Basics-of-PLCs-English-version-website-education.pdf

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Table of ContentsPreface

PLCs in a glance 07

Basic PLC Operation 071- Processor Operating Cycle 07

2- Input modules 08

3- Output modules 084- Programming device 08

Ladder Diagrams 09Hard-wired Control 10

Replacing hard-wired control with a PLC 11The advantage of using a PLC as a controller 11

Different PLC models made by Siemens 12

1- SIMATIC S5 12

2-SIMATIC S7 12 3-LOGO! 13

MicroLogix 1500 PLC 13

Number system – decimal & binary 14

Logic 0, Logic 1 15

Actuator 16Discrete Inputs 16

Analog Inputs 18

Discrete Outputs 18Analog outputs 18

CPU (Central Processing Unit) 19

Programming Languages 19

Ladder Logic programming 20Reading Ladder Logic Diagrams 21

Function Block Diagrams (FBD) 22

Hardware 23

PLC memory size 231- RAM/ ROM / EPROM / Firmware 23

2- Memory structure 24

3- Program space 244- Data space 24

Software 25

Cables 26

S7-200 & A-B MicroLogix 1500 PLCs 26

PLC models 27Optional cartridge 28

Expansion modules 28

Understanding Controller Status indicators 28Using a DIN rail to Install PLC and Expansion units 29

External power supplies 30

I/O numbering 31

Inputs / Outputs 32

Super Capacitor 32PLC Display and HMI units 32

Siemens TD200 33

Computer network 33PROFIBUS connection 33

Powerful AS-Interface connection 33

Contact & coil symbols (Ladder format) 34

Input/Output & contact programming examples 36Functions mostly used in PLC programming 38


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1- AND function 38

2- OR function 39

3- Latch function 40

4- Inverse function 40

FBD (Function Block Diagram) programming language 41STL (Statement List) programming language 41

Normally Open (NO) & Closed Contacts (NC) 42

Boolean Algebra PLC programming 43Status Function 45

Force Function 46

Sequence 47

3 phase motor starter program 48Wiring motor Starter circuit with PLC 50

Expanding the previous problem 51

Introduction to Analog signals 53

Analog Output signals 54Introduction to A-B & Siemens S7-200 Timers 55

Hard-wired time Delay relay 56

A-B MicroLogix 1500 & SIMATIC S7-200 Timers 56 1-Timer On-Delay (TON) 56

2- Timer Off-Delay (TOF) 563- Timer Retentive On-Delay Timer (RTO) 56

An On-Delay timer programming examples & application 57

A-B Off-Delay timer 57

An Off -Delay (TOF) application example 58A-B Retentive On-Delay Timer programming 59

SIEMENS SIMATIC S7-200 timers 60

1- SIMATIC On-Delay timer application example 61

2-How to calculate PT? 61

3- S7-200 Off-Delay timer (ladder Logic symbol and functions) 61

4- S7-200 Retentive On-Delay timer (ladder Logic symbol and functions) 62A-B SLC & Siemens S7-200 Counters 63

1-Up-counter (CTU) definition, symbol & an application example 64

2- Down-counter (CTD) definition and symbol& and application example 66SIEMENS SIMATIC S7-200 Counters 66

1-Regular Counters 66

A- CTUD ~ Count Up/Down counter 66

B-CTU ~ Count Up counter 66

C- CTD ~ Count Down counter 66

2- High-Speed Counters 66Sample problem 68

Pulse Train Output (PTO) function 69

Pulse Width Modulation (PWM) function 70Transmit function 71

Introduction to Network Communication 72

AS-Interface (AS-i) the Actuator Sensor Interface 72

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PLCs in a glance

A PLC or a Programmable Logic Controller, is a programmable controller which

is considered as a member of computer family is mainly designed to be used in anindustrial field to sense different incoming signals, process them (or make decision) and

issue commands based on the special software program in its memory. This course isdesigned to supply you with basic information on the functions and configurations of

PLCs.

Figure 1 illustrates Some I/O field devices connected to a Siemens S7-200 PLC

(Courtesy of Siemens Industrial Automation)

Basic PLC Operation

Processor Operating Cycle ~ in general

All hardware associated with a PLC falls into one of two functional areas. Theactual intelligence of the PLC is derived from electronic computer-based hardware,

which comprises the processor, or CPU, portion of the system as represented in

figure 2. The processor section of a PLC includes a power supply, a microprocessor orspecial-purpose electronic circuitry, as well as a computer-type memory for the storage of

programming instructions and system data. All activity of the PLC system is handled by

the processor. The processor is responsible for the analysis of incoming as well as previously stored data, and for responding to that information according to a detailed

control plan stored within the unit by the user.

Most PLC systems offer the standard relay, latch, timing, counting, and simple

mathematical functions of addition, subtraction, multiplication, and division as part oftheir capabilities.

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Figure 2

From figure 2, notice that a typical PLC system can be divided into four sections:

1-Programming Device

2-Micrrocessor + Memory Unit

3-Power Supply3-Input/Output Sections

Input modules accept a variety of analog or digital signals from various sensors and

convert them to logic signals that can be used by the CPU.

The CPU makes decisions and executes control instructions based on the program in its

memory.

Output modules convert control instructions from the CPU into signals that can be usedto control various field devices

A programming device is used to install the instructions that determine what the PLCwill do in response to specific inputs

Processor Operating Cycle ~ in general

During each operating cycle, the PLC processor does the following continually:1-Reads current input module statuses and updates Input Image Table.

2-PLC processor continually solves user Logic program based on current

Input Image Table statuses3- PLC’s processor updates Output Images Table statuses based on solution of user

Logic Program.

4- Based on the result on step # 3, PLC processor continually activates or deactivatesI/O module status according to Output Image Table status.

And then goes to the next rung. But if it is the end of the ladder logic program, and the

last statement is “END” instruction, it does some other tasks such as communication andhousekeeping tasks and then goes back to step 1 and continues executing ladder logic

program for the second time. It does so over and over until the time that the PLC is taken

off line or shut down manually.

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Based on what is said, notice that a single operating cycle or scan which is illustrated in

figure 3, can be divided into two distinct parts- the I/O scan and the program scan.

Figure 3

Basic PLC Operation

Ladder Diagrams

Ladder diagrams are specialized schematics commonly used to documentindustrial control logic systems. They are called “ladder diagrams” because they resemble

a ladder, with two vertical rails (supply power) and as many as rungs (horizontal lines) as

there are control circuits to represent. If we want to draw a simple ladder diagram

showing a lamp that is controlled by a hand switch, it would look like this:

Figure 4 displays a simple ladder diagram without power supply shown

The L1 and L2 designations refer to the two poles of a 120 VAC supply, unlessotherwise noted, L1 is the hot conductor, and L2 is the ground (neutral) conductor.

Typically in industrial relay logic circuits, but not always, the operating voltage

for the switch contacts and relay coils will be 120 volts AC. Lower voltage AC and evenDC systems are sometimes built and documented according to Ladder diagrams.

Ladder diagrams are used to describe the logic of electrical control systems.

There are differences in the way ladder logic was implemented in computerized form ascompared to hard-wired. The basic component of the control system is the control relay

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which is a solenoid that operates a number of switches or contacts. The contacts come

normally open and normally closed, normal being when the relay is not energized. Relayscome in various breeds like time delay and latching types. Other components of the

control system are the field devices such as push buttons, limits switches, lights, and

controlled devices like motor starters and solenoid operated valves.

