering system, manufacturers prepare their own system. The Allen Bradley numbering system is Y : e. s / b. The meaning of each character has been shown in table

Program Format

The basic program format for a PLC is very similar to an electrical ladder diagram. This format is used because the ladder diagram has been the working language of electricians for many years. It is also used because the computer scans the program in a sequential manner. Devices in a schematic electrical diagram are described as being open or closed. PLC ladder instructions are typically referred to as either true or false .When a PLC solves the user program, it is said to be solving ladder logic.

Ladder logic programming has some basic rules. They are:

· The power will flow from left hand side bus rail to the right hand side bus rail

· Each rung must start with a contact from the left and end with an output on the right

· Outputs cannot be connected to the left bus rail

· Contacts cannot be connected to right bus rail

· Only one output may be placed on each rung

· Each output can be used once in a program

· Inputs with the same terminal number can be used many times

Introduction to Logic

Equivalent Ladder Diagram of AND Gate

Equivalent Ladder Diagram of OR gate

Equivalent Ladder Diagram of NOT Gate

Equivalent Ladder Diagram of XOR Gate

Equivalent Ladder Diagram of NAND Gate

Equivalent Ladder Diagram of NOR Gate

Equivalent Ladder Diagram to Demonstrate De Morgan’s Theorem

Assume the inputs are connected to I:0/1 and I:0/2 and the output terminals are O:0/1 and O:0/2

1^ST Theorem: Complement of a logical sum is equal to the logical product of the complements

Ladder Diagram-

2^nd Theorem: The complement of a logical product is equal to the logical sum of the complement

Ladder Diagram-

Problems –

Design a 4:1 multiplexer using ladder logic. Assume the inputs are connected to I:0/1 and I:0/2, I:0/3 and I:0/4; control signals are connected to I:0/5 and I:0/6 and the output terminal is O: 0/1

A multiplexer is a circuit with many input nut only one output. By applying a suitable control signal, any input can be steered to the output

Truth Table:

Circuit Diagram:

Ladder Diagram:

Problem 2: Design a 1:4 demultiplexer using ladder logic. Assume the inputs are connected to I:0/1, control signals are connected to I:0/2 and I:0/3 and the output terminals are O:0/1, O:0/2, O:0/3 and O:0/4

A demultiplexer is a circuit with one input and many outputs. By applying proper control signal, the input signal can be steered to one of the output lines

Truth Table

Ladder Diagram

Problem 3: A selection committee comprises four members including the chairman. In order for a candidate to be selected, he or she has to have the support of at least two members. The chairman, however, can push any candidate through. If each member is provide with a switch, design a logic that will ring a bell when a candidate is selected.

Assume the switch of the chairman is represented by CH, that of the the first member is by A, the second member by B and the third member by C, The inputs and the output are co0nnected to the PLC as follows:

Truth Table:

Ladder Diagram:

Problem 4: A railway station has 3 platforms A, B and C. A train is coming into the station. It has to be given entrey to platform A if A is empty. If both A and B are occupied then it has to be given entry to platform C. If all the platform are full then the train has to wait. Design the necessary logic diagram.

The input signals for the conditions described in the problem are as follows:

Ta – Train present at A

Tb – Train present at B

Tc – Train present at C

Pa – Go to platform A

Pb – Go to platform B

Pc – Go to platform C

W – Wait

The logic of the PLC can be written as

I:0/1 – Train at A

I:0/2 – Train at B

I:0/3 – Train at C