Unit 9: Describe the Operation of Programmable Logic Control

(PLC) and Distributed Control System (DCS)

Module 9.1
Describe the Operation of Programmable Logic Control (PLC)

Objectives
Terminal Objective
9.1.1: Given access to
Identify the major components of a PLC and manufacturer's
describe their functions. documentation, the
9.1.2: trainee will correctly
Convert numbers from decimal to binary, describe the operation of
BCD, and hexadecimal. Programmable Logic
9.1.3: System
Identify typical discrete and analog inputs and outputs.
9.1.4:
Read a basic ladder logic diagram & identify PLC terminology.
9.1.5:
Identify the basic requirements to create a PLC.
9.1.6:
Describe the operation of analog inputs & outputs.
9.1.7:
Describe the operation of timers and counters.
9.1.8:
Interpret programming a PLC.

Introduction

A PLC (i.e. Programmable Logic Controller) is a device that was

invented to replace the necessary sequential relay circuits for machine
control. The PLC works by looking at its inputs and depending upon their
state, turning on/off its outputs. The user enters a program, usually via
software, that gives the desired results.

PLCs are used in many "real world" applications. If there is an

industry present, chances are good that there is a plc present. If you are
involved in machining, packaging, material handling, automated assembly
or countless other industries you are probably already using them. If you
are not, you are wasting money and time. Almost any application that
needs some type of electrical control has a need for a plc.

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 For example, let's assume that when a switch turns on we want to
turn a solenoid on for 5 seconds and then turn it off regardless of how
long the switch is on for. We can do this with a simple external timer. But
what if the process included 10 switches and solenoids? We would need
10 external timers. What if the process also needed to count how many
times the switches individually turned on? We need a lot of external
counters.

As you can see the bigger the process the more of a need we have
for a PLC. We can simply program the PLC to count its inputs and turn the
solenoids on for the specified time.

Compared with electromechanical relay systems, PLC’s offer the

following additional advantages.

 Ease of programming and reprogramming in the plant.

 A programming language that is based on relay wiring symbols
familiar to most plant personnel.
 High reliability and minimum maintenance.
 Small physical size.
 Ability to communicate with computer systems in the plant.
 Moderate to low initial investment cost.
 Rugged construction.
 Ability to withstand harsh environments.
 Expandability.

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Objective 9.1.1
Identify the major components of a PLC and describe their
functions.

PLCs
Programmable Logic Controllers (PLCs), also referred to as programmable
controllers, are in the computer family. They are used in commercial and
industrial applications. A PLC monitors inputs, makes decisions based on
its program, and controls outputs to automate a process or machine. This
course is meant to supply you with basic information on the functions and
configurations of PLCs.

Basic PLC Operation

PLCs consist of input modules or points, a Central Processing Unit (CPU),
and output modules or points. 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.

The CPU makes decisions and executes control instructions based on

program instructions in memory. Output modules convert control
instructions from the CPU into a digital or analog signal that can be used
to control various field devices (actuators).

A programming device is used to input the desired instructions. These

instructions determine what the PLC will do for a specific input. An
operator interface device allows process information to be displayed and
new control parameters to be entered.

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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).

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Hard-Wired Control
Prior to PLCs, many of these control tasks were solved with contactor or
relay controls. This is often referred to as hardwired control. Circuit
diagrams had to be designed, electrical components specified and
installed, and wiring lists created.

Electricians would then wire the components necessary to perform a

specific task. If an error was made the wires had to be reconnected
correctly. A change in function or system expansion required extensive
component changes and rewiring.

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Advantages of PLCs
The same, as well as more complex tasks can be done with a PLC. Wiring
between devices and relay contacts is done in the PLC program. Hard-
wiring, though still required to connect field devices, is less intensive.
Modifying the application and correcting errors are easier to handle. It is
easier to create and change a program in a PLC than it is to wire and
rewire a circuit.
Following are just a few of the advantages of PLCs:
• Smaller physical size than hard-wire solutions.
• Easier and faster to make changes.
• PLCs have integrated diagnostics and override functions.
• Diagnostics are centrally available.
• Applications can be immediately documented.
• Applications can be duplicated faster and less expensively.

Siemens PLCs S7-200

Siemens makes several PLC product lines
in the SIMATIC® S7 family. They are:
S7-200, S7-300, and S7-400. The S7-
200 is referred to as a micro PLC because
of its small size. The S7-200 has a brick
design which means that the power
supply and I/O are on-board.

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The Siemens S7-200 can be used on smaller, stand-alone
applications such as elevators, car washes, or mixing machines. It can
also be used on more complex industrial applications such as bottling and
packaging machines.

Siemens PLCs 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.
Both PLCs are modular and expandable.
The power supply and I/O consist of separate
modules connected to the CPU. Choosing either
the S7-300 or S7-400 depends on the
complexity of the task and possible future
expansion. Your Siemens sales representative
can provide you with additional information on
any of the Siemens PLCs.

Siemens PLCs S7-S7-400

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Objective 9.1.2
Convert numbers from decimal to binary, BCD, and hexadecimal.

Number Systems
Since a PLC is a computer, it stores information in the form of ON or OFF
conditions (1 or 0), referred to as binary digits (bits).
Sometimes binary digits are used individually and sometimes they are
used to represent numerical values.

Decimal System
Various number systems are used by PLCs. All number systems have the
same three characteristics: digits, base, weight.

The decimal system, which is commonly used in everyday life, has the
following characteristics:

Ten digits: 010, 110, 210, 310, 410, 510, 610, 710, 810, 910, Base 10

Weights Powers of base 10: 105 104 103 102 101 100

Binary System
The binary system is used by programmable controllers.

The binary system has the following characteristics:

Two digits: 02, 12, Base 2

Weights Powers of base 2: 23 22 21 20

In the binary system 1s and 0s are arranged into columns. Each column is
weighted.
The first column has a binary weight of 20. This is equivalent to a decimal
1.