Figure 5 displays a standard motor control circuit (Start /Stop Circuit)

O.L

MM

2 3

3 ~mo t o r

L1 L2 L3 N

M M M

Figure 6 displays Hard-Wired Control of an actual 3 phase motor

with the controlling circuit on the right side of the figure

Hard-Wired Control

Prior to invention of PLCs, many control tasks were done with contactors and

relays hard-wired together. Circuits first had to be designed and drawn up. Then

components were specified and installed, and wiring list is created. Electricians wouldthen wire it all together. If something went wrong, the designers and electricians had to

rework the installation. If changes were made later, they could be time-consuming and

expensive. PLCs can perform the same tasks as hard-wired controls, and more complexfunctions as well. The connections between field devices and relay contacts take place in

the PLC instead of with external wiring. Hard wiring, though still required to connect

field devices, is less extensive. Installations are easier to modify, since this often involveschanging only the PLC program.

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Replacing hard-wired control with a PLC

Now what we would like to do is to replacing control circuit in figure 5 with a

PLC and use special program to control start / stop function of the 3 phase electro motor

later. Of course nobody uses a PLC just to control start / stop function. This example isonly for educational purposes. See figure 7.

Figure 7 illustrates a typical PLC connected to input

& output field devices.

The advantage of using a PLC as a controller is

1- controlling circuitry is smaller

2- ease of troubleshooting3- less need to repair

4- a plc can execute much complicated functions in the controlling systems

5- A plc can easily establish communication with other process control systems

The all above advantages caused the PLCs to win the war against the old way of

doing things with hardwired circuitry using bulky relays, timers and other field devices.

In short, the first PLCs were invented on 1968, and in year 1970 communicating circuitry

was added to them. In 1980 communication protocols were standardized and eventually,in year 1990, programming languages were standardized (IEC1131).

IEC1131 standard

In year 1979, a group of experts under the name IEC (International Electro-

technical Commission) gathered to investigate ways to standard PLC parameters as long

as the hardware, software programming language and communication systems are

concerned. These studies and research took about 12 years and finally IEC1131 standardwas formed. IEC1131 standard is simply rules and regulations which must be considered

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by PLC manufactures and cover different aspects of PLCs. IEC1131 suggests the

specifications set forward as long as the hardware design, programming language,troubleshooting of hardware, installation and testing is concerned.

Different PLC models made by Siemens

Siemens categorizes its range of PLCs under the name SIMATIC ®. Some of

these PLCs are built in Compact form meaning the PLC box contains power supply,

CPU, input and output models. All these different part are located in plc box andconsidered as a unit. On the other hand, the second model of PLC is built in Modular

form. In this form of application, the end user can choose which model he needs for his

particular need.

SIMATIC S5

These PLCs which are relatively older version of Siemens models. Some of the

models were built is compact form such as S5-90U or S5-95U with limited range ofapplication and functionality. The other models, such as S5-100U or S5-115U were built

in Modular form which could be used to control middle range of application. For broaderrange of application, models S5-155U and 135U could be used from the same family.

Siemens STEP 5 software is used to program SIMATIC S5 PLCs.

Figure8 illustrates few models of

SIEMENS SIMATIC S5 ® models of PLCs


SIMATIC S7 PLCs

SIMATIC S7 PLCs are next generation of controllers built after S5 series. S7

series come in three different models which are: S7-200 (compact), S7-300 (modular)

and S7-400 (also modular). S7-200 is used to control a relatively small system, The S7-

200 is ideal for smaller stand-alone applications such as elevators, car washes, or mixingmachines. It can also be used to advantage with more complex industrial applications,

such as bottling and packaging machines.

S7-300 for mid-range applications and finally S7-400 can be used for broader range ofcontrol systems. These applications require a greater number of I/O points. Both the 300

and 400 are modular and expandable. The power supply and I/O functions are contained

in separate modules that connect to the CPU module. Choosing between the S7-300 andS7-400 depends on the complexity of the task and on possible future expansion.

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Figure 9 illustrates S7-300 and S7-400 PLCS


Logo! Logic Modules

SIMATIC S7-200 ® & LOGO! ® are both inexpensive and simple controllers

which are used for small control projects (inside of buildings or small machineries).LOGO is a compact PLC which is programmed by the keyboard on its panel or one can

use LOGO! Soft Comfort ® software to program it via a PC and download the ladderlogic program into the LOGO! PLC’s memory. S7-200 micro PLC is offered by fivedifferent CPUs that can be expanded with a wide range of individual modules.

Programming is based on the easy to use software STEP- 7 Micro/WIN ®. Hence, the

SIMATIC S7-200 is a reliable, fast and flexible controller in the field of microautomation. Figures 10 shows picture of both controllers and 11 MicroLogix 1500 with

two side I/O modules on the right.

Figure 10 illustrates LOGO! and S7-200 PLCs


MicroLogix 1500 PLC

Bulletin 1764 MicroLogix controllers are the most expandable members of theMicroLogix family which are made by Allen-Bradley. This controller fits many

applications that traditionally called for larger and more expensive controllers.MircoLogix 1500 is a compact controller which means a processor, base unit with power

supply and embedded I/O, all are packed inside of the controller box. This controller

packs the best features of a modular system into an inexpensive, small footprint. Figure11 illustrates a MicroLogix 1500 PLC with two extra I/O modules connected to it.

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Figure 11 illustrates MicroLogix 1500®(Courtesy of Allen Bradley)

Number Systems – Decimal & BinaryEverywhere, except for computer-related operation, the main system of

mathematical notation today is the decimal, which is a based”10” system. As in othersystems, the position of a symbol in a base”10” number denotes the value of that symbol

in terms of exponential values of the base. That is, in the decimal system, the quantity

represented by any of the ten symbols used: 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9, depends on its

position in the number. Unlike the decimal system, only two digits “0” and “1” suffice torepresent a number in the binary system. The binary system plays a crucial role in

computer science and technology.

Flip-flops-electronic devices that can only carry two distinct voltages at their outputs andthat can be switched from one state to the other state by an impulse signal, which can also

be used to represent binary numbers; the two voltages correspond to the two digits.Optical and magneto-optical storage devices use two distinct levels of light reflectance or polarization to represent 0 or 1.

Hence, arithmetic operations in the binary system are extremely simpler than doing it indecimal system as long as computer hardware design is concerned.

Since a PLC is a computer, it stores information in the form of 1 = High (or ON) and 0 =

Low (OFF), conditions (1 or 0), referred to as binary digits or bits. Sometimes single bits

are used to represent ON and OFF conditions. At other times they are combined torepresent numerical values. All number systems have three characteristics: digits, base,

and weight. The decimal system, based on the number 10, has these characteristics:

Ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Base: 10

Weights: 1, 10, 100, 1000,…(powers of base 10)

The binary system which is used by PLCs, it also has these characteristics:

Two digits: 0, 1

Base: 2

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Weights: 1, 2, 4, 8, 16, 32, 64, 128, …(powers of base 2)

Logic 0, Logic 1

PLCs use both digital and analog signals, but the CPU itself can only understand

digital signals. These signals are either ON or OFF. The binary number system is used torepresent digital signals, since binary numbers can be represented with only two digits,

that is 1 or ON and 0 or OFF. Binary 1 indicates that a signal is present, or a switch is

ON. Binary 0 indicates that the signal is not present, or the switch is OFF.

