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STEP 2000
Basics of PLCs
Table of Contents
Introduction
...
4
Number Systems
...
14
Basic Requirements
...
28
Connecting External Devices
...
41
Discrete Inputs/Outputs
...
61
Timers
...
71
High-Speed Instructions
...
78
Review Answers
...
85
1
Introduction
Welcome to another course in the STEP 2000 series, Siemens
Technical Education Program, designed to prepare our
distributors to sell Siemens Energy & Automation products
more effectively
...
Upon completion of Basics of PLCs you should be able to:
•
•
Convert numbers from decimal to binary, BCD, and
hexadecimal
•
Identify typical discrete and analog inputs and outputs
•
Read a basic ladder logic diagram and statement list
•
Identify operational differences between different S7-200
models
•
Identify the proper manual to refer to for programming or
installation of an S7-200 PLC
•
Connect a simple discrete input and output to an S7-200
•
Select the proper expansion module for analog inputs and
outputs
•
2
Identify the major components of a PLC and describe
their functions
Describe the operation of timers and counters
This knowledge will help you better understand customer
applications
...
You should complete Basics of Electricity
before attempting Basics of PLCs
...
In addition you may wish to complete
Basics of Control Components
...
If you are an employee of a Siemens Energy & Automation
authorized distributor, fill out the final exam tear-out card and
mail in the card
...
Good luck with your efforts
...
Other trademarks are the property of their respective owners
...
They are
used in commercial and industrial applications
...
This course is meant
to supply you with basic information on the functions and
configurations of PLCs
...
An input accepts a
variety of digital or analog signals from various field devices
(sensors) and converts them into a logic signal that can be used
by the CPU
...
Output
modules convert control instructions from the CPU into a digital
or analog signal that can be used to control various field devices
(actuators)
...
These instructions determine what the PLC will do
for a specific input
...
Pushbuttons (sensors), in this simple example, connected to
PLC inputs, can be used to start and stop a motor connected to
a PLC through a motor starter (actuator)
...
This is often referred to as hardwired control
...
Electricians would then wire the components necessary to
perform a specific task
...
A change in function or system
expansion required extensive component changes and rewiring
...
Wiring between devices and relay contacts is done in
the PLC program
...
Modifying the application and
correcting errors are easier to handle
...
Following are just a few of the advantages of PLCs:
•
•
•
•
•
•
6
Smaller physical size than hard-wire solutions
...
PLCs have integrated diagnostics and override functions
...
Applications can be immediately documented
...
Siemens PLCs
Siemens makes several PLC product lines in the SIMATIC® S7
family
...
S7-200
The S7-200 is referred to as a micro PLC because of its small
size
...
The S7-200 can be used on
smaller, stand-alone applications such as elevators, car washes,
or mixing machines
...
S7-300 and S7-400
The S7-300 and S7-400 PLCs are used in more complex
applications that support a greater number of I/O points
...
The power supply and I/O
consist of separate modules connected to the CPU
...
Your Siemens sales
representative can provide you with additional information on
any of the Siemens PLCs
...
Sometimes binary digits are used individually and sometimes
they are used to represent numerical values
...
All number systems
have the same three characteristics: digits, base, weight
...
The binary system is used by programmable controllers
...
)
In the binary system 1s and 0s are arranged into columns
...
The first column has a binary weight of
0
2
...
This is referred to as the
least significant bit
...
The next column, for example, has a weight
1
of 2 , which is equivalent to a decimal 2
...
The number in the far left
hand column is referred to as the most significant bit
...
This
is equivalent to a decimal 128
...
1)
2)
3)
Search from least to most significant bit for 1s
...
Add the column values
...
The decimal value of the fourth column from
the right is 8, and the decimal value of the fifth column from
the right is 16
...
The sum of all the weighted columns that contain a 1 is the
decimal number that the PLC has stored
...
The decimal value of the fourth column from
the right is 8, and the decimal value of the sixth column from
the right is 32
...
Bits, Bytes, and Words
Each binary piece of data is a bit
...
Two bytes, or 16 bits, make up one word
...
The binary system is a
system in which there are only two numbers, 1 and 0
...
Binary 0
indicates that the signal is not present, or the switch is Off
...
BCD is
commonly used with input and output devices
...
The
binary numbers are broken into groups of four bits, each group
representing a decimal equivalent
...
10
Hexadecimal
Hexadecimal is another system used in PLCs
...
)
The ten digits of the decimal system are used for the first ten
digits of the hexadecimal system
...
A = 10
B = 11
C = 12
D = 13
E = 14
F = 15
The hexadecimal system is used in PLCs because it allows the
status of a large number of binary bits to be represented in a
small space such as on a computer screen or programming
device display
...
To convert a decimal number to a
hexadecimal number the decimal number is divided by the base
of 16
...
Twelve is
equivalent to C in hexadecimal
...
The decimal value of a hexadecimal number is obtained by
multiplying the individual hexadecimal digits by the base 16
weight and then adding the results
...
0
16 = 1
1
16 = 16
B = 11
11
Conversion of Numbers
12
The following chart shows a few numeric values in decimal,
binary, BCD, and hexadecimal representation
...
Identify the following:
2
...
3
...
4
...
In order to
understand the ideas and concepts of PLCs, an understanding
of these terms is necessary
...
Sensors are connected to
the input of a PLC
...
An electrical signal is sent
from the pushbutton to the PLC indicating the condition (open/
closed) of the pushbutton contacts
...
Actuators are connected to the PLC output
...
Depending on the output PLC signal the
motor starter will either start or stop the motor
...
