Rail (+V)
Rail (0 V)
A
B
Variable in ladder logic programming is called TAG.
TAG is linked to ladder logic instruction.
Created variables TAG and their types are gathered in Datablock.
C
1 2
3
4 5
6
Ladder fundamentals
Terminology
BOOL 1 = TRUE = HIGH = ON
BOOL 0 = FALSE = LOW = OFF
NO & NC CONTACTS
NO
INPUT = 1 ➔ | | ➔ OUTPUT = 1
NC
INPUT = 1 ➔ |\| ➔ OUTPUT = 0
TRUTH TABLES
INPUTS
OUTPUT
A
B
X
0
0
0
0
1
1
1
0
1
1
1
0
TRUTH TABLES
INPUTS
OUTPUT
A
B
C
X
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
1
0
0
1
1
0
1
0
1
1
0
0
1
1
1
1
Boolean Algebra
• Multiplication = Logical AND
• Addition = Logical OR
• ! = logical inversion (NOT)
Example
• X = AB (X equals A AND B)
• Y = A + !B (Y equals A OR NOT B)
The normally open (NO) Contact
• Basic Instruction: If A, Then X
• If Boolean tag A is true then make tag X true
else, make tag X false
• Bool algebra X = A
TAG.1A
TAG.1X
The normally open (NO) Contact
• Basic Instruction: If A, Then X
• If Boolean tag A is true then make tag X true
else, make tag X false
• Bool algebra X = A
TAG.1A
TAG.1X
The normally open (NO) Contact
• Basic Instruction: If A, Then X
• If Boolean tag A is true then make tag X true
else, make tag X false
• Bool algebra X = A
TAG.1A
TAG.1X
The normally open (NO) Contact
• Basic Instruction: If A, Then X
• If Boolean tag A is true then make tag X true
else, make tag X false
• Bool algebra X = A
TAG.1A
TAG.1X
The normally closed (NC) Contact
• Basic Instruction: IF NOT (A), Then X
• If Boolean tag A is NOT true then make tag X true
else, make tag X false
• Bool algebra X = !(A)
• NOTE: NOT is represented by an NC contact
TAG.2A
TAG.2X
The normally closed (NC) Contact
• Basic Instruction: IF NOT (A), Then X
• If Boolean tag A is NOT true then make tag X true
else, make tag X false
• Bool algebra X = !(A)
• NOTE: NOT is represented by an NC contact
TAG.2A
TAG.2X
Combining Contacts: The AND Instruction (IF (A AND B), Then X
Truthe Table:
• Basic Instruction: AND- If (A AND B), Then X
A
B
X
• If A & B are true then make X true
0
0
0
else, make X false
0
1
0
1
0
0
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional series NO contact for each
tag. For example A AND B AND C AND D can be achieved by Adding a 2 series NO contacts.
TAG.3A
TAG.3B
TAG.3X
Combining Contacts: The AND Instruction (IF (A AND B), Then X
Truthe Table:
• Basic Instruction: AND- If (A AND B), Then X
A
B
X
• If A & B are true then make X true
0
0
0
else, make X false
0
1
0
1
0
0
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional series NO contact for each
tag. For example A AND B AND C AND D can be achieved by Adding a 2 series NO contacts.
TAG.3A
TAG.3B
TAG.3X
Combining Contacts: The AND Instruction (IF (A AND B), Then X
Truthe Table:
• Basic Instruction: AND- If (A AND B), Then X
A
B
X
• If A & B are true then make X true
0
0
0
else, make X false
0
1
0
1
0
0
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional series NO contact for each
tag. For example A AND B AND C AND D can be achieved by Adding a 2 series NO contacts.
TAG.3A
TAG.3B
TAG.3X
Combining Contacts: The AND Instruction (IF (A AND B), Then X
Truthe Table:
• Basic Instruction: AND- If (A AND B), Then X
A
B
X
• If A & B are true then make X true
0
0
0
else, make X false
0
1
0
1
0
0
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional series NO contact for each
tag. For example A AND B AND C AND D can be achieved by Adding a 2 series NO contacts.
TAG.3A
TAG.3B
TAG.3X
Combining Contacts: The OR Instruction (IF (A OR B), Then X
Truthe Table:
• Basic Instruction: OR- If (A OR B), Then X
A
B
X
• If A OR B is true then make X true
0
0
0
else, make X false
0
1
1
1
0
1
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional parallel NO contact for
each tag. For example A OR B OR C OR D can be achieved by Adding a 2 parallel NO contacts.
TAG.4A
TAG.4B
TAG.4X
Combining Contacts: The OR Instruction (IF (A OR B), Then X
Truthe Table:
• Basic Instruction: OR- If (A OR B), Then X
A
B
X
• If A OR B is true then make X true
0
0
0
else, make X false
0
1
1
1
0
1
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional parallel NO contact for
each tag. For example A OR B OR C OR D can be achieved by Adding a 2 parallel NO contacts.
TAG.4A
TAG.4B
TAG.4X
Combining Contacts: The OR Instruction (IF (A OR B), Then X
Truthe Table:
• Basic Instruction: OR- If (A OR B), Then X
A
B
X
• If A OR B is true then make X true
0
0
0
else, make X false
0
1
1
1
0
1
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional parallel NO contact for
each tag. For example A OR B OR C OR D can be achieved by Adding a 2 parallel NO contacts.
TAG.4A
TAG.4B
TAG.4X
Combining Contacts: The OR Instruction (IF (A OR B), Then X
Truthe Table:
• Basic Instruction: OR- If (A OR B), Then X
A
B
X
• If A OR B is true then make X true
0
0
0
else, make X false
0
1
1
1
0
1
1
1
1
• Bool algebra X = AB
• NOTE: Can be expanded to contain more input tags by adding an additional parallel NO contact for
each tag. For example A OR B OR C OR D can be achieved by Adding a 2 parallel NO contacts.
