Chapter 5: Timers and Counters - Kent State University
Chapter 5: Timers and Counters
Computer Aided Manufacturing TECH 4/53350
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Ladder Logic
Learning objectives
Describe the use of timers and counters in ladder logic
Describe such terms as retentive, cascade, delay on and delay off
Explain the operation of TON, TOF, and RTO timers
Explain the use of CTU and CTD timers
Utilize timers and counters in ladder logic
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Timers
Overview
Timers are used to delay actions
Keep an output on for a specified time after an input turns off Keep an output off for a specified time before it turns on
Timing functions are vital in PLC applications
Cycle times are critical in many processes
Many PLCs use block-type timers and counters
Compliance with IEC 61131-3 standards
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Block-Type Timer
Example
TON
I:2
TIMER ON DELAY
EN
Timer
T4:0
Time Base Preset
0.1 50
DN
Accum
0
Above Timer is labeled T4:0
Timer Attributes
Time Base: Timers are typically programmed with several different time bases
1 second, 0.1 second, 0.01 second, 0.001 second Suppose time base is set to 0.1 and Delay increments is set to 50
timer has 5 seconds delay (0.1 x 50)
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Block-Type Timer
Example
TON
I:2
TIMER ON DELAY
EN
Timer
T4:0
Time Base Preset
0.1 50
DN
Accum
0
Preset Attribute: Preset value is the number of time increments timer must count
before changing the state of the output
Time Delay = Preset value x Time Base ( refer to previous example) Preset can be constant or a variable
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