This is referred to as the least significant bit. The binary weight is doubled
with each succeeding column.

The next column, for example, has a weight of 21, which is equivalent to
a decimal 2.

The decimal value is doubled in each successive column. The number in

the far left hand column is referred to as the most significant bit.

In this example, the most significant bit has a binary weight of 27. This is
equivalent to a decimal 128.

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Most Significant Bit Last Significant
Bit

128=27 64=26 32=25 16=24 8=23 4=22 2=21

1=20
0 0 0 1 1 0 0 0

Converting Binary to Decimal

The following steps can be used to interpret a decimal number from a
binary value.
1) Search from least to most significant bit for 1s.
2) Write down the decimal representation of each column containing a 1.
3) Add the column values.

In the following example, the fourth and fifth columns from the right
contain a 1. 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 decimal
equivalent of this binary number is 24. The sum of all the weighted
columns that contain a 1 is the decimal number that the PLC has stored.

In the following example the fourth and sixth columns from the right
contain a 1. 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. The
decimal equivalent of this binary number is 40.

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Bits, Bytes, and Words
Each binary piece of data is a Bit.
Eight bits make up one Byte.
Two Bytes, or 16 bits, make up one word.

Logic 0, Logic 1

Programmable controllers can only

understand a signal that is ON or OFF
(present or not present).

The binary system is a system in which there

are only two numbers, 1 and 0. Binary 1
indicates that a signal is present, or the
switch is ON.

Binary 0 indicates that the signal is not

present, or the switch is OFF.

Binary-Coded Decimal (BCD)

Binary-Coded Decimal (BCD) are decimal numbers where each digit is
represented by a four-bit binary number.

BCD is commonly used with input and output devices. A thumbwheel

switch is one example of an input device that uses BCD.

The binary numbers are broken into groups of four bits, each group
representing a decimal equivalent. A four-digit thumbwheel switch, like
the one shown here, would control 16 (4 x 4) PLC inputs.

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Hexadecimal
Hexadecimal is another system used in PLCs. The hexadecimal system has
the following characteristics:
16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
Base 16
Weights Powers of base 16 (1, 16, 256, 4096 ...)

The ten digits of the decimal system are used for the first ten digits of the
hexadecimal system. The first six letters of the alphabet are used for the
remaining six digits.
A = 10 D = 13
B = 11 E = 14
C = 12 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. Each hexadecimal digit
represents the exact status of four binary bits.

To convert a decimal number to a hexadecimal number the decimal

number is divided by the base of 16.

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To convert decimal 28, for example, to hexadecimal:

Decimal 28 divided by 16 is 1 with a remainder of 12. Twelve is equivalent

to C in hexadecimal. The hexadecimal equivalent of decimal 2810 is 1C16.

To convert a hexadecimal number to decimal number is obtained by

multiplying the individual hexadecimal digits by the base 16
weight and then adding the results.
In the following example the hexadecimal number 2B16 is converted to its
decimal equivalent of 4310.
160 = 1
161 = 16
B = 11

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Conversion of Numbers
The following chart shows a few numeric values in decimal, binary, BCD,
and hexadecimal representation.

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Review 1

1. What is another name for programmable logical controllers (PLCs)?

2. Which one of the following is an advantage of PLCs?

a. Easier and faster to make changes

b. Diagnostics are hardly available
c. Applications can’t be duplicated
d. Larger size than Hard wired solutions

3. Which one of the S7 product families is referred to as a micro PLC?

a. S7 – 100
b. S7 – 200
c. S7 - 300
d. S7 – 400

4. Which one of the following is not a product line in the SIMATIC S7

family?
a. S7 - 100
b. S7 - 200
c. S7 - 300
d. S7 – 400

5. From which devices do inputs accept variety of digital or analog signals

a. CPU
b. Sensors
c. Actuators
d. Relays
6. Mention two 2 advantages of S7-300 & S7–400 PLCs?
a. Micro & modular
b. Rigid & modular
c. Small & expandable
d. Modular and expandable

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7. Mention the three characteristics of all number systems?
a. Digits, base, power
b. Base, power, function
c. Digits, base, weight
d. Weight, power, base

8. PLCs store the information in the form of:

a. Digits or numbers
b. On or off
c. Positive or negative
d. Analog or digital

3. What is the base of binary systems?

a. 10
b. 16
c. 8
d. 2

4. In the equivalent binary system of the decimal number 6410, what is

the weight of the most significant bit?
a. 27
b. 26
c. 28
d. 210

5. In the equivalent binary system of the decimal number 128, what is

the weight of the least significant bit?
a. 21
b. 2-1
c. 20
d. 22

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6. What does each binary piece of data represent?
a. One byte
b. 8 bits
c. A bit
d. One word

7. Eight (8) bits make:

a. One word
b. One bit
c. Two bytes
d. One byte

8. Two Bytes equal to:

a. 16 bits
b. One word
c. Eight bytes
d. One bit

9. 16 bits make up:

a. One binary
b. One word
c. One byte
d. Two bits

10. Convert the decimal number 10 to hexadecimal equivalent number?

a. 11
b. 7E
c. D
d. A

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11. Convert the decimal number 16 to BCD representation?
a. 0001 0110
b. 1 0000
c. 10
d. 1FE

12. Convert the decimal number 127 to binary representative?

a. 0001 0010 0111
b. 111 1111
c. 7F
d. 12

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Objective 9.1.3
Identify typical discrete and analog inputs and outputs.

Terminology
The language of PLCs consists of a commonly used set of terms; many of
which are unique to PLCs. In order to understand the ideas and concepts
of PLCs, an understanding of these terms is necessary.