Figure 12

Sensors

A Sensor is a device, which responds to an input quantity by generating afunctionally related output usually in the form of an electrical or optical signal for use by

PLCs. Sensors are connected to the inputs of PLCs. One example is a pushbutton. Anelectrical signal is sent from the pushbutton to a PLC input, indicating the condition of

the pushbutton’s contacts which is either un-pressed or pressed (un-activated or

activated).

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Figure 13

Actuator

An actuator is a transducer that accepts a signal and converts it to a physical

action. In other words, an actuator causes an action to occur relating to the data that wassent to it. Actuators convert electrical signals from PLC outputs into physical conditions.

in our case, we use contactors as “actuators” which are activated any time PLC’s output

terminal is “1 or ON” and un-activated when output terminal is “0 or OFF”.

Figure 14

Discrete Inputs

Discrete inputs are inputs to a PLC that require an on or off signal. Pushbuttons,

toggle switches, limit switches, proximity switches, and contact closures are examples ofdiscrete sensors. These discrete sensors may be connected to PLC discrete inputs. In theON condition, the state of a discrete input may be referred to as a logic 1 or “ON” or

“High”. In the OFF condition it is referred to as logic 0, or low.

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Figure 15 discrete input examples – momentary contact / Normally

Open and close Pushbutton switches (NO or NC)

Normally open switches are the type used to turn on or off when it is pressed or

un-pressed accordingly. When the switch is pressed, the circuit becomes closed and

functions. In the following figure, when the switch is not depressed, no voltage is presentat the PLC input. This is the OFF condition. When the button is depressed, 24 VDC is

applied to the PLC input. This is the ON condition. Hence, normally closed switches perform just the opposite. When the switch is pressed, the circuit becomes open.

Figure 16

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Analog Inputs

While a digital input can tell us about discrete changes in the physical world, suchas whether a lamp is on or off, there are times when this is not enough. Sometimes we

want to know how much is the weight of a load “placed on” a Force- sensitive resistors

(FSRs) that change resistance based on a changing force applied to the surface of the

sensor; thermistors that change resistance in response to changing heat; and many more.To measure varying signals we need to have analog inputs. Typical analog inputs vary

from 0 to 20 milliamps, or 0 to 10 volts. In the accompanying example, a level

transmitter monitors the level of liquid in a tank. Depending on the type of leveltransmitter, the voltage on the PLC input either increases or decreases as the liquid level

increases.

Figure 17

Discrete Outputs

The word “discrete” means a signal that has two states, ON and OFF. Therefore, a

discrete output, or a digital output, is either ON or OFF. Solenoids, contactor cols, andlamps are examples of actuator devices normally connected to discrete outputs. In the

accompanying example, the lamp can be turned on or off by the PLC output.

Figure 18

Analog Outputs

The word “analog” means a continuously variable signal. Analog signals differ

from digital signals in that small fluctuations in the analog signal are meaningful. Hence,an analog output signal varies continuously. The analog signal voltage level might be as

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simple as a 0-10 VDC that drives an analog meter. Examples of analog meter outputs are

speed, weight, and temperature. The output signal may also be used on more complexapplications such as a current to pneumatic transducer that controls an air-operated flow-

control valve.

Figure 19

CPU

PLCs are often defined as miniature industrial computers that contain hardware

and software that is used to perform control functions. A PLC consists of two basic

sections: the central processing unit (CPU) and the input/output interface system. TheCPU, which controls all PLC activity, can further be broken down into the processor and

memory system. The input/output system is physically connected to field devices

(switches, sensor, etc) and provide the interface between the CPU and the information

provider (inputs) and controllable devices (outputs). To operate, The CPU monitors theinputs and makes decisions based on instructions held in its program memory. It performs

relay, counting, timing, data comparison, and sequential operations.Programs are typically created in ladder logic, a language that closely resembles a relay- based wiring schematic, and are entered into the CPU’s memory prior to operation and

changes only when a change is made to the control program.

Figure 20

Programming Languages

In short, when it comes to PLC programming languages, it defines 5 different

types of data which can be used by the programming languages. IEC1131 standards sixdifferent programming languages for PLCs.

1- IL (Instruction List)

2- FBD (Function Block Program)

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3- LD (Ladder Diagram)

4- ST (Structured Text)5- SFC (Sequential Function Control)

6- CFC (Continuous Function Chart)

Figure 21 illustrates a sample of each programming language

Ladder Logic Programming

Ladder logic is the main programming method used for PLCs. Ladder logic has

been developed to mimic relay logic. The decision to use the relay Logic diagrams was astrategic one. By selecting ladder logic as the main programming method, the amount ofre-training needed for engineers and trades people was greatly reduced. The reason it’s

called “Ladder logic” is the program is drawn pictorially and looks like a ladder.

There are several techniques used to look at and understand programs. Among them areladder logic, statement lists, and function block diagrams. Ladder logic programming

uses components that resemble elements used in line diagram. Line diagrams are used to

describe hard-wired control systems. The accompanying diagram is an example of aladder logic program.

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Figure 22

Reading Ladder Logic Diagrams

In a ladder logic program, the left vertical line represents the power of energized

conductor. The other end of all output elements or instructions represent the neutral or

return circuit path (the right vertical line that represents the neutral on hard-wireddiagrams is omitted). Ladder logic diagrams are read from left to right and from top to

bottom. The ladder’s rungs are referred to as networks. A network may have several

control elements, but only one output coil.

Figure 23

Ladder Logic and Statement List (or Instruction List)

I0.0, I0.1, and Q0.0 are the first instruction combination located in the first rung.

It means If inputs I0.0 AND I0.1 are energized, output relay Q0.0 energizes.In the second instruction combination, means if either I0.4 OR I0.5 is energized, output

relay Q0.1 will be energized.

A statement list is another way of viewing a program. The operations are shown on theleft. The operands are shown on the right. Comparing the statement list and ladder logic

diagram shows that they have a similar structure, and define the same program.

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Figure 24

Function Block Diagrams (FBD)

Function Block Diagrams provide another way to develop PLC control

programs. Each function has a name to designate its specific task, and is indicated by a

rectangle with its function name inside. Inputs are shown on the left and outputs on theright. The accompanying function block diagram performs the same function as the

previous ladder logic diagram and statement list.

Figure 25

Software

PLC programs are typically written in a special application on a personalcomputer, then it is downloaded by a direct-connection cable or over a network to the

PLC. The program is stored in the PLC either in battery-backed-up RAM or some other

non-volatile flash memory. Each PLC manufacture designees a special type of programming software that can be used by end users to develop PLC control programs.

Often, a single PLC can be programmed to replace thousands of relays. As an example,

Siemens has developed S7 software for the S7 family of PLC. To program PLCs made by

Allen Bradley one might need to purchase software developed by Rockwell Automation.Although in some cases, third party companies also developed and market software for

other manufactures brand of PLCs.

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http://en.wikipedia.org/wiki/RAMhttp://en.wikipedia.org/wiki/Flash_memoryhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Flash_memoryhttp://en.wikipedia.org/wiki/RAM


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Hardware

Hardware is the actual equipment. The PLC, the Programming device, and the

connecting cable are examples of hardware.