TAG.4A
TAG.4B
TAG.4X
Best Practice: Multiple Outputs
• Multiple outputs can be used in a network, they may be placed in
series or parallel with each other, best practice is to place them in
parallel with each other.
TAG.5A
TAG.5B
TAG.5Y
TAG.5X
TAG.5Z
Best Practice: Multiple Outputs
• Multiple outputs can be used in a network, they may be placed in
series or parallel with each other, best practice is to place them in
parallel with each other.
TAG.5A
TAG.5B
TAG.5Y
TAG.5X
TAG.5Z
Best Practice: Multiple Outputs
• Multiple outputs can be used in a network, they may be placed in
series or parallel with each other, best practice is to place them in
parallel with each other.
TAG.5A
TAG.5B
TAG.5Y
TAG.5X
TAG.5Z
Exercise: Three Input AND Instruction
• Write a ladder logic to represent a 3 input AND instruction which takes
inputs A, B and C, and has output X. Note that input C should be inverted.
• Boolean algebra for this Instruction: X=(A)(B)(!C)
• Verbal Reading of this instruction: X equals A AND B AND NOT C
Question for this assignment:
• How many input combinations produced a TRUE output for X?
• How many input combinations are possible for a 3 input instruction?
(Hint: for Boolean/ binary logic, the number of input combinations is 2^n,
where n is the number of inputs.
Exercise: Three Input AND Instruction
• Write a ladder logic to represent a 3 input AND instruction which
takes inputs A, B and C, and has output X. Note that input C should be
inverted.
X = (A)(B)(!C)
A
B
C
X
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
1
1
1
1
0
Exercise: Three Input OR Instruction
• Write a ladder logic to represent a 3 input OR instruction which takes
inputs A, B and C, and has output X. Note that input C should be inverted.
• Boolean algebra for this Instruction: X=A+B+(!C)
• Verbal Reading of this instruction: X equals A OR B OR NOT C
Question for this assignment:
• Which instruction is more likely to produce a TRUE output between the
AND instruction and the OR instruction?
• How many input combinations are possible for a 3 input instruction?
(Hint: for Boolean/ binary logic, the number of input combinations is 2^n,
where n is the number of inputs.
Exercise: Three Input OR Instruction
• Write a ladder logic to represent a 3 input OR instruction which takes
inputs A, B and C, and has output X. Note that input C should be
inverted.
X = (A)(B)(!C)
A
B
C
X
0
0
0
1
0
0
1
0
0
1
0
1
0
1
1
1
1
0
0
1
1
0
1
1
1
1
0
1
1
1
1
1
Combining Contacts: The XOR Instruction
(IF (A XOR B), Then X
• If either A or B is true, but not both at the
same time, then make X true (if only one input
is true, make X true)
Else, make X false
• Equivalent Bool Algebra X = (!A)(B) + (A)(!B)
TAG.8A
TAG.8B
TAG.8A
TAG.8B
Truthe Table:
A
B
X
0
0
0
0
1
1
1
0
1
1
1
0
TAG.8X
Combining Contacts: The XOR Instruction
(IF (A XOR B), Then X
• If either A or B is true, but not both at the
same time, then make X true (if only one input
is true, make X true)
Else, make X false
• Equivalent Bool Algebra X = (!A)(B) + (A)(!B)
TAG.8A
TAG.8B
TAG.8A
TAG.8B
Truthe Table:
A
B
X
0
0
0
0
1
1
1
0
1
1
1
0
TAG.8X
Combining Contacts: The XOR Instruction
(IF (A XOR B), Then X
• If either A or B is true, but not both at the
same time, then make X true (if only one input
is true, make X true)
Else, make X false
• Equivalent Bool Algebra X = (!A)(B) + (A)(!B)
TAG.8A
TAG.8B
TAG.8A
TAG.8B
Truthe Table:
A
B
X
0
0
0
0
1
1
1
0
1
1
1
0
TAG.8X
Combining Contacts: The XOR Instruction
(IF (A XOR B), Then X
• If either A or B is true, but not both at the
same time, then make X true (if only one input
is true, make X true)
Else, make X false
• Equivalent Bool Algebra X = (!A)(B) + (A)(!B)
TAG.8A
TAG.8B
TAG.8A
TAG.8B
Truthe Table:
A
B
X
0
0
0
0
1
1
1
0
1
1
1
0
TAG.8X
Lab Assignment: Three Input XOR Instruction
• Write a ladder logic to represent a 3 input XOR Instruction which
takes inputs A, B and C, and has an output X.
• Boolean Algebra for this instruction X = ABC
• Equivalent Boolean Algebra for this Instruction:
X=(A)(!B)(!C)+((!A)(!B)(C)+(!A)(B)(!C)+(A)(B)(C)
Verbal Reading of this instruction: X equals A XOR B XOR C
Questions for this assignment
1. True or False? An XOR instruction produces a TRUE output when it
has an ODD number of TRUE inputs?
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
TAG.10A
TAG.10X
TAG.10B
TAG.10X
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
Press
TAG.10A
TAG.10X
TAG.10B
TAG.10X
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
Press
TAG.10A
TAG.10X
TAG.10B
TAG.10X
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
Unpress
TAG.10A
TAG.10X
TAG.10B
TAG.10X
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
Press
TAG.10A
TAG.10X
TAG.10B
TAG.10X
The Latch Circuit
• Often used for Basic Motor Start/Stop
• If Input A is made TRUE, output X should become true and remain
true regardless of whether input A is true or false.
• Output X should only become false if input B is made true.
• Note: Outputs may be used as inputs.
TAG.10A
TAG.10X
TAG.10B
TAG.10X