Sensor
A sensor is a device that converts a physical condition into an electrical
signal for use by the PLC. Sensors are connected to the input of a PLC. A
pushbutton is one example of a sensor that is connected to the PLC input.
An electrical signal is sent from the pushbutton to the PLC indicating the
condition (open/ closed) of the pushbutton contacts.

Actuators
Actuators convert an electrical signal from the PLC into a physical
condition. Actuators are connected to the PLC output.
A motor starter is one example of an actuator that is connected to the PLC
output. Depending on the output PLC signal the motor starter will either
start or stop the motor.

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Discrete Input
A discrete input also referred to as a digital input, is an input that is either
in an ON or OFF condition.
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 ON condition a discrete input may

be referred to as logic 1 or logic high.

In the OFF condition a discrete input may

be referred to as logic 0 or a logic low.

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A Normally Open (NO) pushbutton is used in the following example. One
side of the pushbutton is connected to the first PLC input. The other side
of the pushbutton is connected to an internal 24 VDC power supply. Many
PLCs require a separate power supply to power the inputs.

In the open state, no voltage is present at

the PLC input. This is the OFF condition.
When the pushbutton is depressed, 24 VDC is
applied to the PLC input.
This is the ON condition.

Analog Inputs
An analog input is an input signal that has a continuous signal.
Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20
milliamps, or 0 to 10 volts.

In the following example, a level transmitter monitors the level of liquid in

a tank.

Depending on the level transmitter, the signal to the PLC can either
increase or decrease as the level increases or decreases.

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Discrete Outputs
A discrete output is an output that is either in an ON or OFF condition.

Solenoids, contactor coils, and lamps are examples of actuator

devices connected to discrete outputs.

Discrete outputs may also be referred to as digital outputs.

In the following example, a lamp can be turned on or off by the PLC

output it is connected to.

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Analog Outputs
An analog output is an output signal that has a continuous signal. The
output may be as simple as a 0-10 VDC level that drives an analog meter.

Examples of analog signals meter outputs are speed, weight, and

temperature.

The output signal may also be used on more complex applications such as
a current-to pneumatic transducer that controls an air-operated flow-
control valve.

CPU
The central processor unit (CPU) is a microprocessor system that contains
the system memory and is the PLC decision making unit.

The CPU monitors the inputs and makes decisions based on instructions
held in the program memory. The CPU performs relay, counting, timing,
data comparison, and sequential operations.

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Objective 9.1.4
Read a basic ladder logic diagram & identify PLC terminology.

Ladder Logic
A program consists of one or more instructions that accomplish a task.
Programming a PLC is simply constructing a set of instructions. There are
several ways to look at a program such as ladder logic, statement lists, or
function block diagrams.
Ladder logic (LAD) is one programming language used with PLCs. Ladder
logic uses components that resemble elements used in a line diagram
format to describe hard-wired control. Refer to the STEP 2000 course
Basics of Control Components for more information on line diagrams.

Ladder Logic Diagram

The left vertical line of a ladder logic diagram represents the power or
energized conductor. The output element or instruction represents the
neutral or return path of the circuit. The right vertical line, which
represents the return path on a hard-wired control line diagram, is
omitted. Ladder logic diagrams are read from left-to-right, top-to-bottom.
Rungs are sometimes referred to as networks. A network may have
several control elements, but only one output coil.

In the example program shown example I0.0, I0.1 and Q0.0 represent the
first instruction combination. If inputs I0.0 and I0.1 are energized, output
relay Q0.0 energizes. The inputs could be switches, pushbuttons, or
contact closures. I0.4, I0.5, and Q1.1 represent the second instruction
combination. If either input I0.4 or I0.5 are energized, output relay Q0.1
energizes.

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PLC Scan
The PLC program is executed as part of a repetitive process referred to as
a scan. A PLC scan starts with the CPU reading the status of inputs. The
application program is executed using the status of the inputs. Once the
program is completed, the CPU performs internal diagnostics and
communication tasks.

The scan cycle ends by updating the

outputs, then starts over.
The cycle time depends on the size of
the program, the number of I/Os, and
the amount of communication
required.

Software
Software is any information in a form
that a computer or PLC can use.
Software includes the instructions or
programs that direct hardware.

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Hardware
Hardware is the actual
equipment. The PLC, the
programming device, and
the connecting cable are
examples of hardware.

Memory Size
Kilo, abbreviated K, normally refers to 1000 units. When talking about
computer or PLC memory, however, 1K means 1024.
This is because of the binary number system (210=1024). This can be
1024 bits, 1024 bytes, or 1024 words, depending on memory type.

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RAM
Random Access Memory (RAM) is memory
where data can be directly accessed at any
address. Data can be written to and read from
RAM. RAM is used as a temporary storage area.
RAM is volatile, meaning that the data stored in
RAM will be lost if power is lost. A battery
backup is required to avoid losing data in the
event of a power loss.

ROM
Read Only Memory (ROM) is a type of memory that
data can be read from but not written to. This
type of memory is used to protect data or
programs from accidental erasure. ROM memory is
nonvolatile. This means a user program will not
lose data during a loss of electrical power. ROM is
normally used to store the programs that define
the capabilities of the PLC.

EPROM
Erasable Programmable Read Only Memory
(EPROM) provides some level of security against
unauthorized or unwanted changes in a program.
EPROMs are designed so that data stored in
them can be read, but not easily altered.
Changing EPROM data requires a special effort.
UVEPROMs (ultraviolet erasable
programmable read only memory) can only be
erased with an ultraviolet light. EEPROM
(electronically erasable programmable read only
memory), can only be erased electronically.

Firmware
Firmware is user or application specific software burned into EPROM and
delivered as part of the hardware. Firmware gives the PLC its basic
functionality.