Figure 26

PLC Memory Size

Memory in a PLC system is divided into the program memory which is usually

stored in EPROM/ROM, and the operating memory. The RAM memory is necessary for

the operation of the program and the temporary storage of input and output data. Typicalmemory sizes of PLC systems are around 1kb (when talking about computer or PLCmemory, 1 k refers to 1024 units) for small PLCs. This can be 1024 bits, 1024 bytes, or

1024 words, depending on memory type. Few kb for medium sizes and greater than 10-

20 kb for larger PLCs depending on the requirements. Many PLC would support easymemory upgrades fro future expansions.

While it is common for PLCs to measure their memory capacity in words, it is important

to know the number of bits in each word. A PLC that uses 8-bit words would have half

the memory capacity of a PLC that uses 16-bit words. For example a PLC that uses 8-bitwords with an 8K word capacity has 8 x 8 x 1024 = 65,536 bits of memory whereas if it

is using 16-bit words, it now has 16 x 8 x 1024 = 131,072 bits of storage with the same

8K memory. So, it is important to know the word size of any given PLC before memorysize cab be accurately compared.

Table in Figure 27

RAM / ROM / EPROM / Firmware

Random Access Memory (RAM) is the most common type of volatile memory

used in all computers. Information can be written into, or read from, a RAM chip, and it

is often referred to as read/write memory. Random access refers to the ability of anylocation or address in the memory to be accessed or used. Ram is used for both the user

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memory and storage memory in all PLCs. Since RAM is volatile, it must be battery

backup to retain or protect the stored program. RAM chips are manufactured with varioustechnology and CMOS-RAM is one of the most popular one. CMOS-RAM has very low

current drain when not being accessed (15µ amperes!). So a CMOS-RAM can be battery

backed up with a lithium batter which is rated 2.95 V at 1.75 ampere/hours and normally

can hold or protect a program for 60 days!Read Only Memory (ROM) is a common type of nonvolatile memory and it

means that the information stored in memory can be read only, and cannot be changed.

PLC manufacturer places information in the ROM for internal use and operation of thePLC, and it is not supposed to be changed or altered.

Erasable programmable Read Only memory (EPROM) is other type of

nonvolatile memory. An EPROM is ideally suited when program storage is to be semi permanent, or additional security is needed to prevent unauthorized program changes.

The EPROM chip has a quartz window over a silicon material that contains the electronic

integrated circuit. This window is normally covered by an opaque material, but when it is

removed, and the circuitry exposed to ultraviolet light, the memory content can be erased

and then it can be reprogrammed with a special equipment called “EPROMProgrammer”.

Firmware is user – or application specific software programmed into specialmemory contained in the hardware itself. In our case, it can be programmed into an

EPROM and delivered as part of the PLC hardware. It gives the PLC its basic

functionality.

Figure 28

Memory Structure

The memory of the most PLCs is divided into three areas:

1- Program space, data space, and configurable space.2- Data space

3- Configurable space

Program space contains the ladder program instruction programmed by the user.

The instructions are entered either by a programming device, hand-held or desktop-type ,

or a system computer. This area of memory controls the way data space and I/O pointsare used. LDA (Ladder logic) or STL (statement list) instructions are written using a

programming device and then loaded into this memory area.

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Data space is where the status (ON or OFF) of all input and output devices is

stored. Numeric values timers and counters (preset and accumulated), numeric values forarithmetic instruction, and the status of internal relays also are stored in this area of

memory.

Configurable parameter space stores either the default or modified configuration

parameters.

Figure 29

Figure 29 show how the written ladder logic program after being change to some

binary codes is transferred into the PLC’s memory and is going to be executed by thePLC at hand.

Minimum hardware and software Requirements to develop ladder logic control

programs

In order to modify or edit a program, you need the following.

1- PLC

2- Programming device3- Programming software

4- Connector cable

A programming device (PG) is needed to enter, modify, and troubleshoot the

PLC programs, or to check the condition of the process. Once the program has been

entered and the PLC is running, the PG may be disconnected. It is not necessary for the

PG to be connected for the PLC to operate. A PG on the other hand can be used tomonitor the PLC program while the program is running. Programming devices come in

three types: hand-held, dedicated desktop, and computer.

Software

Many companies such as Siemens and Rockwell Software have developed

software for programming PLCs. A software program, running on a PC, may be used tocreate a program for the PLC. This programming software is typically specific to one

PLC or family of PLCs. As an example, RSLogix ® software created by Rockwell

Software for programming the Allen-Bradley family of PLCs. This software in variousversions, can be used to program PLC-5, SLC 500, or the MicroLogix ® family of

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processors. It is Microsoft Window ® based and very user friendly. The S7-200 also uses

a windows-based program called step 7-micro/Win32 ®, which may be installed on a PCin the same way as any computer software.

Cables

A special cable is needed when a personal computer is used as the programmingdevice to download the developed programs into the PLC’s memory. This cable, called a

PC/PPI cable, allows the serial interface of the PLC to communicate with the serial

interface of a PC. Some of these connecting cables have DIP switches on the PC/PPI toselect the appropriate speed (baud rate) for passing information between the PLC and

computer.

The S7-200 and Allen-Bradley MicroLogix 1500 Micro PLCs

Examples of basic programming techniques of typical PLCs are discussed and

illustrated in this section of this manual. For the sake of educational purposes, Many of

the examples used in the text are based on the Allen-Bradley MicroLogix as well as the

S7-200 micro PLC which the smallest member of the SIMATIC family of programmablecontrollers . These two manufacturers are considered to have a large share of U.S.A and

Europe PLC market accordingly.In both controllers, the CPUs are integral to the motherboards and inputs and outputs

connect them to the system being controlled. The inputs monitor field devices, such as

switches and sensors. Outputs control devices such as contactors, signal lamps or pumps.The programming port is used to connect to the programming device.

Figure 30 illustrates Allen-Bradley MicroLogix 1500 ®

PLC with hard-wired I/O terminals. Courtesy of Allen Bradley

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Figure 31 illustrates the graphical picture of the Allen- Bradley

MicroLogix 1500 ® PLC

Figure 32 illustrates graphical SIMATIC

S7-200 PLC with hard-wired I/O terminals.

PLC Models

There model descriptions published by manufacturer of any brand of PLC which

gives the end user information about the CPU types, power supplies available to model.

Model description also indicates the type of inputs / outputs (if they are relay type ortransistor ones, AC or DC.). Other features related to each PLC such as amount of

memory, data backup time, number of I/O and expansion modules available for each

model can also be obtained from the said documents.

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Optional Cartridge

The S7-200 provides a super capacitor that maintains the integrity of the RAM

after power has been removed. Depending on the model of the S7-200, the super

capacitor can maintain the RAM for several days. The S7-200 also supports an optional

cartridge that extends the amount of time the RAM can be maintained after power has been removed from the S7-200. The battery cartridge provides power only after the super

capacitor has been drained.

Expansion modules

S7-200 PLCs are expandable and various expansion modules are designed to give

these PLCs extra capacity to add additional digital/analog inputs, outputs and otherfunctions such as protecting over-voltage analog input modules.

Expansion modules are connected to the base unit with a ribbon connector.

The number of expansion modules that one can connect it to a PLC also depends on the

model of that particular PLC. For example for S7-200 CPU 224 the limit is maximum

number of 7 units and even then one needs to check the power budget to be sure he doesnot overload the CPU power output. Check specification sheets when it comes to

expansion modules.

