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 Standard
IEC (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 00001 AND 00002
AND NOT
00003
OUT
01000
00004
END(01)

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

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

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

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

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

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