Programmable Logic Controllers (PLC) - Part 4

Programmable Logic Controllers (PLC) - Part 4

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

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)

Logical AND ladder diagram

• The logical AND function is constructed by series combinations of digital (discrete) inputs

 Two (or more) series components

Logical OR ladder diagram

• The logical OR function is constructed by parallel combinations of digital (discrete) inputs

Two (or more) parallel components

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

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

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

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

Properly formatted outputs

• An output energize instruction (OTE) referencing a specific output bit should appear only once in a ladder logic program

Properly formatted outputs -1

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

Properly formatted outputs-2

• 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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