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4-1
Dr. D. J. Jackson Lecture 4-1Electrical & Computer Engineering
Programmable Logic
Controllers
Basic Ladder Logic Programming
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

4-2
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)
Conveyor motor control system
sensor_A
sensor_B
actuator_C

4-3
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
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
Logical NOT
• The logical NOT function is constructed by
referencing the input signal with a normally closed
contact (XIO instruction)
Complex Boolean expressions
• More complex Boolean expressions can be
formulated with various serial-parallel combinations
of XIC and XIO instructions
– NAND, NOR, XOR, XNOR

4-5
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
Start-stop-seal ladder diagram
Initial state START pushbutton pressed
START pushbutton released STOP pushbutton pressed

4-6
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
Start-stop-seal circuit example

4-7
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)
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

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

4-9
• Only one output energize instruction (OTE) should
appear in a rung of ladder logic
• 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

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