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Putting it together
The memory of the S7-200 is divided into three areas: program space,
data space, and configurable parameter space.
• Program space stores the ladder logic (LAD) or statement list (STL)
program instructions. This area of memory controls the way data space
and I/O points are used. LAD or STL instructions are written using a
programming device such as a PC, then loaded into program memory of
the PLC.
• Data space is used as a working area, and includes memory locations for
calculations, temporary storage of intermediate results and constants.
Data space includes memory locations for devices such as timers,
counters, high-speed counters, and analog inputs and outputs. Data space
can be accessed under program control.
• Configurable parameter space, or memory, stores either the default or
modified configuration parameters.

Basic Requirements

In order to create or change a program, the following items are needed:

• PLC
• Programming Device
• Programming Software
• Connector Cable

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PLC
Throughout this course we will be
using the S7-200 because of its
ease of use.

Programming Devices
The program is created in a programming device (PG) and then
transferred to the PLC. 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.

A personal computer (PC), with STEP 7 Micro/WIN installed, can also be

used as a programming device with the S7-200.

Software
A software program is required in order to tell the
PLC what instructions it must follow. Programming
software is typically PLC specific. A software
package for one PLC, or one family of PLCs, such
as the S7 family, would not be useful on other
PLCs. The S7-200 uses a Windows based software
program called STEP 7-Micro/WIN32. The PG 720
and PG 740 have STEP 7 software pre-installed.
Micro/WIN32 is installed on a personal computer in
a similar manner to any other computer software.

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Connector Cables PPI (Point-to-Point Interface)
Connector cables are required to transfer data from the programming
device to the PLC.
Communication can only take place when the two devices speak the same
language or protocol.
Communication between a
Siemens programming device
and the S7-200 is referred to
as PPI protocol (point -to- point
interface).
An appropriate cable is required
for a programming device such
as a PG 720 or PG 740.

The S7-200 uses a 9-pin, D-connector. This is a

straight-through serial device that is compatible
with Siemens programming devices (MPI port)
and is a standard connector for other serial
interfaces.

A special cable, referred to as a PC/PPI

cable, is needed when a personal computer
is used as a programming device.

This cable allows the serial interface of the

PLC to communicate with the RS-232 serial
interface of a personal computer.

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.

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Review 2

1. A switch or a pushbutton is a ____________ input.

2. A lamp or a solenoid is an example of a ___________ output.

3. The ____________ makes decisions and executes control instructions

based on the input signals.

4. ____________ ____________ is a PLC programming language that

uses components resembling elements used in a line diagram.

5. A ____________ consists of one or more instructions that accomplish a

task.

6. Memory is divided into three areas: ____________, ____________,

and ____________ ____________ space.

7. When talking about computer or PLC memory, 1K refers to

____________ bits, bytes, or words.

8. Software that is placed in hardware is called ____________.

9. Which of the following is not required when creating or changing a PLC

program?
a. PLC
b. Programming Device
c. Programming Software
d. Connector Cable
e. Printer

10. A special cable, referred to as a ____________ cable, is needed when

a personal computer is used as a programming device.

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Objective 9.1.5
Identify the basic requirements to create a PLC .

S7-200 Micro PLCs

The S7-200 Micro PLC is the smallest member of the SIMATIC S7 family of
programmable controllers. The central processing unit (CPU) is internal to
the PLC. Inputs and outputs (I/O) are the system control points. Inputs
monitor field devices, such as switches and sensors. Outputs control other
devices, such as motors and pumps. The programming port is the
connection to the programming device.

S7-200 Models
There are four S7-200 CPU types: S7-221, S7-222, S7-224, S7-226, and
S7-226XM and three power supply configurations for each type.

Models Power Supply Input Types Output Tyoes

Discretion
CPU 221
CPU 221

CPU 221 DC/DC/DC 6 Inputs/4 Outputs 6ES7 211--0AA23--0XB0

CPU 221 AC/DC/Relay 6 Inputs/4 Relays 6ES7 211--0BA23--0XB0
CPU 222 DC/DC/DC 8 Inputs/6 Outputs 6ES7 212--1AB23--0XB0
CPU 222 AC/DC/Relay 8 Inputs/6 Relays 6ES7 212--1BB23--0XB0
CPU 224 DC/DC/DC 14 Inputs/10 Outputs 6ES7 214--1AD23--0XB0
CPU 224 AC/DC/Relay 14 Inputs/10 Relays 6ES7 214--1BD23--0XB0
CPU 224XP DC/DC/DC 14 Inputs/10 Outputs 6ES7 214--2AD23--0XB0
CPU 224XP AC/DC/Relay 14 Inputs/10 Relays 6ES7 214--2BD23--0XB0
CPU 226 DC/DC/DC 24 Inputs/16 Outputs 6ES7 216--2AD23--0XB0
CPU 226 AC/DC/Relay 24 Inputs/16 Relays

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The model description indicates the type of CPU, the power supply, the
type of input, and the type of output.

S7-200 Features
The S7-200 family includes a wide variety of CPUs and features.
This variety provides a range of features to aid in designing a cost-
effective automation solution. The following table provides a summary of
the major features, many of which will be covered in this course.

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Mode Switch and Analog Adjustment
When the mode switch is in the RUN position the CPU is in the run mode
and executing the program. When the mode switch is in the STOP position
the CPU is stopped. When the mode switch is in the TERM position the
programming device can select the operating mode.

The analog adjustment is used to increase or decrease values stored in

special memory. These values can be used to update the value of a timer
or counter, or can be used to set limits.

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Optional Cartridge
The S7-200 supports an optional memory cartridge that provides a
portable EEPROM storage for your program. The cartridge can be used to
copy a program from one S7-200 PLC to a like S7-200 PLC.

In addition, two other cartridges are available. A real-time clock with

battery is available for use on the S7-221 and S7-222. The battery
provides up to 200 days of data retention time in the event of a power
loss. The S7-224 and S7-226 have a real-time clock built in. Another
cartridge is available with a battery only.