Understanding controller Status Indicators

The controller status LEDs provide a mechanism to determine current status of the

controller if a programming device is not present or connected. When yellow one is on, it

means CPU is in the STOP mode. When the mode is set t to RUN, the green RUN

indicator will be lit. Red led means system is in fault status. Activation of any terminals

of input or output will cause the related Green led to be turned on. Application of theseindicators is very useful when testing or debugging control logic ladder programs. See

figure 33.

Figure 33

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Using a DIN Rail to Install a PLC and Expansion units

PLCs usually can be installed inside of cabinets in one of two ways. The base

unit and expansion I/O DIN rail latches lock in the open position so that an entire system

can be easily attached to or removed from the DIN rail. Most PLCs also have holes on panel which can be used to mount the PLC inside the any control cabinet.

Figure 34

External Power Supplies

Depending on the CPU model, Allen-Bradley model 1764-24BWA ® can beconnected only to 120/230 VAC power source. I/O numbering and power supply

terminals are all indicated on top and bottom terminal block layouts. Following figure

illustrates the way the A-B PLC is wired to 120 VAC supply. S7-200 PLCs can beconnected to either 24 VDC or 120/230 VAC. Power sources. On the other hand, S7-222

DC/DC/DC would be connected to 24 VDC. But S7-200 AC/DC/Relay model issupposed to be connected to 120/230 VAC power source. See next two figures.

Figure 35

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Figure 36

I/O Numbering

The inputs and outputs are all identified by their addresses, the notation useddepending on the PLC manufacturer. These addresses are used by the CPU to determinewhich inputs are present and which outputs need to be turned on or off. This is the

address of the input or output in the memory of the PLC. Siemens precedes input

numbers by I and outputs by Q. The first number in the designator identifies the byte, andthe second number the bit, of the I/O address. With the Siemens SIMATIC, the inputs

and outputs are arranged in groups of 8. Each 8 group is termed a byte and each input oroutput with an 8 is term a bit Thus, Q2.0 means an output at bit 0 in byte 2. With larger

PLCs having several racks of input and output channels, the racks are numbered. With

the AB, the rack containing the processor is given the number 0 and the addresses of theother racks are numbered 1, 2, 3, etc. according to how set-up switches are set. So, in

input address I:012/03 for example, “I” indicates an input, rack 01, module 2 and

terminal 03.

Figure 36a, Siemens and Allen Bradley addressing

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Inputs

Input devices, such switches, pushbuttons, and other sensor devices are connectedto the terminal strip under the bottom cover of the PLC. In figure 37, two pushbuttons,

proximity sensor and limit switch, all are considered as input devices.

Figure 37

Outputs

The output ports of a PLC are of the relay type or opto-isolator with transistor or

TRIAC types depending on the devices connected to them which are to switched on or off.These output devices are connected to the terminal strip located under the top cover of

the S7-200 PLC. When testing a program, it is not necessary to connect output devices.The LED status indicators “on” if an output is active. In the following figure, the red

signal lamp and the contactor to power motor are considered as output devices.

Figure 38

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Super Capacitor

The Super capacitor is an electrochemical energy storage applied in “power”

industries. Compared with battery, a super Capacitor has one-tenth of energy, but delivers

over 10 times power due to ultra low ESR (Equivalent Series Resistance). They also have

a much higher power density than batteries or fuel cells. These capacitors can provide backup power for maintaining charge for a long period of time and protect data stored in

PLC’s RAM during power losses. The RAM is typically backed up for 50 hours on the

S7-221 and 222, and for 72 hours on the S7-224 and 226.

PLC Reference Manuals

A reference manual is a document often organized aphetically, designed as aquick reference for experience users. A reference manual typically contains the most

frequently referenced subset of information including basic setup instructions,

troubleshooting for the most commonly encountered problems, and or prominent

features. Both Siemens and AB have lot’s of information in their websites regarding their

line of products.www.ab.com ab.rockwellautomation.com/Programmable-Controller s

www.siemens.com/ www.automation.siemens.com/_en/s7-200/index.htm www.automation.siemens.com/.../simatic-s7.../s7-200/.../Default.aspx

PLC Display and HMI (human-machine interaction) Units

A user interface is a system by which people (users) interact with a machine. Theuser interface includes hardware (physical) and software (logical) components. User

interfaces exist for various systems, and provide a means of:

1- Input, allowing the users to manipulate a system2- Output, allowing the system to indicate the effects of the users manipulation.

Generally, the goal of human-machine interaction engineering is to produce a user

interface which makes it easy, efficient, and enjoyable to operate a machine in the way

which produces the desired result. This generally means that the operator needs to provide minimal input to achieve the desired output, and also that the machine minimizes

undesired outputs to the human. Usually PLCs are designed such that could be used to

communicate with a variety of external devices such as HMI devices. HMI devices can

display messages read from the PLCs connected to and allow for adjustment of programvariables, provides forcing ability, and permits setting of time and date. The summarized

description of few HMI devices made by Rockwell Automation and Siemens ismentioned here for your information.Mfr. Part Number: 1760-DUB made by Rockwell Automation.

Description: Allen Bradley 1760-DUB multi-function Pico GFX-70 Display unit with

Keypad, displays text, date, and time, as well as custom bitmaps.Mfr. Part number 20-HIM-C3 S/A made by Rockwell Automation

Allen Bradley 20-HIM-A3 PwerFlex Architecture Class Hand-Held Human Interface

Module, LCD Display, Full Numeric Keypad, Series C

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Allen Bradley 2711-B5A2 PanelView 550 monochrome Terminal 5.5 inch, Keypad &touch screen, DH-485 communication Ports, AC power, Series F. combination for

convenient and flexible operator input: RS232 printer port, alarms, alarm lists, triggered

messages, and triggered states of a multi state indicator, field-replaceable backlights.

AB 1201-HJ2 programming terminal LCD Display

Siemens LOGO! Text display 6ED1 055-4MH00-0BA0The new LOGO! TD text display panel provides an affordable HMI for equipment

builders and their customers, even on the simplest relay control systems. By having a

display panel with built-in operator functions and diagnostic messages customized fortheir process, end users can now make quick adjustment or easy troubleshoot….

Siemens TD200

The Siemens TD 200 is the proven HMI device for the SIMATIC S7-200. In

addition to the display of alarm texts, it enables interventions in the control program (e.g.set point value changes) or the setting of inputs and outputs. The TD 200 is suitable for

simple operation tasks with the SIMATIC S7-200 PLC. The focus is on the display ofalarm texts. The low total height and device depth make it the unit of choice even in

cramped space conditions.

Computer network

A computer network is a collection of hardware components and computers

interconnected by communication channels that allow sharing of resources andinformation. The SIMATIC S7-200 micro PLC provides a full range of communication

capabilities. The integrated RS485 interfaces can be operated at data transmission ragesfrom 1.2 to 187.5 k. baud. A total of 31 units can be interconnected using a

communication cable.

PROFIBUS connection

All CPUs from 222 upwards can be run via the EM277 communications modulesas a norm slave on a PROFIBUS DP network with a transmission rate of up to 12 Mbit/s.

the open feature of the S7-200 to higher level PROFIBUS DP control levels ensures you

can integrate individual machines into your production line. With the EM277 expansionmodule, you can implement PROFIBUS capability of individual machines equipped with

S7-200.

