This document provides an overview of programmable logic controllers (PLCs) and ladder logic programming. It discusses how PLC programs are loaded into microprocessor-based systems using machine code. The IEC 61131-3 standard defines common PLC programming languages like ladder diagrams and function block diagrams. Ladder diagrams resemble electrical ladder diagrams and are read from left to right. They use contacts to represent inputs and coils to represent outputs. Basic logic functions like AND, OR, and NOT are used to write simple PLC programs in ladder logic.

1. Programmable Logic
Controller
Ladder Diagram

2. Introduction
• Programs for microprocessor based systems
have to be loaded into them in machine code
(sequence of binary number to represent the
program instruction)
• Assembly language based on the use of
mnemonic, (eg. LD to indicate the operation
required to load data)
• Assembler is used to translate the mnemonics
into machine code.

3. IEC 61131-3 Standard
• IEC 61131-3 (International Electrotechnical
Commission) is the standard of PLC
programming language
• Examples of PLC Programming language are;
Ladder Diagrams, Instruction List (like
mnemonic), Structured Text (like C
Programming) and Functional Block Diagrams.

4. Ladder Diagram
• Ladder diagrams are specialized schematics commonly
used to document industrial control logic systems.
• They are called "ladder" diagrams because they
resemble a ladder, with two vertical rails (supply power)
and as many "rungs" (horizontal lines) as there are
control circuits to represent.
• If we wanted to draw a simple ladder diagram showing
a lamp that is controlled by a hand switch, it would look
like this:

5. Ladder Diagram
• The left and right uprights represent power. If we
connect the left and right uprights through a load, power
can flow through the rung from the left upright to the
right upright
• Ladder logic diagrams are read from left-to-right, top-to-
bottom.
• Rungs are sometimes referred to as networks

6. Ladder Diagram
• The load(s) must always be located
nearest the grounded power conductor in
the ladder diagram
• The PLC then runs the ladder and
monitors the input continually and controls
output. This called scanning

7. Ladder Diagram Symbols
• The main function of PLC program is to
control outputs based on the condition of
inputs
• The symbols used in ladder logic
programming can be divided into two
broad categories: Contacts (inputs) and
Coils (outputs)

8. Contacts
• Most inputs to a PLC are simple devices that are either on
(true) or off (false). These inputs are sensors and switches
that detect part presence, empty or full status, and so on.
• Contacts can be thought of as switches. The two basic kinds
of switches are normally open and normally closed:
– A normally open switch does not pass current until it
closed
– A normally closed switch allows current flow until it closed

9. Coil
• Coils are output symbols. There are many
types of real-world output devices: motors,
lights, pumps, counters, timers and relays
• The PLC examines the contacts (inputs) in
the ladder and turns the coils (outputs) on
or off, depending on the condition of the
inputs

10. Basic Programming Instruction
• Programmable control is based on the
basic logic function (AND, OR, NOT) to
form the instruction.
• Parallel contacts are equivalent to an OR
gate.
• Series contacts are equivalent to an AND
gate.
• Normally-closed contacts are equivalent to
a NOT gate (inverter).

11. Mnemonic Code
• When using programming console, ladder
diagram need to be converted into
mnemonic code for entering the
programming instructions.
• The operand or data, is the item to be
operated on by the operation.
• Address is the memory storage location of
the operand.

12. PLC Processor Memory Size
The size of the programmable controller relates to the
amount of user program that can be stored.
The 1 K word memory
size shown can store
1,024 words, or 16,380
(1,024 x 16) bits of
information using
16-bit words or 32,768
(1,024 x 32) using
32 bit words.

13. Instructions Description
LD A starting instruction for logic line
AND This is used to connect two or more inputs in serial
OR This is used to connect two or more inputs in parallel
AND LD This is used to connect two blocks in serial
OR LD This is used to connect two blocks in parallel
NOT NOT invert its inputs
Often used to form an NC (Normally Closed) input or
output.
NOT can be used with LD, OUT, AND or OR
OUT This is used to connect output /designated operand bit
END This is used to indicate the end of program.
The last instruction in any program must be an END
instruction.

14. AND Logic Function
Address Instructions Data
00000 LD 00000
00001 AND 00001
00002 AND NOT 00002
00003 OUT 01000
00004 END(01)

15. OR Logic Function
Address Instructions Data
00000 LD 00000
00001 OR 00001
00002 OR NOT 00002
00003 OUT 01000
00004 END(01)

16. Combining The Logic Functions
END
Address Instruction Data
0000 LD 0001
0001 AND 0002
0002 OR 0003
0003 AND 0004
0004 AND NOT
0005 OUT 1000
END(01)
0001 0002 0004 0005 1000
0003

17. AND LD
END
Address Instruction Data
0000 LD 0001
0001 OR 0002
0002 LD 0003
0003 OR NOT 0004
0004 AND LD
0005 OUT 1000
0006 END(01)
0001 0003 1000
0002 0004

18. OR LD
END
Address Instruction Data
0000 LD 0001
0001 AND 0002
0002 LD 0003
0003 AND NOT 0004
0004 OR LD
0005 OUT 1000
0006 END(01)
0001 0002
0003 0004
1000

19. Combining AND LD and OR LD
END
Address Instruction Data
0000 LD 00001
0001 AND NOT 00002
0002 LD NOT 00008
0003 AND 00003
0004 OR LD
0005 LD 00004
0006 AND 00005
0007 LD 00006
0008 AND 00007
0009 OR LD
0010 AND LD
0011 OUT 1000
0012 END(01)
0001 0002 0004 0005 1000
0007
0006
0003
0008

20. A Systematic Approach of Control
System Design using a PLC
1. Determine the Machine Sequence of
Operation
2. Assignment of Inputs and Outputs
3. Writing of the Program
4. Programming into Memory
5. Running the System