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Programmable Logic
Controllers
PLC Timer Functions
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
Outline
- Introduction
- PLC Timer Functions
- Examples of Timer Function Applications
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
Objectives
- Describe PLC retentive and delay timer functions.
- List and describe major timing functions that are commonly used in circuits and processes.
- Apply PLC functions and PLC circuitry to process control for timing functions.
- Apply PLC timers in multiple timing problems
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
pp y p g p that combine two or more of the basic timing functions.
- Apply PLC timers for the control of processes.
Introduction
- The most commonly used process control device after coils and contacts is the timer.
- The most common timing function is TIME DELAY-ON, which is the basic function.
- There are also many other timing configurations, all of which can be derived from one or more of the basic TIME DELAY-
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
ON functions.
- PLCs have the one basic function timer capability in multiples.
Timer Variables
- One major advantage of the PLC timer is that its time may be a programmable variable time as well as a fixed time.ll f d
- The variable time interval may be in accordance with a changing register value.
- Another advantage of the PLC timer is that its timer accuracy, repeatability, and
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
reliability are extremely high because it is based on solid-state technology.
Non-retentive Timers
- A single-input timer called a non-retentive timer is used in some PLCs. - Energizing I:1/0 causes the timer to run for 4 seconds. - At thAt the end of 4 seconds the output (DN) goes on. When the input is d f 4 d th t t (DN) Wh th i t i de-energized, the output goes off and the timer resets to 0. - If the input I:1/0 is turned off during the timing interval (for example, after 2.7 seconds), the timer resets to 0. - TON is the basic non-retentive timer in Allen-Bradley PLCs
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
Timer Information
- The timer table contains all information for that timer
- /EN: Timer is enabled (i.e. the input rung is energized)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
- /TT: Timer is timing
- /DN: Timer is done
- .PRE: Timer preset value (point at which the timer stops timing)
- .ACC: Timer accumulator (accumulated time value)
Timer Delay Off (TOF)
- The TOF timer functions the opposite of the TON timer.
- De-Energizing I:1/0 causes the timer to run for 4.5 seconds. The DN bit is initially set.
- At the end of 4.5 seconds the output (DN) goes off. When the input is energized the timer resets to 0.
- If the input I:1/0 is turned on during the timing interval (for example, after 2.7 seconds), the timer resets to 0.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
Examples Of Timer
Function Applications
- On delay
- Output B comes on at a specific set time after output A is turned on. When A is turned off, B also goes off.
- Limited on time
- A and B go on at the same time. B goes off after specific set time period, but A remains on.
- One-shot operation
- Output B goes on for a specified time after output A is turned on. Output B will run for its specified time interval even if A is turned off during the B timing interval.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
g g
- Interval time within a cycle
- We may require that an output come on 7.5 seconds after system startup, remain on for 4.5 seconds, and then go off and stay off. The interval would repeat only after the system is shut off and then turned back on.
On Delay Timer Function
- The first example is the simplest form of time delay.
- When the circuit is turned on, one action takes place.
- A specified time later, another action occurs.A specified time later another action occurs
- O:2/1 energizes exactly 8 seconds after O:2/0 energizes, provided I:1/0 remains energized
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
Limited On-Time Timer Function
- This example illustrates a situation in which two inputs go on at the same time.
- Then, one of them is to go off after a preset period of time., g p p
- One output, O:2/0, stays on; the other output, O:2/1, turns off at the end of the timing interval.
- Resetting is accomplished by turning I:1/0 off.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
One-shot Operation Timer
Function
- The next example is a one-shot system.
- The output comes on after its specified time period even if the input is turned off during the timing periodthe input is turned off during the timing period.
- I:1/0 must be opened and re-closed to reset the process.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 5-