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Expansion Modules
The S7-200 PLCs are expandable. Expansion modules contain additional
inputs and outputs. These are connected to the base unit using a ribbon
connector.

The ribbon connector is protected by a cover on the base unit.

Side-by-side mounting completely encloses and protects the ribbon
connector.

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Available Expansion
The S7-221 comes with 6 digital inputs and 4 digital outputs.
These are not expandable. The S7-222 comes with 8 digital inputs and 6
digital outputs. The 222 will accept up to 2 expansion modules. The S7-
224 comes with 14 digital inputs and 10 digital outputs. The 224 will
accept up to 7 expansion modules. The S7-226 and S7-226XM come with
24 digital inputs and 16 digital outputs. The 226 and 226XM will accept up
to 7 expansion modules.

Status Indicators
The CPU status indicators reflect the current mode of CPU operation. If,
for example, the mode switch is set to the RUN position, the green RUN
indicator is lit. When the mode switch is set to the STOP position, the
yellow STOP indicator is lit.

The I/O status indicators represent the On or Off status of corresponding

inputs and outputs. When the CPU senses an input is on, the
corresponding green indicator is lit.

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Installing
The S7-200 can be installed in one of two ways. A DIN clip allows
installation on a standard DIN rail. The DIN clip snaps open to allow
installation and snaps closed to secure the unit on the rail. The S7-200
can also be panel mounted using installation holes located behind the
access covers.

External Power Supply

An S7-200 can be connected to either a 24 VDC or a 120/230 Sources
VAC power supply depending on the CPU. An S7-200 DC/DC/ DC would be
connected to a 24 VDC power supply.

An S7-200 AC/DC/Relay would be connected to a 120 or 230 VAC power

supply.

37 PCSTP
I/O Numbering
S7-200 inputs and outputs are labeled at the wiring terminations and next
to the status indicators. These alphanumeric symbols identify the I/O
address to which a device is connected. This address is used by the CPU
to determine which input is present and which output needs to be turned
on or off. I designates a discrete input and Q designates a discrete
output. The first number identifies the byte, the second number identifies
the bit. Input I0.0, for example, is byte 0, bit 0.
I0.0 = Byte 0, Bit 0
I0.1 = Byte 0, Bit 1
I1.0 = Byte 1, Bit 0
I1.1 = Byte 1, Bit 1
The following table identifies the input and output designations.

Inputs
Input devices, such as switches, pushbuttons, and other sensor devices
are connected to the terminal strip under the bottom cover of the PLC.

38 PCSTP
Input Simulator
A convenient method of testing a program is to wire toggle switches to
the inputs. Input simulators with prewired toggle switches are available
for the S7-200s. Switches are wired between the 24 VDC power supply
(L+) and the inputs. For example, the switch on the far left is wired
between the first input (0.0) and L+. When the switch is closed, 24 VDC is
applied to the input. This is referred to as a logic 1. When the switch is
open, 0 VDC is applied to the input. This is referred to as a logic 0.

Outputs
Output devices, such as relays, are connected to the terminal strip under
the top cover of the PLC. When testing a program, it is not necessary to
connect output devices. The LED status indicators signal if an output is
active.

39 PCSTP
Optional Connector
An optional fan-out connector allows for field wiring connections to remain
fixed when removing or replacing an S7-221 or 222.
The appropriate connector slides into either the input, output, or
expansion module terminals.

Removable Terminal Strip

The S7-224 and S7-226 do not have an optional fan-out connector.
Instead, the terminal strips are removable. This allows the field wiring
connections to remain fixed when removing or replacing the S7-224 and
S7-226.

Super Capacitor
A super capacitor, so named because of its ability to maintain a charge for
a long period of time, protects data stored in RAM in the event of a power
loss. The RAM memory is typically backed up on the S7-221 and 222 for
50 hours, and on the S7-224 and 226 for 190 hours.

40 PCSTP
Review 3
1. The five models of S7-200 are ____________ , ____________ ,
____________ , ____________, and ____________ .

2. Which of the following is not available on an S7-221?

a. Mode Switch
b. Expansion Port
c. Programming Port
d. Status Indicators

3. An S7-222 can have a maximum of ____________ expansion modules

and an S7-224 can have a maximum of ____________ expansion
modules.

4. An S7-222 has ____________ DC inputs and ____________ DC

outputs.

5. An S7-224 has ____________ DC inputs and ____________ DC

outputs.

6. The fourth output of an S7-200 would be labeled ____________ .

7. S7-200 can be panel mounted or installed on a ____________ rail.

41 PCSTP
Connecting External Devices

TD200
The S7-200 programming port can be used to communicate with a variety
of external devices. One such device is the TD200 text display unit. The
TD200 displays messages read from the S7-200, allows adjustment of
designated program variables, provides the ability to force, and permits
setting of the time and date. The TD200 can be connected to an external
power supply or receive its power from the S7-200.

Freeport Mode
The programming port has a mode called freeport mode.
Freeport mode allows connectivity to various intelligent sensing devices
such as a bar code reader.

42 PCSTP
Printer
Freeport mode can also be used to connect to a non-SIMATIC printer.

Interconnection
It is possible to use one programming device to address multiple S7-200
devices on the same communication cable. A total of 31 units can be
interconnected without a repeater.

43 PCSTP
Objective 9.1.6
Describe the operation of analog inputs & outputs.

Discrete Inputs/Outputs
To understand discrete control of a programmable controller the same
simple lamp circuit illustrated with forcing will be used. This is only for
instructional purposes as a circuit this simple would not require a
programmable controller. In this example the lamp is off when the switch
is open and on when the switch is closed.

Wiring
To accomplish this task, a switch is wired to the input of
the PLC and an indicator light is wired to output terminal.

