PLC Timers
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PLC files
The SLC 500 organizes the CPU memory into sections called
“files”
There can be additional user defined files above the basics listed,
but we won’t need more.
Types of Timers
There are two major classifications of timers.
Retentive timers can be started and stopped any number of times and will retain the elapsed time. Non-retentive
timers will reset if stopped before completing their timing function.
Two types of non-retentive timers we use are Time delay off and Time delay On.
Parts of a Timer
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Timebase
Preset
Accumulator
Enable
Reset
Timebase
In our lives we use different timebases. A second, minute, hour, day, month, etc are all
examples of timebases. If we agree ahead of time to use minutes for our timebase then we
can say there is one minute per increment. A 50 minute class would have 50 increments.
The SLC500 has two timebases; seconds and hundredths of a second (10 mSec) increments.
Preset
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Is the number of timebase increments required before the timer activates. For
example, when using a microwave oven, you enter the cook time. The
cooking time is the PRESET.
Accumulator
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The accumulator holds the elapsed time since the timer was activated. The
Preset value can be a constant, but the accumulator must be a variable since it
is constantly changing.
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In the timer below, the accumulator has a value of 475. In 25 more increments
or 0.25 seconds the timer will activate
Enable
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The timer will only run when the enable bit is active. The enable line is used
to start and stop the timer.
Enable line
Reset
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After the timer is finished it turns on the DONE bit. This will remain on until
the timer is Reset. When the Reset bit is activated the accumulator is reset to a
value of zero.
In this example, the timer done bit is used to turn on an output.
Retentive - RTO
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Will retain the accumulated value if the enable becomes false
Useful if you want to measure the total time, regardless of interruptions
SLC 500 instruction mnemonics
Non-Retentive
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Will zero the accumulator if the enable becomes false. This is useful for
events that don’t get interrupted. For example, in a heat treat process, it might
be necessary to put a part in an oven and hold the oven temperature above a
400 degrees for two minutes. A part will only be considered good if the
temperature stays above 400 degrees for the entire two minutes. A
temperature probe is setup to go high if the temp is above 400 degrees. This
signal will control the run line on the timer. If the timer times out the part is
good, otherwise we have a bad part.
Example of the difference
• A person can hold their breath for 20
minutes.
• The “catch”, hold your breath for 15
seconds of every minute until the total time
measured is 20 minutes
Which timer does this illustrate? Retentive or Non-retentive
Time delay off - TOF
• Might be used to keep an exhaust fan
running for a while after someone signals
for a process to stop
WARNING!
Do not use the RES instruction to reset a
TOF instruction. A RES always clears the
status bits and the accumulated value. This
could result in unpredictable machine
operation or injury to personnel.
Time delay off example
Time delay on - TON
• The output will energize after a period of
time
• Might be used to provide a 2 second OSHA
safety start delay before machinery starts to
move
Timer example
Programming Timers
• You can have up to 255 timers, starting at
T4:0 thru T4:255
• The Reset coil is T4:x/RES, where x is the
timer from 0 to 255
• The Done bit is T4:x/DN or T4:x/13
• Maximum # of increments is 32767
Timer -internal structure
• Each timer uses 3 - 16 bit words
Timer Data File Element
Bit
15
14
13
Word 0
EN
TT
DN
Word 1
Preset Value
Word 2
Accumulated Value
12
11
Timer issues
• In the next example, you will probably not
see output #1 fire
• How can we tell if it did?
• Look at the latch and how it is used
• Timing accuracy is limited to +/- 1 timebase
increment, therefore in time sensitive
applications use the 10 mSec timebase.
Timer example #2
Turn the timer back off at the end of the scan
Clocks
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By using two timers together you can generate a “clock” that will have the
frequency and duty cycle you want
Duty cycle is the On time / Period
Period = 1 / frequency
The “clock” ladder logic might be used to turn on a pump for a 2 minute duration
every 18 minutes. The total period is 18+2 or 20 minutes, the duty cycle is 2 / 20 or
10%
Timer example #3
Special Timer contacts
• The Status File - S2 provides access to a
free running clock
• A contact with the reference of S2:4/0 is a
20mSec with 50% duty cycle
• S2:4/1 is a 40 mSec clock
• For bit values from 2 thru 15 the clock
doubles (20, 40, 80, 160 mSec….)
• S2:4/15 is a 655360 mSec clock
Exercises
• Construct a clock that will create a 1 scan
pulse every 24 hours
• Construct a clock that will make B3:1/1 a 1
second clock with 50% duty cycle
• Construct a 1 year clock
• Construct a program that will create a pulse
on output #1, 5 sec in duration that will
start 15 seconds after input #1 is turned off.