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Programming A PLC
STEP 7-Micro/WIN32 is the program software used with the
S7-200 PLC to create the PLC operating program
...
These
instructions are divided into three groups: standard instructions,
special instructions, and high-speed instructions
...
Standard instructions include; timer, counter,
math, logical, increment/decrement/invert, move, and block
instructions
...
Special
instructions include shift, table, find, conversion, for/next, and
real-time instructions
...
These include high-speed
counters, interrupts, output, and transmit instructions
...
A few of the more common instructions
necessary for a basic understanding of PLC operation will
be discussed
...
Refer to the SIMATIC S7-200 Programmable Controller
System Manual for detailed information concerning these
instructions
...
Offline programming allows the user to edit the ladder diagram
and perform a number of maintenance tasks
...
On-line programming requires the PLC to be connected
to the programming device
...
In addition, status of the input/output
elements can be monitored
...
Symbols
In order to understand the instructions a PLC is to carry out, an
understanding of the language is necessary
...
Contacts
One of the most confusing aspects of PLC programming for
first-time users is the relationship between the device that
controls a status bit and the programming function that uses
a status bit
...
Symbolically, power flows through these contacts
when they are closed
...
The normally closed contact (NC) is true (closed)
when the input or output status bit controlling the contact is 0
...
When a coil is energized, it causes a corresponding
output to turn on by changing the state of the status bit
controlling that output to 1
...
Boxes
Boxes represent various instructions or functions that are
executed when power flows to the box
...
Entering Elements
Control elements are entered in the ladder diagram by
positioning the cursor and selecting the element from a lists
...
2
...
Network 1
I0
...
1
Q0
...
2
43
An AND Operation
Each rung or network on a ladder represents a logic operation
...
Two contact closures and one output coil are placed
on network 1
...
0, I0
...
0
...
In this example I0
...
1 (input 2) must be
true in order for output Q0
...
It can also
be seen That I0
...
1 must be true for Q0
...
Another way to see how an AND function works is with
a Boolean logic diagram
...
In this case
there are two inputs
...
It can be seen from the table that both inputs must be a
logic 1 in order for the output to be a logic 1
...
0
Q0
...
1
Input 2
0
1
0
1
Output 1
0
0
0
1
I0
...
1
0
1
0
1
Q0
...
It can be
seen that if either input I0
...
3 (input 4), or both are true, then output Q0
...
Another way to see how an OR function works is with a
Boolean logic diagram
...
The OR function is represented by a number of inputs
on the left side
...
The output is
represented on the right side
...
Or (O) Function
Or (O) Function
Input 3
I0
...
1
I0
...
4
0
0
1
1
I0
...
1
0
1
1
1
45
Testing a Program
Once a program has been written it needs to be tested and
debugged
...
The program is first downloaded from the programming
device to the CPU
...
The simulator switches are operated and the resulting
indication is observed on the output status indicator lamps
...
The standard method of showing a
ladder element is by indicating the circuit condition it produces
when the device is in the deenergized or non operated state
...
0) is programmed as a
normally open (NO) contact
...
0)
...
In the
example shown the toggle switch connected to input 1 has
been closed
...
0) and activate the output (Q0
...
The
lamp will illuminate
...
It can be used to temporarily override the input
or output status of the application in order to test and debug
the program
...
The force function can be used to skip
portions of a program by enabling a jump instruction with a
forced memory bit
...
0) and turn on the output light
...
With input 1 forced high the output light will illuminate
...
The
element is also highlighted because it is on
...
48
Discrete Inputs/Outputs
To understand discrete control of a programmable controller the
same simple lamp circuit illustrated with forcing will be used
...
In this example
the lamp is off when the switch is open and on when the
switch is closed
...
49
The following drawing illustrates the sequence of events
...
A lamp is wired
to the output module
...
The CPU
scans the inputs
...
0 receives
a binary 0
...
0 to send a binary 0 to the output
module
...
When it finds the switch closed I0
...
This instructs Q0
...
Program Instruction
When the switch is open the CPU receives a logic 0 from input
I0
...
The CPU sends a logic 0 to output Q0
...
When the switch is closed the CPU receives a logic 1 from
input I0
...
The CPU sends a logic 1 to output Q0
...
0
...
50
Motor Starter Example
The following example involves a motor start and stop circuit
...
In this example a motor started (M) is wired in series with
a normally open momentary pushbutton (Start), a normally
closed momentary pushbutton (Stop), and the normally closed
contacts of an overload relay (OL)
...
51
This closes the associated M and Ma (auxiliary contact located
in the motor starter) contacts
...
The motor will run until the normally closed
Stop button is depressed, or the overload relay opens the OL
contacts, breaking the path of current flow to the motor starter
and opening the associated M and Ma contacts
...
52
Program Instruction
A normally open Start pushbutton is wired to the first input
(I0
...
1), and normally closed overload relay contacts (part
of the motor starter) are connected to the third input (I0
...
The first input (I0
...
1), and third input (I0
...
I0
...
I0
...
Output Q0
...
In addition, a normally open set
of contacts associated with Q0
...
A motor starter is connected to output
Q0
...
When the Start pushbutton is depressed the CPU receives a
logic 1 from input I0
...
This causes the I0
...
All three inputs are now a logic 1
...
0
...
53
When the Start pushbutton is pressed, output Q0
...
0 is
solved, the contact will close and output Q0
...
The motor will continue to run until the Stop pushbutton is
depressed
...
1 will now be a logic 0 (false)
...
0
...
54
When the Stop pushbutton is released I0
...
Expanding the Application
The application can be easily expanded to include indicator
lights for RUN and STOP conditions
...
1 and a STOP indicator
light is connected to output Q0
...
55
It can be seen from the ladder logic that a normally open output
Q0
...
1 and a normally
closed Q0
...
2 on network
3
...
0 is off
...
0 contacts on Network 2 are open and the RUN indicator,
connected to output Q0
...
The normally closed Q0
...
2 is on
...
0 is now a logic
high (On)
...
0 contacts on Network 2 now
switch to a logic 1 (closed) and output Q0
...
The normally closed Q0
...
2 is now off
...
3
...
An access door to the motor, or its
associated equipment, is one example of a limit switch’s use
...
3 are open and the motor will not start
...
Input I0
...
58
Expansion
The PLC program can be expanded to accommodate many
commercial and industrial applications
...
Overtravel limit switches can be added along with proximity
switches for sensing object position
...
The
applications are only limited by the number of I/Os and amount
of memory available on the PLC
...
Identify the following symbols:
a
...
____________
c
...
3
...
0 will be true (logic 1)
when ____________ or ____________ is true, and when
____________ is true