The following drawing illustrates the sequence of events. A switch is wired

to the input module of the PLC. A lamp is wired to the output module. The
program is in the CPU. The CPU scans the inputs. When it finds the switch
open I0.0 receives a binary 0. This instructs Q0.0 to send a binary 0 to
the output module. The lamp is off. When it finds the switch closed I0.0
receives a binary 1. This instructs Q0.0 to send a binary 1 to the output
module, turning on the
lamp.

44 PCSTP
Program Instruction
When the switch is open the CPU receives a logic 0 from input I0.0. The
CPU sends a logic 0 to output Q0.0 and the light is off.

Motor Starter Example

The following example involves a motor start and stop circuit. The line
diagram illustrates how a normally open and a normally closed pushbutton
might be used in a control 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).

Momentarily depressing the

Start pushbutton completes the
path of current flow and
energizes the motor starter
(M).

45 PCSTP
This closes the associated M and Ma (auxiliary contact located in the
motor starter) contacts. When the Start button is released a holding
circuit exists to the M contactor through the auxiliary contacts Ma. 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.

This control task can also be accomplished with a PLC.

46 PCSTP
Program Instruction
A normally open Start pushbutton is wired to the first input (I0.0), a
normally closed Stop pushbutton is wired to the second input (I0.1), and
normally closed overload relay contacts (part of the motor starter) are
connected to the third input (I0.2). The first input (I0.0), second input
(I0.1), and third input (I0.2) form an AND circuit and are used to control
normally open programming function contacts on Network 1. I0.1 status
bit is a logic 1 because the normally closed (NC) Stop Pushbutton is
closed. I0.2 status bit is a logic 1 because the normally closed
(NC) overload relay (OL) contacts are closed. Output Q0.0 is also
programmed on Network 1. In addition, a normally open set of contacts
associated with Q0.0 is programmed on Network 1 to form an OR circuit.
A motor starter is connected to output Q0.0.

When the Start pushbutton is depressed the CPU receives a logic 1 from
input I0.0. This causes the I0.0 contact to close. All three inputs are now
a logic 1. The CPU sends a logic 1 to output Q0.0. The motor starter is
energized and the motor starts.

47 PCSTP
When the Start
pushbutton is
pressed, output
Q0.0 is now true
and on the next
scan, when
normally open
contact Q0.0 is
solved, the contact
will close and
output Q0.0 will
stay on even if the
Start pushbutton
has been released.

The motor will

continue to run
until the Stop
pushbutton is
depressed. Input
I0.1 will now be
a logic 0 (false).
The CPU will
send a binary 0
to output Q0.0.
The motor will
turn off.

When the Stop

pushbutton is
released I0.1
logic function
will again be
true and the
program ready
for the next time
the Start
pushbutton is
pressed.

48 PCSTP
Expanding the Application
The application can be easily
expanded to include indicator lights
for RUN and STOP conditions. In
this example a RUN indicator light
is connected to output Q0.1 and a
STOP indicator light is connected to
output Q0.2.

It can be seen from the

ladder logic that a normally
open output Q0.0 is
connected on Network 2 to
output Q0.1 and a normally
closed Q0.0 contact is
connected to output Q0.2 on
network 3. In a stopped
condition output Q0.0 is off.
The normally open Q0.0
contacts on Network 2 are
open and the RUN indicator,
connected to output Q0.1
light is off. The normally
closed Q0.1 on Network 3
lights are closed and the STOP indicator light, connected to output Q0.2 is
on.

When the PLC starts the

motor output Q0.0 is now a
logic high (On). The normally
open Q0.0 contacts on
Network 2 now switch to a
logic 1 (closed) and output
Q0.1 turns the RUN indicator
on. The normally closed Q0.0
contacts on Network 3 switch
to a logic 0 (open) and the
STOP indicator light connected
to output Q0.2 is now off.

49 PCSTP
Adding a Limit Switch
The application can be
further expanded by adding
a limit switch with normally
open contacts to input I0.3.

A limit switch could be used

to stop the motor or
prevent the motor from
being started. An access
door to the motor, or its
associated equipment, is
one example of a limit
switch’s use. If the access
door is open, the normally
open contacts of LS1
connected to input I0.3 are
open and the motor will not
start.

When the access door is

closed, the normally open
contacts on the limit
switch (LS1) are closed.
Input I0.3 is now on
(logic 1), and the motor
will start when the Start
pushbutton is pressed.

50 PCSTP
Expansion
The PLC program can be expanded to
accommodate many commercial and
industrial applications. Additional
Start/Stop pushbuttons and indicator
lights can be added for remote
operation, or control of a second motor
starter and motor.
Over travel limit switches can be added
along with proximity switches for
sensing object position. In addition,
expansion modules can be added to
further increase the I/O capability. The
applications are only limited by the
number of I/Os and amount of memory
available on the PLC.

Review

51 PCSTP
Objective 9.1.7
Describe the operation of timers and counters.

Timers
Timers are devices that count increments of time. Traffic lights are one
example where timers are used. In this example timers are used to control the
length of time between signal changes.

Timers are represented by boxes in ladder logic. When a timer receives an

enable, the timer starts to time. The timer compares its current time with the
preset time. The output of the timer is a logic 0 as long as the current time is
less than the preset time. When the current time is greater than the preset
time the timer output is a logic 1. S7-200 uses three types of timers: On-
Delay (TON), Retentive On-Delay (TONR), and Off-Delay (TOF).

S7-200 Timers
S7-200 timers are provided with resolutions of 1 millisecond, 10 milliseconds,
and 100 milliseconds. The maximum value of these timers is 32.767 seconds,
327.67 seconds, and 3276.7 seconds, respectively. By adding program
elements, logic can be programmed for much greater time intervals.

