PLC Architecture

Control System Training
PLC Architecture

Classification: Restricted
Agenda:
• Programmable Logic Controller(PLC)
• Types of PLC’s
• PLC architecture
• Scan cycles
• Scan patterns
• PLC programming
• Ladder diagram programming
• Latch and Unlatch
• DCS architecture

Programmable Logic Controller
• The National Electrical Manufacturers Association (NEMA) defines a PLC as a
"digitally operating electronic apparatus which uses a programmable memory
for the internal storage of instructions by implementing specific functions,
such as logic, sequencing, timing, counting, and arithmetic to control through
digital or analog I/O modules various types of machines or processes.’’
• In simple terms PLC is a solid-state industrial control device which receives
signals from user supplied controlled devices, such as sensors and switches,
implements them in a precise pattern determined by ladder-diagram-based
application progress stored in user memory, and provides outputs for control
of processes or user-supplied devices, such as relays or motor starters.
• PLCs come in different types, and it is generally chosen for an application
depending upon the number of inputs and outputs in a process.

TYPES OF PLCS
The general classification of PLC based upon the number of inputs and outputs
is
• Fixed type PLC
• Modular type PLC
• Rack type PLC
Fixed type PLC: In this type of PLC all the components of the PLC are as a single
unit. The number of I/O supported by the PLC is decided by the manufacturer
and cannot be changed. This type of PLC can support a small number of I/Os.

Programmable Logic Controller
• Modular Type PLC: In modular type PLC the number of I/Os can be increased
by the addition of modules to the existing PLC. In modular type PLC the
number of I/O supported can be increased to few hundreds by adding I/O
modules.
• Rack Type PLC: In rack type PLC all the components of the PLC are as separate
modules and are assembled to form one unit by mounting the individual
components on a rack. This PLC can support upto thousands of I/Os.

BRANDS OF PLC
• There are lot of PLC manufacturers, some of the commonly known brands of
PLC are,
• Allen Bradley(AB)
• Siemens
• ASEA Brown Boveri
• Omoron
• GE Fanuc
• DELTA
• Mitsubishi
• Messung
• Keyence
• In all the brands there may be the different types of PLCs available.

PLC ARCHITECTURE

PLC ARCHITECTURE
MEMORY UNIT: The memory of the PLC is divided into two types
• Program memory : The Ladder Logic program downloaded will be stored
in this memory.
• Data memory: The data from the I/O modules will be stored in this
memory section

INPUT MODULE FUNCTIONING
• The input from the field device is received on the input module.
• The input signal could be digital input signal (0/1) or it could be analog input
signal ( 4 to 20 mA, 0 to 10V)
• The PLC is a digital device and the CPU of the PLC can withstand voltage of
5V.
• The Input module receives the input signal from the field devices which
passes through the isolation barrier in the module and is then convert to the
required 5V and then sent to the PLC.
• The isolation barrier prevents the external events from interfering with the
PLC and causing a module failure.
• In the PLC it is stored in the data memory as a binary value1 for 5V and binary
value 0 for 0V.

OUTPUT MODULE FUNCTIONING
• The value of either 0 or 1 stored in the data memory after execution of the
program is sent to the output module.
• The output module also contains the isolation barrier which is used to protect
the PLC from external interference.
• The signal from the PLC is converted to corresponding analog or digital signal
according to the module to which it is connected.

PLC ARCHITECTURE
• The input signal from the input module is stored in the data memory.
• This data is then used for execution of the program that is downloaded into
the PLC.
• The output status which is there on the program output is stored in the data
memory of the PLC.
• The data from this memory is sent to the output module which is used to
control the field devices.
• This process of reading the input value , executing the program and writing
the output value is called as scanning.
• ISOLATION BARRIER : Optical Isolator provides electrical isolation between
field wiring and the PLC internal circuitry .
• It consists of Light-Emitting Diode & Photoelectric transistor .
• When Switch Close d =>Current flow through LED =>Generates Light =>
Triggers current flow through Photoelectric transistor => Sensed by PLC
digital logic.
• Circuitry to the LHS of isolator powered from the field and Circuitry to the
RHS is powered from internal PLC voltage source

PLC ARCHITECTURE
The PLC scans the process continuously because the inputs can change at any
time.