Powerful AS-Interface connection

The CP 243-2 turns all CPUs from 222 upwards into powerful masters on the AS-

interface network. According to the new AS-interface specification v 2.1, you can

connect up to 62 stations, making even analog sensors easy to integrate. With AS-interface, you can connect up to 62 stations, making even analog sensors easy to

integrate. With AS-interface, you can connect up to 248 DIs + 186 DOs in the maximum

configuration. The max, number of 62 stations can include up to 31 analog modules. The

configuration of the slaves and reading/writing of data is supported by the handy AS-interface Wizard.

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Figure 39

Programming a PLC with a Computer

Many companies have developed software for programming PLCs. For purposesof illustrating how the software works, and the relative case of programming, we have

selected Step 7-Micro/Win ® created by Siemens for programming S7-200 family of processors. The software is Windows® based and very user friendly.

With Step 7-Micro/Win software one can create any control program by arranging manyinstructions that are arranged in a logical order.

Typical PLC Programming Instructions

Contact & coil Symbols (ladder format)

Generally, not all PLC manufacturers use the same notation and format when

labeling inputs, outputs, and the diagrams. Figure 40 illustrates a ladder logic diagram fora simple circuit, where a single switch is controlling an output for two different

manufacturers of PLCs: (A) Allen Bradley and (B) Siemens.

Figure 40

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In Ladder Logic programming format, different electrical symbols are used to

write the program. For example, a contactor is consisted of a coil and some contactswhich are usually either normally open (NO) or normally close (NC).

The main function of the PLC program is to control outputs based on the condition of

inputs. The symbols used in ladder logic programming can be divided into two broad

categories: contacts (inputs) and coils (outputs). Most inputs to a PLC are simple devicesthat are either on or off. These inputs are sensors and switches that are either “ON”

(pressed pushbutton) or “OFF” (inactive).

Coils are output symbols. Outputs can take various forms: motors, lights, pumps,counters, timers, relays, and so on. A coil is simply an output or a “Load”. The PLC

examines the contacts in a ladder and turns the coils on or off depending on the condition

of the inputs. Figure 41 shows the two common symbols for contacts and a basic coil.

Figure 41

Figures 42 & 43 show a few possible inputs & output devices accordingly. A few

possible inputs including (from left to right), proximity switch, limit switch, floater

switch, limit switch, 3 different types of pushbuttons.

Figure 42

Next figure shows a few possible output devices. A few possible outputs including (from

left to right), contactor, motor, solenoid valve, signal light, pump, and pneumatic valve.

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Figure 43

Input/output & Contact programming examples

Following are 2 representative examples of PLC programming using contacts and coils.For these examples, both PLC and relay logic solutions are shown.

EXAMPLE 1

The first example is a simple circuit with one toggle switch as a contact and one output as

a Lamp. As the switch is pressed or not, the output goes on or off (figure 44). Next two

figures show the relay logic (45B) and ladder logic (45C) diagrams.

Figure 44

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Figure 45

EXAMPLE 2

The second example is a start / stop seal-in circuit. When the start button is

pressed, the coil energizes. When the button is released, the coil remains on. It is held on

by a sealing contact that is in parallel with the start button. The seal contact closes whenthe output coil goes on. If the stop button is pressed, the coil goes off and stays off. Also

if the control power goes off, the coil goes off too. The advantage of this example over

the first one is that when failed control power returns, stat pushbutton must be pressed to

reenergize the coil.

Figure 46 connection diagram

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Figure 47 Relay Logic diagram

Figure 48 Ladder Logic program

Functions mostly used in PLC programming

In this section we discuss some functions that are mostly used in programming

PLCs. And also short samples of written codes are represented for each function. Thesefunctions are: AND, OR, Latch, Inverse ..etc.

AND functionIn figure 49 let’s assume that we wish to activate output K1 any time 3 inputs

are activated in the same time. In this case we need to use AND function.

Figure 49 displays the Circuit and logic diagram for AND function. Ladder diagram ofAND function is also given. Anytime S1=S2=S3= 1 (all pushbuttons depressed) > K1 =

1 or High or on.

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Figure 49

OR function

In figure 50, K1 = 1 if any of inputs S1, S2 or S3 to be closed. Figure 51 displays Circuit

and logic diagram, and the LDA and STL programs respectively.

Figure 50

Figure 51 LDA and STL programs of OR function

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Latch Function

This function has lots of application. Let’s assume we wish to have a circuit

that anytime inputs S1 and S2 to be activated, K1 > ON and if S1 = 0 (OFF), K1 stays

ON (activated). K1 becomes un-activated if S2 = OFF (open). This function is used tostart electric motors in industry fields. In practice, a normally open switch is used instead

of S1, and a normally closed switch instead of S2.In figure 44 notice that Q2.0 is used as parallel contact to I0.1 or in motor circuit, K1 is

used as a parallel contact to switch S1. Usually, most contactors or relays, have someextra contacts that could be used with the same name (of the contactor) in the circuit

diagrams. As an example, K1 represent a contactor coil which is activated with say

120/240 volt AC. It has also an extra contact which is parallel to S1 switch (which alsolabeled as K1).

Notice that only one bit of memory is allocated to each contact (or addresses such as

Q2.0) anytime as an example I0.1 = 0 means it is OFF or I0.1 = 1, it is ON. Since anyPLC usually has many banks of memory, therefore, a large number of contacts are

available to a software developer to use.

Figure 52 displays circuit and logic diagrams of Latch function.

Inverse function

We wish to have a circuit diagram to have its output K1 = 1 any time S1 = S2

= 1 but S3 = 0 (or OFF). Circuit Diagram of 53 displays such a circuit. Now, to simulateS3 = 0 situation, we may use an Inverse function in our ladder logic program. Figure 53,

displays the ladder logic program for SIMATIC S7-200 PLC.

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Figure 53

The NOT gate is an electronic circuit that produces an inverted version of the

input at its output. It is also known as an inverter. If the input variable is A, the

inverted, output is known as NOT A. This is also shown as with a bar over the top.

In ladder logic programming, we may use a normally close contact, to simulate theInverse function. Figure 53 displays the how contact I0.3 represents inverse function of K

in the given LDA or STL software program.

FBD (Function Block Diagram) programming language

Function Block Diagram is a graphical programming language which enables the

user to rapidly program both Boolean and analogue expressions. The FBD editor offersfast programming by placing boxes that each block does some type of logical function.

Figure54 illustrates a small program written in FBD format to function as an AND Logic

circuit.

Figure 54

STL (Statement List) programming language

In programming in STL language (or format), mnemonic codes are used, each

code corresponding to a ladder element. The codes used differ to some extent frommanufacturer to manufacturer. A program written in STL format, consists of different

statements grouped to form a list of instructions. Each instruction (statement) is usually

made of one logical block such as AND, OR, NOT … etc which is shown by an Englishletter. English letter A, stands for logical block, AND: AN for NOT: O for OR and

finally”=” for OUT. A program can also consist of some other software devices such as

Flip Flops, Counters, Timers.. etc. As an example of STL programming, figure 55displays a simple two inputs AND circuit.

Figure 55 shows the program for two input AND circuit

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Normally Open (NO) & Closed (NC) contacts

This contact gets activated anytime an input device such as a push button switchis depressed. In this case, a “1” or “HGH”, or if the input is a DC type, 24 VDC appears

at the PLC’s input port channel. Obviously, if the contact gets de-activated, input voltage

level changes into “0” or “low“, or “ 0 VDC”. Figure56 illustrates, symbol used to

indicated a “NO” contact in ladder logic program. This contact functions apposite to whatwas said about Normally Open contact. It means anytime, the push button is not

activated, input signal is considered as a “HIGH” or “on” or =1, and when it is activated,

input signal to PLC is “LOW” or “off” or = 0.