Hard-Wired Timing Circuit

Timers used with PLCs can be compared to timing circuits used in hard-wired
control line diagrams. In the following example, a normally open (NO) switch
(S1) is used with a timer (TR1). For this example the timer has been set for 5
seconds. When S1 is closed, TR1 begins timing. When 5 seconds have
elapsed,
TR1 will close its associated normally open TR1 contacts, illuminating pilot
light PL1. When S1 is open, de-energizing TR1, the TR1 contacts open,
immediately extinguishing PL1. This type of timer is referred to as ON delay.
ON delay indicates that once a timer receives an enable signal, a

52 PCSTP
predetermined amount of time (set by the timer) must pass before the timer’s
contacts change state.

On-Delay (TON)
When the On-Delay timer (TON) receives an enable (logic 1) at its input (IN),
a predetermined amount of time (preset time - PT) passes before the timer bit
(T-bit) turns on. The T-bit is a logic function internal to the timer and is not
shown on the symbol. The timer resets to the starting time when the enabling
input goes to a logic 0.

In the following simple timer example, a switch is connected to input I0.3, and
a light is connected to output Q0.1.

When the switch is closed input 4 becomes a logic 1, which is loaded into
timer T37. T37 has a time base of 100 ms (.100 seconds). The preset time

53 PCSTP
(PT) value has been set to 150. This is
equivalent to 15 seconds (.100 x 150 ).
The light will turn on 15 seconds after the
input switch is closed. If the switch were
opened before 15 seconds had passed,
then reclosed, the timer would again
begin timing at 0.

A small sample of the flexibility of PLCs is

shown in the following program logic. By
reprogramming the T37 contact as a
normally closed contact, the function of
the circuit is changed to cause the
indicator light to turn off only when the
timer times out. This function change was
accomplished without changing or
rewiring I/O devices.

Retentive On-Delay (TONR)

The Retentive On-Delay timer (TONR) functions in a similar
manner to the On-Delay timer (TON). There is one difference.
The Retentive On-Delay timer times as long as the enabling
input is on, but does not reset when the input goes off. The
timer must be reset with a RESET (R) instruction.

The same example used with the On-Delay timer will be used with the
Retentive On-Delay timer. When the switch is
closed at input I0.3, timer T5 (Retentive timer)
begins timing. If, for example, after 10 seconds
input I0.3 is opened the timer stops. When input
I0.3 is closed the timer will begin timing at 10
seconds. The light will turn on 5 seconds after input
I0.3 has been closed the second time. A RESET
(R) instruction can be added. Here a pushbutton is
connected to input I0.2. If after 10 seconds input
I0.3 were opened, T5 can be reset by momentarily
closing input I0.2. T5 will be reset to 0 and begin timing from 0 when input I0.3
is closed again.

54 PCSTP
Off-Delay (TOF)
The Off-Delay timer is used to delay an output off for a fixed period of time
after the input turns off. When the enabling bit turns on the timer bit turns on
immediately and the value is set to 0.When the input turns off, the timer
counts until the preset time has elapsed before the timer bit turns off.

S7-200 Timers
The S7-200s have 256 timers. The specific T number chosen for the timer
determines its time base and whether it is TON, TONR, or TOF.

Timer Example
In the following example a tank will be filled with two chemicals, mixed, and
then drained. When the Start Button is pressed at input I0.0, the program
starts pump 1 controlled by output Q0.0. Pump 1 runs for 5 seconds, filling the
tank with the first chemical, then shuts off. The program then starts pump 2,
controlled by output Q0.1. Pump 2 runs for 3 seconds filling the tank with the
second chemical. After 3 seconds pump 2 shuts off. The program starts the
mixer motor, connected to output Q0.2 and mixes the two chemicals for 60
seconds. The program then
opens the drain valve
controlled by output Q0.3,
and starts pump 3
controlled by output Q0.4.
Pump 3 shuts off after 8
seconds and the process
stops. A manual Stop
switch is also provided at
input I0.1.

55 PCSTP
Review 5

1. Analog signals are converted into a ____________ format by the PLC.

2. Three types of timers available in the S7-200 are On- Delay,

____________ On-Delay, and ____________- Delay.

3. The maximum time available on a 100 millisecond time base timer is

____________ seconds.

4. A count of 25 on a 10 millisecond time base timer represents a time of

__________ milliseconds.

5. There are ____________ timers in the S7-200.

Counters

Counters used in PLCs serve the same function as mechanical counters.

Counters compare an accumulated value to a preset value to control circuit
functions. Control applications that commonly use counters include the
following:
• Count to a preset value and cause an
event to occur
• Cause an event to occur until the
count reaches a preset value
A bottling machine, for example, may
use a counter to count bottles into
groups of six for packaging.

Counters are represented by boxes in ladder logic. Counters increment/

decrement one count each time the input transitions from off (logic 0) to on
(logic 1). The counters are reset when a RESET instruction is executed. S7-
200 uses three
types of counters:
up counter (CTU),
down counter
(CTD), and
up/down counter
(CTUD).

56 PCSTP
S7-200 Counters
There are 256 counters in the S7-200, numbered C0 through C255. The same
number cannot be assigned to more than one counter. For example, if an up
counter is assigned number 45, a down counter cannot also be assigned
number 45. The maximum count value of a counter is ±32,767.

Up Counter
The up counter counts up from a current value to a
preset value (PV). Input CU is the count input. Each time
CU transitions from a logic 0 to a logic 1 the counter
increments by a count of 1. Input R is the reset. A preset
count value is stored in PV input. If the current count is
equal to or greater than the preset value stored in PV,
the output bit (Q) turns on (not shown).

Down Counter
The down counter counts down from the preset value
(PV) each time CD transitions from a logic 0 to a logic
1. When the current value is equal to zero the counter
output bit (Q) turns on (not shown). The counter
resets and loads the current value with the preset
value (PV) when the load input (LD) is enabled.

Up/Down Counter
The up/down counter counts up or down from the preset value each time
either CD or CU transitions from a logic 0 to a logic 1. When the current value
is equal to the preset value, the output QU turns on. When the current value
(CV) is equal to zero, the output QD turns on. The counter loads the current
value (CV) with the preset value (PV) when the load input (LD) is enabled.

