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Programmable Logic
Controllers
Basic Ladder Logic Programming
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
Outline
• Boolean statements and ladder logic
equivalents
- Logical AND
- Logical OR
- Logical NOT
• Commonly used ladder logic sequences
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
• Properly formatted outputs
Boolean logic control programs
- Boolean logic control programs examine and control on and off states - Boolean here is used interchangeably with the wordBoolean 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)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
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
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
Logical NOT
- The logical NOT function is constructed by referencing the input signal with a normally closed contact (XIO instruction)contact (XIO instruction)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
Complex Boolean expressions
- More complex Boolean expressions can be formulated with various serial-parallel combinations of XIC and XIO instructionsof XIC and XIO instructions - NAND, NOR, XOR, XNOR
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
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 af
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
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
• 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
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
START pushbutton released STOP pushbutton pressed
Interlock circuits
- Interlocks can prohibit output(s) from energizing under a certain condition
- Example: O:2/0 should not energize if O:2/1 isExample: O:2/0 should not energize if O:2/1 is energized (and vice versa)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
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 ofOne 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
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
- 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
Formatting considerations
E-stop conditions
Normal Stop Start Interlocks (if any) Outputs
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
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 programladder logic program
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 4-