Figure 56

A PLC can only scan its input ports and register if any of them it is HIGH or

LOW , but it has no way to know if a NO or NC pushbutton is connected to any of itsinput port. As an example, in figure 57, if S1 in CKT 1 is depressed, light bulb turns on

and turns off if it changes its state.

In the second circuit, (CKT2) again if we download USER PROG. 1 and execute it and

then depressed SP, light bulb goes on and turns off if pushbutton is de-activated.BUT notice if we download the second USER PROG. 2 and execute it, as soon as PLC

starts to execute the program, light bulb turn on and stays on as long as pushbutton is nottouched otherwise, it turn off. So the conclusion is that with the same pushbutton, (an

input device), we get different result just by changing the software and this is beauty of

using a PLC in control processing circuits.

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Figure 57

Boolean Algebra PLC programming

We can program a PLC based on the Boolean algebra system, which is ashorthand method of writing logic gate diagrams. Complex gate diagrams can be

analyzed easily when they are written in Boolean form. The analysis is covered in digital

logic texts. Here we cover only the PLC programming aspects of Boolean algebra.

The symbols used in the Boolean algebra system are illustrated in figure 58. Examples oftypical usage and the meaning of the Boolean expression in words are also given.

Figure 59 shows some typical gates and how they would be represented in Boolean form.

Figure 58

A conversion example

The example is about a motor control circuit with two start and stop pushbuttons.When any of the two start button is depressed, the motor runs. By sealing, it continues

to run even when the start button is released. Depressing any of the two stop

pushbuttons, causes the motor to stop running. Figures 59, 60, 61 and 62, illustrate thesolution in four different formats: (59) Gate logic (60) Boolean expression. (61) Relay

logic (62) PLC logic (LDA)

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Figure 59

Figure 60

Figure 61

Figure 62

Example 1 is a start/stop seal- in circuit. When the start (I0.0) button is depressed,the coil (LED in figure 1-48) turns on. When it is released, the coil remains on. It is held

on by a sealing contact that is in parallel with the start button. The seal contact closes

when the output coil goes on. If the stop button is depressed (I0.1), the coil goes off and

stays off. Also, if the control power goes off, the LED (coil) goes off. When failed powerreturns, start button (I0.0) must be depressed to reenergize the coil. Figure 54 illustrates

the circuit in Gate Logic and LDA version of the given example1.

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Figure 63

Figure 64

As it was mentioned previously, the origin of PLCs are in the use of relays to

perform automatic functions and the wiring diagrams for relay logic circuits witch

resembled a ladder. In fact, PLCs replace the physical relays with imaginary ones thatare part of the written control program being executed within a PLC. PLCs input/output

terminals are physically connected to a set of input/output field devices such as sensors or

switches and control the on or off status of the output contacts. The control program

within the PLC determines the way the inputs control outputs. As it was illustrated infigure 21, there are few methods of programming a PLC and in this book we will use

LDA and FBD format of programming.Status Functions

After a program has been loaded and is running in the PLC, the status of ladder

elements can be monitored using the STEP 7-micro/WIN32 software. The standard

method of showing ladder elements is in the de-energized or non-operated state. Whenviewing ladder diagrams in status mode, control elements that are active are highlighted.

Figure 65

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Figure 66

Force Function

Many PLCs have the capability to carry out a FORCE function. The function is

essentially an override control that is considered as another useful tool for program

debugging. When turned on, it can lead to feeding the process program with incorrectinformation. It is used to temporarily override the input or output status of the

application during testing. It can be used to override discrete output points or to skip

portions of a program. The function must be used with utmost caution in conjunction with a

working process. Figure 67 shows in our test circuit, toggle switch is “off”, hence I0.0 =

off so Q0.0 = 0 or “off”.

Figure 67

Circuit in figure 68 shows the same circuit but this time by use of Force function, I0.0 is

turned “on” so the AND circuit consisting of I0.0 and I0.1 is set to 1 therefore, Q0.0 = 1

and the lamp which is wired to Q0.0 output is turned on or set to 1. As you noticed, in

figure 68 turning FORCE on, changed the statues of the contact (I0.0 = 1) on.

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Figure 68

You noticed that turning FORCE function on, changes the status of a contact orcoil. Meaning If it is a normally open contact, it will close (turn on), and if it is normally

closed contact, it will open (turn off). If you force a coil or function, it will go on when

forced. See figure 69.

Figure 69

SequenceFor the simple lamp circuit, the following is the sequence of events. Using its

program, 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 remains off.When the switch is closed, I0.0 receives a binary 1, which instructs Q0.0 to send a binary

1 to the output module. This turns the lamp on.

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Figure 70

Figure 71

3 Phase Motor Starter Diagram

Figure 72 shows the diagram of a motor start and stop circuit. The diagram shows

how a normally open and a normally closed pushbutton might be wired to start and stop

a 3 phase motor. The motor starter coil (CR) is wired in series with both pushbuttons.

The starter’s auxiliary contact, CR, is wired in parallel with the normally openpushbutton. See figure 72.

Figure 72

When “START pushbutton “is pressed, current flows through STOP pushbutton ,START pushbutton , CR coil of contactor , and overload protection circuit to “N” pole

of power supply. CR coil is activated, causing main contacts of CR and auxiliary contact

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of CR to close. See figure 73. Hence, when START pushbutton is released, motor is

going to continue being ON. See figure 74.

Figure 73

Figure 74 – STOP pushbutton is released

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Wiring Motor Starter circuit with PLC

Figure 75 shows the motor starter control task which can also be accomplished with a

PLC.

Figure 75

Writing a Ladder Logic Program for circuit for figure 75

Figure 76 shows the control ladder logic program written for circuit in figure 75.

For the sake of clarity, in figure 75, I replaced the good old START pushbutton with a

toggle switch you can see the status of the contacts inside of the switch better. Figure 76,shows the status of PLC after the developed program is download into PLC memory and

RUN command is executed. Since, I0.1 and I0.2 both seen as NC contacts, programming

software displays them as turned ON field devices. As it was mentioned previously,when PLC scans the inputs, Ladder Logic program tells the PLC that those two inputs

I0.1 and I0.2 are NC contactors and that is the PLC’s way to show those as turned “on”

elements. Since logically, neither of I0.0 nor Q0.0 is “on”, then current can not flow

through none of those devices to activate the main contactor (CR). That is why Q0.0 = 0or off so the 3 phase motor stays “off”. Figure 77 shows the circuit status when START

toggle switch is activated.

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Figure 76

Figure 77 shows the circuit status when START toggle switch is activated.

Figure 77

As explained circuit in figure 77, when toggle switch is pressed, current flow through

AND circuit consisting I0.1, I0.0 and I0.2. All these three inputs are used in AND

configuration to control Q0.0. Also, a normally open set of contacts associated with Q0.0is programmed to form an OR circuit. Output Q0.0 is used to control the motor starter.

Expanding the previous problem

Now that we have mastered writing Ladder Logic programming, let’s do another

one with a little more number of inputs and outputs devices wired to our simple PLC.There is a mach table which has three pushbutton switches S1, S2 and S3. Design circuit

diagram and control program such that if these switches be hit at the same time, only the

one which is hit first, to be turned on. Figure 78 shows the relay logic of the problem.

