This document discusses basic ladder logic programming concepts for programmable logic controllers (PLCs). It covers Boolean logic and ladder logic equivalents like logical AND, OR, and NOT. Common ladder logic sequences like start-stop-seal circuits and basic interlocks are described. The document provides examples of ladder logic diagrams and emphasizes the importance of properly formatting outputs and ladder logic rungs for readability and clarity.

1. 4-1
Dr. D. J. Jackson Lecture 4-1Electrical & Computer Engineering
Programmable Logic
Controllers
Basic Ladder Logic Programming
Dr. D. J. Jackson Lecture 4-2Electrical & Computer Engineering
Outline
• Boolean statements and ladder logic
equivalents
– Logical AND
– Logical OR
– Logical NOT
• Commonly used ladder logic sequences
– Start-stop-seal circuits
– Basic interlocks
• Properly formatted outputs

2. 4-2
Dr. D. J. Jackson Lecture 4-3Electrical & Computer Engineering
Boolean logic control programs
• Boolean logic control programs examine and control
on and off states
– Boolean here is used interchangeably with the word
“discrete”
• Each control program (ladder diagram sequence) can
contain one or more conditionals
• Example
– If (a part is on the conveyor) AND (there is not a
box in the chute) THEN (turn the conveyor motor on)
• In terms of sensors and actuators this becomes
– If (sensor_A is ON) AND (sensor_B is NOT ON) THEN
(turn actuator_C ON)
Dr. D. J. Jackson Lecture 4-4Electrical & Computer Engineering
Conveyor motor control system
sensor_A
sensor_B
actuator_C

3. 4-3
Dr. D. J. Jackson Lecture 4-5Electrical & Computer Engineering
Logical AND ladder diagram
• The logical AND function is constructed by series
combinations of digital (discrete) inputs
– Two (or more) series components
I:1/0 AND I:1/1
I:1/0 AND I:1/1 AND I:1/2
Dr. D. J. Jackson Lecture 4-6Electrical & Computer Engineering
Logical OR ladder diagram
• The logical OR function is constructed by parallel
combinations of digital (discrete) inputs
– Two (or more) parallel components
I:1/0 OR I:1/1
I:1/0 OR I:1/1 OR I:1/2

4. 4-4
Dr. D. J. Jackson Lecture 4-7Electrical & Computer Engineering
Logical NOT
• The logical NOT function is constructed by
referencing the input signal with a normally closed
contact (XIO instruction)
Dr. D. J. Jackson Lecture 4-8Electrical & Computer Engineering
Complex Boolean expressions
• More complex Boolean expressions can be
formulated with various serial-parallel combinations
of XIC and XIO instructions
– NAND, NOR, XOR, XNOR

5. 4-5
Dr. D. J. Jackson Lecture 4-9Electrical & Computer Engineering
Start-stop-seal circuits
• For PLC systems without latch and unlatch
instructions, a circuit is needed that will allow
a process to start, continue to run after a
start button is released, and stop under
control of another button
– A circuit that implements this functionality is
commonly referred to as a start-stop-seal circuit
• A feedback path (i.e. a contact) that
references the output is normally used to
seal around the start contact
Dr. D. J. Jackson Lecture 4-10Electrical & Computer Engineering
Start-stop-seal ladder diagram
Initial state START pushbutton pressed
START pushbutton released STOP pushbutton pressed

6. 4-6
Dr. D. J. Jackson Lecture 4-11Electrical & Computer Engineering
Start-stop-seal variations
• In practice several start and/or several stop
buttons can be used in a process
• Start buttons (with XIC instructions) can be
used
– In series if it is required that ALL be pressed
before a process starts
– In parallel if pressing ANY start button is to start a
process
• Stop buttons (with XIO instructions) are
normally used in series if pressing ANY stop
button is to stop a process
Dr. D. J. Jackson Lecture 4-12Electrical & Computer Engineering
Start-stop-seal circuit example

7. 4-7
Dr. D. J. Jackson Lecture 4-13Electrical & Computer Engineering
Interlock circuits
• Interlocks can prohibit output(s) from energizing
under a certain condition
• Example: O:2/0 should not energize if O:2/1 is
energized (and vice versa)
Dr. D. J. Jackson Lecture 4-14Electrical & Computer Engineering
Formatting considerations
• Ladder logic rungs should be formatted so the reader
can easily infer the meaning of the intended logic
• One mechanism to help this is the grouping of
related signals within an area on a given rung of
logic
• For example:
– Group signals together that have some common intent
• Start signals
• Stop signals
• Emergency stop signals (E-stop)
• Interlocks
– Controls that might have greater importance (i.e. E-stop)
might be located on the left hand side of the rung if possible

8. 4-8
Dr. D. J. Jackson Lecture 4-15Electrical & Computer Engineering
Formatting considerations
E-stop
conditions
Normal
Stop Start Interlocks (if any) Outputs
This is also a good example of instruction and rung documentation.
Dr. D. J. Jackson Lecture 4-16Electrical & Computer Engineering
Properly formatted outputs
• An output energize instruction (OTE) referencing a
specific output bit should appear only once in a
ladder logic program

9. 4-9
Dr. D. J. Jackson Lecture 4-17Electrical & Computer Engineering
Properly formatted outputs
• Only one output energize instruction (OTE) should
appear in a rung of ladder logic
Dr. D. J. Jackson Lecture 4-18Electrical & Computer Engineering
Properly formatted outputs
• If more than one output is to be controlled by a
certain rung of ladder logic, the output energize
(OTE) instructions can be placed in parallel