THE SCAN CYCLE
PLCs operate by continually scanning programs and repeat this process many
times per second. When a PLC starts, it runs checks on the hardware and
software for faults, also called a self-test. If there are no problems, then the
PLC will start the scan cycle. The scan cycle consists of three steps: input scan,
executing program(s), and output scan.
Input Scan: A simple way of looking at this is the PLC takes a snapshot of the
inputs and solves the logic. The PLC looks at each input card to determine if it
is ON or OFF and saves this information in a data table for use in the next
step. This makes the process faster and avoids cases where an input changes
from the start to the end of the program.
Execute Program (or Logic Execution): The PLC executes a program one
instruction at a time using only the memory copy of the inputs the ladder
logic program. For example, the program has the first input as ON. Since the
PLC knows which inputs are ON/OFF from the previous step, it will be able to
decide whether the first output should be turned ON.
Output Scan: When the ladder scan completes, the outputs are updated using
the temporary values in memory. The PLC updates the status of the outputs
based on which inputs were ON during the first step and the results of
executing a program during the second step. The PLC now restarts the
process by starting a self-check for faults.

SCAN PATTERNS

PLC PROGRAMMING
• There are 5 programming languages available , but the commonly used
language is ladder diagram for PLC

PLC PROGRAMMING
LADDER LOGIC
• Ladder logic is the main programming method used for PLC's. Ladder logic has
been developed to mimic relay logic. By selecting ladder logic as the main
programming method, the amount of retraining needed for engineers and
trades people was greatly reduced.
• The first PLC was programmed with a technique that was based on relay logic
wiring schematics. This eliminated the need to teach the electricians,
technicians and engineers how to program - so this programming method has
stuck and it is the most common technique for programming in today's PLC.
INSTRUCTION LIST
• There are other methods to program PLCs. One of the earliest techniques
involved mnemonic instructions. These instructions can be derived directly
from the ladder logic diagrams and entered into the PLC through a simple
programming terminal.

PLC PROGRAMMING
SEQUENTIAL FUNCTION CHARTS (SFC)
• SFC have been developed to accommodate the programming of more
advanced systems. These are similar to flowcharts, but much more powerful.
This method is much different from flowcharts because it does not have to
follow a single path through the flowchart.
STRUCTURED TEXT (ST)
• Programming has been developed as a more modern programming language.
It is quite similar to languages such as BASIC and Pascal.
• Structured Text (ST) is a high level textual language that is a Pascal like
language. It is very flexible and intuitive for writing control algorithms.
FUNCTION BLOCK DIAGRAM (FBD)
• FBD is another graphical programming language. The main concept is the
data flow that start from inputs and passes in block(s) and generate the
output.

LADDER DIAGRAM PROGRAMMING
Introduction to Ladder Logic
Ladder logic uses graphic symbols similar to relay schematic circuit diagrams.
Ladder diagram consists of two vertical lines representing the power rails.
Circuits are connected as horizontal lines between these two verticals.
Ladder diagram features
Power flows from left to right.
Output on right side can not be connected directly with left side.
Contact can not be placed on the right of output.
Each rung contains one output at least.
Each output can be used only once in the program.
A particular input a/o output can appear in more than one rung of a ladder.
The inputs a/o outputs are all identified by their addresses, the notation used
depending on the PLC manufacturer.

PLC PROGRAMMING

LADDER DIAGRAM PROGRAMMING

LATCH AND UNLATCH
• When input devices like Push Buttons are used the input contacts are not
continuous but momentarily.
• With this momentary contacts devices like motor are run continuously. For
this
purpose the concept of latch
is used.
• To break this continuous
supply to output a NC switch
is used to open the circuit
condition. This is called as
unlatch condition.

QUESTIONS
question1
• X0 – START pushbutton to Start Oil Pump Motor
• X1 – START pushbutton to Stop Main Motor
• X2 – STOP pushbutton to Stop Oil Pump Motor
• X3 – STOP pushbutton to Stop Main Motor
question2
• X0 – Start Switch.
• X1 – Low level float sensor. X1 = ON
when the liquid level reaches X1.
• X2 – High level float sensor. X2 = ON
when the liquid level reaches X2.
X3 – Stop Switch.
• X10 - EMERGENCY STOP button. X10 = ON when the button is pressed.

DCS ARCHITECTURE

Topics to be covered in the next session:
• PLC programming languages
• Ladder logic
• Ladder diagram programming
• PLC programming

Thank you!

PLC Architecture

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