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Indra Nisja
PLC Program Execution PLCs monitor input devices,
execute instructions, and update output devices sequentially during the processor scan cycle
The steps of the scan cycle are:
Update the input image
Reads the CPU for instructions
Update the output terminals
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Ladder Diagram Programming Language The programming language most commonly used with
programmable logic controllers is the ladder diagram
Ladder diagram programming is built into the software of most PLCs
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Ladder Diagram Programming
Ladder logic language closely resembles hardwired relay circuits
The symbols represent an instruction set that perform various logic and on-off functions
There are five categories of instructions
Relay Logic
Timers
Counters
Data Manipulation
Arithmetic
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Relay Logic Instructions These are the most common instructions found
in PLCs
The relay logic instructions are:
Examine On -| |-
Energize Output -( )-
Branching - used to implement parallel inputs
Examine Off -|/|-
Latch On Output -(L)-
Unlatch Output -(U)-
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Timers are internal instructions to the PLC
Timers are activated by changes in the logic continuity of the rung
Types of timers found are:
Timer-On Delay
Timer-Off Delay
Retentive Timer-On Delay
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A separate address file for timers is provided in PLCs
File addresses begin at T4:0
Once the address is entered, the following characteristics are entered:
Time base
Preset value
Accumulated value
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Timer Words Each timer in a PLC uses
three words to store data
The second and third words store the preset and accumulated values
The first word contains status bits related to time status
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EN = Enable bit
TT = Timing bit
DN = Done bit
The Timer On-delay begins timing when rung conditions go true
When the accumulated value equals the preset value, the timer stops timing and the output is energized as bit 13 is set
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When the condition of the rung goes false, the timer off-delay begins timing
When the accumulated value equals the preset value, the output is energized
When the rung goes true, the counter is reset to zero
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The accumulated value of a retentive time is held until a reset command is given, regardless of rung conditions
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If the time valued needed exceeds the maximum value of a single timer, timers may be cascaded using the done bit of previous timers
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Nonretentive Timer
A single input timer called a non-retentive timer is used in some PLCs.
Energizing IN001 caused the timer to run for 4 seconds.
At the end of 4 seconds the output goes on.
When the input is de-energized, the output goes off and
the timer resets to 0
If the input IN001 is turned off during the timing interval (for
example. After 2.7 seconds), the timer resets to 0.
Def. Nonretentive timer is timer that reset when de-energized
The block format below includes the Enable/Reset line which allows
the timer to run when energized. When de-energized, the timer is kept
at 0 or reset to 0.
• The upper line causes the timer to run when the timer is enabled.
• When enabled, the timer runs as long as the run input is
energized.
• If Run is de-energized while the timer is running, the timing stops
where it is and does not reset to 0
• If both IN001 (Run) and IN002, (Enable/Reset) open and close at
the same time, the timer function is the same manner as the
non-retentive timer.
For format (a), suppose that IN002 is closed and IN001 is turned
on. After 6 seconds, IN001 is opened. The timer retains a count
of 6. Timing has not reached preset value 0f 14 seconds, and
the timer output still off. The timer does not reset unless IN002 is
opened.
Suppose that sometime later IN001 is reclosed. After 8 more
seconds of IN001 being closed, the timer coil will energize, since
6 + 8 = 14.
Format (b) is an alternate.
IN7 is for timing RT31 = RN.
IN8 enables RT31 = RS.
When the timer goes on, its output 31(internal) turns output 78 on.
A special case when the enable and reset are two separate input
rather than a common single input.
Example 1:When the circuit is turned on, one action take place. A specified
time later, another action occurs.
Example 2:A motor and its lubrication pump motor are both running.
Lubrication for main motor bearings is required during motor
coast-down. After the main motor is shut off, the lubricating pump
remains on for a time corresponding to coast-down. In this
example, a lubricating pump remains on for 20 seconds after the
main system is shut down.
Example 3:Two inputs go on at the same time. Then, one of them is go to
off after the preset period of time. One output A stays on; the
other output B turns off at the end of the timing interval.
Resetting is accomplished by turning IN001 and IN002 off.
Single time
interval
Example 4:A multiple application timing system. Three outputs turn on at
the same time. One stays on. Another, M shuts off after 8
seconds. The third output N, shuts off after 14 second.
Example 5:A short voltage pulse is produced every 12 seconds. The timer is
initially turned on by its “time” input. After the timing interval, the
timer turns the output on. When the output goes on, one of its
contacts, TT013, immediately opens and resets the timer to 0.