Similarly, the counter resets and loads the current

value (CV) with zero when the reset (R) is enabled.
The counter stops counting when it reaches preset
or zero.

57 PCSTP
Objective 9.1.8
Interpret programming a PLC.

Programming A PLC
STEP 7-Micro/WIN32 is the program software used with the S7-200 PLC to
create the PLC operating program. STEP 7 consists of a number of
instructions that must be arranged in a logical order to obtain the desired
PLC operation. These instructions are divided into three groups: standard
instructions, special instructions, and high-speed instructions.

Standard Instructions
Standard instructions consist of instructions that are found in most
programs. Standard instructions include; timer, counter, math, logical,
increment/decrement/invert, move, and block instructions.

Special Instructions
Special instructions are used to manipulate data. Special instructions
include shift, table, find, conversion, for/next, and real-time instructions.

High-Speed Instructions
High-speed instructions allow for events and interrupts to occur
independent of the PLC scan time. These include high-speed counters,
interrupts, output, and transmit instructions. It is not the purpose of this
text to explain all of the instructions and capabilities. A few of the more
common instructions necessary for a basic understanding of PLC operation
will be discussed. PLC operation is limited only by the hardware
capabilities and the ingenuity of the person programming it.

Refer to the SIMATIC S7-200 Programmable Controller System

Manual for detailed information concerning these instructions.

Micro/WIN32
The programming software can be run Off-line or On-line. Offline
programming allows the user to edit the ladder diagram and perform a
number of maintenance tasks. The PLC does not need to be connected to
the programming device in this mode. On-line programming requires the
PLC to be connected to the programming device. In this mode program
changes are downloaded to the PLC. In addition, status of the
input/output elements can be monitored. The CPU can be started,
stopped, or reset.

58 PCSTP
Symbols
In order to understand the instructions a PLC is to carry out, an
understanding of the language is necessary. The language of PLC ladder
logic consists of a commonly used set of symbols that represent control
components and instructions.

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. Two of the most common
programming functions are the normally open (NO) contact and the
normally closed (NC) contact. Symbolically, power flows through these
contacts when they are closed. The normally open contact (NO) is true
(closed) when the input or output status bit controlling the contact is 1.
The normally closed contact (NC) is true (closed) when the input or output
status bit controlling the contact is 0.

Coils
Coils represent relays that are energized when power flows to them. 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. That same
output status bit may be used to control normally open and normally
closed contacts elsewhere in the program.

Boxes
Boxes represent various instructions or functions that are executed when
power flows to the box. Typical box functions are timers, counters, and
math operations.

59 PCSTP
Entering Elements
Control elements are entered in the ladder diagram by positioning the
cursor and selecting the element from a list. In the following example the
cursor has been placed in the position to the right of I0.2. A coil was
selected from a pull-down list and inserted in this position.

An AND Operation
Each rung or network on a ladder represents a logic operation. The
following programming example demonstrates an AND operation. Two
contact closures and one output coil are placed on network 1. They were
assigned addresses I0.0, I0.1, and Q0.0. Note that in the statement list a
new logic operation always begins with a load instruction (LD). In this
example I0.0 (input 1) and (A in the statement list) I0.1 (input 2) must
be true in order for output Q0.0 (output 1) to be true. It can also be seen
That I0.0 and I0.1 must be true for Q0.0 to be true by looking at the
function block diagram representation.

60 PCSTP
Another way to see how an AND function works is with a Boolean logic
diagram. In Boolean logic an AND gate is represented by a number of
inputs on the left side. In this case there are two inputs. The output is
represented on the right side. 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.

An OR Operation
In this example an OR operation is used in network 1. It can be seen that
if either input I0.2 (input 3) or (O in the statement list) input I0.3 (input
4), or both are true, then output Q0.1 (output 2) will be true.

Another way to see how an OR function works is with a Boolean logic

diagram. The symbol differs slightly from an AND function. The OR
function is represented by a number of inputs on the left side. In this case
there are two inputs. The output is represented on the right side. It can
be seen from the table that any input can be a logic 1 in order for the
output to be a logic 1.

61 PCSTP
Testing a Program
Once a program has been written it needs
to be tested and debugged. One way this
can be done is to simulate the field inputs
with an input simulator, such as the one
made for the S7-200. The program is first
downloaded from the programming device
to the CPU. The selector switch is placed
in the RUN position. The simulator
switches are operated and the resulting
indication is observed on the output status indicator lamps.

Status Functions
After a program has been loaded and is running in the PLC, the actual
status of ladder elements can be monitored using STEP 7 Micro/WIN32
software. The standard method of showing a ladder element is by
indicating the circuit condition it produces when the device is in the de-
energized or non-operated state. In the following illustration input 1 (I0.0)
is programmed as a normally open (NO) contact. In this condition, power
will not flow through the contacts to the output (Q0.0).

62 PCSTP
When viewing the ladder diagram in the status mode, control elements
that are active, or true (logic 1), are highlighted. In the example shown
the toggle switch connected to input 1 has been closed. Power can now
flow through the control element associated with input 1 (I0.0) and
activate the output (Q0.0). The lamp will illuminate.

Forcing
Forcing is another useful tool in the commissioning of an application. 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
also be used to override discrete output points. The force function can be
used to skip portions of a program by enabling a jump instruction with a
forced memory bit. Under normal circumstances the toggle switch, shown
in the illustration below, would have to be closed to enable input 1 (I0.0)
and turn on the output light. Forcing enables input 1 even though the
input toggle switch is open. With input 1 forced high the output light will
illuminate. When a function is forced the control bit identifier is
highlighted. The element is also highlighted because it is on.

63 PCSTP
The following table shows the appearance of ladder elements in the Off,
forced, and On condition.

64 PCSTP