Pushbuttons, toggle
switches, limit switches, proximity switches, and contact
closures are examples of discrete sensors which are connected
to the PLCs discrete or digital inputs
...
In
the OFF condition a discrete input may be referred to as a logic
0 or a logic low
...
One side of the pushbutton is connected to the first
PLC input
...
Many PLCs require a separate
power supply to power the inputs
...
This is the OFF condition
...
This is the ON condition
...
Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20
milliamps, or 0 to 10 volts
...
Depending on
the level transmitter, the signal to the PLC can either increase or
decrease as the level increases or decreases
...
Solenoids, contactor coils, and lamps are examples
of actuator devices connected to discrete outputs
...
In the
following example, a lamp can be turned on or off by the PLC
output it is connected to
...
The output may be as simple as a 0-10 VDC level that
drives an analog meter
...
The output signal may also
be used on more complex applications such as a current-topneumatic transducer that controls an air-operated flow-control
valve
...
The CPU monitors the inputs and makes decisions
based on instructions held in the program memory
...
17
Programming
A program consists of one or more instructions that accomplish
a task
...
There are several ways to look at a program such
as ladder logic, statement lists, or function block diagrams
...
Ladder logic uses components that resemble
elements used in a line diagram format to describe hard-wired
control
...
STEP 2000
Basics of
Control
Components
Ladder Logic Diagram
18
The left vertical line of a ladder logic diagram represents the
power or energized conductor
...
The right
vertical line, which represents the return path on a hard-wired
control line diagram, is omitted
...
Rungs are sometimes referred
to as networks
...
In the example program shown example I0
...
1 and Q0
...
If inputs I0
...
1 are energized, output relay Q0
...
The inputs could
be switches, pushbuttons, or contact closures
...
4, I0
...
1 represent the second instruction combination
...
4 or I0
...
1 energizes
...
The operation, what is to be done, is shown on the
left
...
A comparison between the statement
list shown below, and the ladder logic shown on the previous
page, reveals a similar structure
...
Function Block Diagrams
Function Block Diagrams (FBD) provide another view of a set of
instructions
...
Functions are indicated by a rectangle
...
The function block diagram shown below
performs the same function as shown by the ladder diagram
and statement list
...
A PLC scan starts with the CPU reading
the status of inputs
...
Once the program is completed, the
CPU performs internal diagnostics and communication tasks
...
The cycle time depends on the size of the program, the number
of I/Os, and the amount of communication required
...
Software includes the instructions or programs that
direct hardware
...
The PLC, the programming
device, and the connecting cable are examples of hardware
...
When talking
about computer or PLC memory, however, 1K means 1024
...
This can be
1024 bits, 1024 bytes, or 1024 words, depending on memory
type
...
Data can be written to and
read from RAM
...
RAM is volatile, meaning that the data stored in RAM will be
lost if power is lost
...
ROM
Read Only Memory (ROM) is a type of memory that data can
be read from but not written to
...
ROM
memory is nonvolatile
...
ROM is normally used to
store the programs that define the capabilities of the PLC
...
EPROMs are designed so that data
stored in them can be read, but not easily altered
...
UVEPROMs (ultraviolet
erasable programmable read only memory) can only be erased
with an ultraviolet light
...
Firmware
Firmware is user or application specific software burned into
EPROM and delivered as part of the hardware
...
21
Putting it Together
The memory of the S7-200 is divided into three areas: program
space, data space, and configurable parameter space
...
This area of memory controls
the way data space and I/O points are used
...
• Data space is used as a working area, and includes memory
locations for calculations, temporary storage of intermediate
results and constants
...
Data space can be
accessed under program control
...
22
Basic Requirements
In order to create or change a program, the following items are
needed:
•
•
•
•
PLC
PLC
Programming Device
Programming Software
Connector Cable
Throughout this course we will be using the S7-200 because of
its ease of use
...
The program for the S7-200 can be
created using a dedicated Siemens SIMATIC S7 programming
device, such as a PG 720 (not shown) or PG 740, if STEP 7
Micro/WIN software is installed
...
24
Software
A software program is required in order to tell the PLC what
instructions it must follow
...
A software package for one PLC, or one family
of PLCs, such as the S7 family, would not be useful on other
PLCs
...
The PG 720 and PG 740 have STEP
7 software pre-installed
...
Connector Cables PPI
(Point-to-Point Interface)
Connector cables are required to transfer data from the
programming device to the PLC
...
Communication between a Siemens programming
device and the S7-200 is referred to as PPI protocol (pointto- point interface)
...
The S7-200
uses a 9-pin, D-connector
...
Programming Device Cable
25
A special cable, referred to as a PC/PPI cable, is needed when a
personal computer is used as a programming device
...
DIP switches on
the PC/PPI cable are used to select an appropriate speed (baud
rate) at which information is passed between the PLC and the
computer
...
A switch or a pushbutton is a ____________ input
...
A lamp or a solenoid is an example of a ___________
output
...
The ____________ makes decisions and executes
control instructions based on the input signals
...
____________ ____________ is a PLC programming
language that uses components resembling elements
used in a line diagram
...
A ____________ consists of one or more instructions
that accomplish a task
...
Memory is divided into three areas: ____________ ,
____________ , and ____________ ____________ space
...
When talking about computer or PLC memory, 1K
refers to ____________ bits, bytes, or words
...
Software that is placed in hardware is called
____________
...
Which of the following is not required when creating or
changing a PLC program?
a
...
c
...
e
...
A special cable, referred to as a ____________ cable,
is needed when a personal computer is used as a
programming device