When the timer is reset to
0, the output is turned off.
Then, since the timer is
also off, TT013 recloses,
restarting the cycle. The
pulsed on time is very
short, one can-scan cycle
time. The process repeats
itself continuously.
Example 7:This example is for a timed interval of a number of seconds after
the start of a process operation. This type of time interval is
some time called an embedded time interval.
This operation uses the
special operation of a
fan. The fan is to come
on 8.7 seconds after a
system is turned on. It is
then to run until 16
seconds after the system
is turned on, which is a
net time of 7.3 second.
Counter Instructions Counters are output instructions internal to the
PLC
Counters are incremented or decremented by changes in the logic continuity of the rung
Types of counters found are: Up Counter
Down Counter
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Counter Programming
A separate file is used for counter functions
Counter instructions begin with the address C5:0
The following information must be entered
Counter and address
Preset value
Accumulated value
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Counter Words
Each counter in a PLC uses three words to store data
The second and third words store the preset and accumulated values
The first word contains status bits related to count status
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Up Counter Application
The count-up instructions are useful for repeating processes
The drilling application at the right makes use of an up counter which then activates a robotic arm
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The Down-Counter
The down counter decrements with each false-true transition of the ladder rung
The down counter is often used to end a cycle
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For the UC, the count starts at 0 and increments by 1
each time IN001 is pulsed on. When the preset value is
reached the output, CR17 goes on.
At the count goes
on beyond the
preset value, the
output stay on.
Opening IN002 at
any time resets the
counter to 0.
The DC operate in
the similar manner
There are three input :
May wish to stop the count for a time without it resetting to 0
and start counting again later where the count left on.
This is accomplished with the Enable and Reset inputs are
separate.
Example 1: Straight Counting in Process.
The counter output goes on after the set count
is received by repetitive pulses to the counter
input.
Either counter will function if its Enable line
is energized. After the count input receives
18 pulses, the CR output will energize.
Example 2: Two counters used with a common register to give
the sum of two counts
Suppose that we
wanted an output
indicator to go on
when 6 of part C
and 8 of part D
are on conveyor.
IN002 and IN003
are proximity
devices that
pulse on when a
part goes by
them.
Example 4: A process where a timed interval is started when
a count reaches a preset value.
Data Manipulation Instructions Data manipulation instructions allow words to
be moved within the PLC
Data manipulation instructions permit more complex operations than relay type instructions
These instructions are divided into three categories:
Data Transfer
Data Conversion
Data Compare
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Data Transfer Instructions
Data Transfer Instructions are implemented by the move (MOV) instruction
Contents from one register are moved to another based on rung conditions
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Data Conversion Data conversion is
available as:
Convert to BCD (TOD)
Convert from BCD (FRD)
Both are output instructions and convert data from or to binary coded decimal
A typical application would be the implementation of BCD encoded thumbwheel switches to input data into a PLC
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Data Compare Instructions
These commands instruct the PLC to compare the numerical contents of two registers and make decisions based upon their values and the results of the comparison
Compare instructions:
Compare Equal (EQU)
Compare Not Equal (NEQ)
Compare Less Than (LES)
Compare Less Than or Equal (LEQ)
Compare Greater (GRT)
Compare Greater Than or Equal (GEQ)
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Arithmetic Functions
Most PLCs have the capability to carry out arithmetic operations
The output of an arithmetic instruction is stored in a specified location
Arithmetic functions available are:
Addition (ADD)
Subtraction (SUB)
Multiplication (MUL)
Division (DIV)
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Writing a Program Use the following steps when developing a PLC program:
Choose the sequence you want the I/O devices to operate in
Write a description and make a drawing showing the sequence and conditions for each operation
Use the description to write the ladder diagram
Connect and label the I/O devices
Make a written record of each address used and what the address represents. Document all counters, timers, data instructions, etc.
Enter the program into the PLC
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Larger-Process Ladder Diagram Construction
Some of the steps in planning a program for a large process are:
1. Define the process to be controlled
2. Make a sketch of the process operation
3. Create a written step sequence listing for the process
4. Add sensors on the sketch as needed to carry out the control sequence
5. Add manual control as needed for process setup or operational checking
6. Consider the safety of the operating personnel and make addition and
adjustments as needed
7. Add master stop switches as required for safe shutdown
8. Create the ladder diagram that will be used as a basis for the PLC program
9. Consider the “what if’s” where the process sequence may go astray.
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