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Figure 79 the related Ladder Logic program, and finally figure 80 the connection of field

devices to the PLC.

Figure 78

Figure 79

Figure 80

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Introduction to Analog signals

An analog is any continuous signal for which the time varying feature (variable)of the signal is a representation of some other time varying quantity. It differs from a

digital signal in terms of small fluctuations in the signal which are meaningful.

Electrically, the property most commonly used is voltage followed closely by frequency,current, and charge. Temperature is the most measured process variable in industrialautomation. The two prime sensors used to measure it are resistance temperature

detectors (RTD) and thermocouples (T/Cs).

RTDs are usually remotely located (they are not located directly next to the PLC).So, wires have to be used to connect them to the monitors or PLCs. Hence, when we need

to connect a RTD to a PLC, then PLCs must be able to work with continuous, or analog,

signals as well as discrete. Typical analog signals range from 0-10 or 0-5 VDC or 4-20mA. From our previous examples, you noticed that all our input devices were discrete,

meaning their produced output voltage was either 0 (off) or 1 (on). Since a PLC can not

process signals in analog form. Therefore, Analog signals must first be converted todigital representation, which is accomplished by an expansion module. Both Siemens andAllen Bradley have designed Expansion analog isolated input and output modules. By

connecting these modules to the related PLCs, user can connect verities of sensor our

output devices which their application requires analog signals. These analog modulesconvert standard voltage and current values to 12-bit digital representation. These digital

values are transferred to the PLC for use in its program. Analog modules are also

available for use with thermocouple and RTD sensors for accurate temperature

measurements.

Analog modules made by Allen Bradley for MicroLogix family of PLCs

1769-IF4I isolated analog input module1769- OF4CI isolated analog output module1769-OF4VI isolated analog output module

Analog modules made by Siemens for S7-200 family of PLCs

EM 231 RTD SIMATIC S7-200 ANALOG INPUT MOD (6ES7 231 – 7PB20- 0XA0)

EM 235 SIMATIC S7-200 ANALOG I/O (6ES7235 – 0KD20-0XA0)

Application Example

Figure 81 shows that a typical expansion module is connected to a PLC via a flat

cable. RTD is a field device that can be used to measure a varying value that in our caseis “current”. Inside circuitry of the expansion device is such that it applies small amount

of voltage to RTD and measures the related current. It then based on the measurement,

calculates the temperature value. Based on the control ladder logic program, PLC maketo decision what to do based on the calculated temperature value.

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Figure 81

Analog Output signals

Analog applications are present in many forms. Figure 82 shows the controller

that controls the amount of fluid in a holding tank by adjusting the valve opening. The

valve functions based on the analog input signal received from the PLC. The valve isinitially open 100% in this case maximum amount of fluid can flow into the tank. As the

fluid level approaches the preset point, the controller modifies the output. Controller

reduces the amount of flow of fluid little by little continuously adjusting the valve tomaintain the fluid level and finally shuts it off. Analog outputs can be provided by

digital-to-analogue converters at the output channel. The input to converter is a sequenceof bits with each bit along a parallel line. Next figure shows the basic function of the

converter. By using a converter module (a suitable expansion module) we can change 8

or 16 bits of PLC output to an analog output module. Typical analog signals range from0-10 or 0-5 VDC or 4-20 mA. Figure 71 shows that analog l output signal of a PLC is

wired to a valve to control the flow of fluid.

Figure 82

Could you explain how does the process in figure 82 work?

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Introduction to A-B MicroLogix and SIMATIC S7-200 Timers

In many control tasks there is a need to control time. For example a motor might

need to be controlled to operate for a particular interval of time, or perhaps be switched

on after some time interval. PLCs thus have timers as built-in devices. Timers countfractions of seconds or seconds using the internal CPU clock.

Hard-Wired Time Delay relay

A Time Delay relay is a combination of an electromechanical output relay and acontrol circuit. The control circuit is comprised of solid state components and timing

circuits that control operation of the relay and timing range. Most common typical time

delay functions include On-Delay, Off Delay, Retrigger able, One Shot Timer and fewother types.

Prior to invention of PLCs, different types of electromechanical timers were only

available to many control tasks which there was a need to control time. For example, a

Stop light control system used to be built with tens of hard-wired electromechanicaltimers.

Timers used with PLCs can be compared to hard-wired timing circuits. In the

following example, we first examine how a Time Delay Relay functions, and then discuss

timers used with PLCs. See figure 83.

Figure 83

According to the timing chart (On Delay timer), when switch is pressed, input

voltage I (enable signal (L1) is applied to timer TR1, timing delay t1 begins (next figure b). Relay contacts T changes state after time delay is complete (let’s assume

predetermined value for t1 = t2 = 10 seconds). Terminal # 1 connects to terminal # 3 and

lamp turns on (120 VAC is applied to the Lamp). Contacts T return to their shelf statewhen input voltage T is removed. Lamp turns off. When I is applied again, after t2= 10

seconds, Lamp turns on and stays on as long as switch is pressed. Lamp turns off as soon

as switch is released.

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Figure 84 shows status of a, un-activated, and b activated timer

A-B MicroLogix 1500 and SIMATIC S7-200 Timers

Generally speaking, 3 most used types of timers may be discussed here. These are:

1- TIMER ON-DELAY (TON)

2- TIMER OFF-DELAY (TOF)

3- RETENTIVE ON-Delay Timer (RTO)

Timers are represented by boxes in ladder logic. When a timer receives an enable signal,it starts timing, and continuously compares its current time with a preset time. Thetimer’s output is logic 0 as long as the current time is less than the preset time. When the

current time exceeds the preset time, the output changes to logic 1.

A-B Timer On- Delay (ladder logic symbol and function)

Figure 85 displays symbol used in ladder logic

programs to represent On-Delay timer

An on-delay timer will wait for a set amount of time (preset value e.g. 20 seconds

in above figure) after a line of ladder logic has been true (Timer on Delay input) before

turning its output on (EN), and it will stay on as long as the line of ladder logic stays at

its true status (as long as Timer On Delay = 1).Figure 85 shows that the timer consists of a timing block containing TIMER address

T4:0, TIM BASE which is set to 1.0 second, the PRESET value which is 20 secondsand finally ACCUM which is = 0.

T4:0 is the timer format in which.T Identifies this as a timer file

4 This is timer file 4 (default file)0 This means timer 0 in file 4 which can be any number from 0 to 255.

In this case, there are 256 timers available in file 4 for the programmer to use

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An On-Delay Timer programming Example

Develop a program that will turn on light O0 for 2 second and off for 5 second.

Figure 86 ladder logic program developed Using an On- Delay timer

TIMER ON-DELAY (TON) Application

Timer on-delay (TON) is used any time we need to start some action after a

certain amount of time passed from the time an input signal is received.

As an example, let’s assume stop light turns from red to green and the driver drives thecar along the road. If he drives 35 mile / hour, (which is actable speed), it takes him only

10 minutes to get to the second stop light. Since he was a good boy driving within the

speed limit, he deserves to receive a green light at the next stop light.

The 10 minutes delay is the on-delay timer’s preset value.

A-B Timer Off-Delay (ladder logic symbol and function)

Figure 87

In winter, you have noticed that any time electric heater is turned off (by the housethermostat), the blower fan motor dos not turn off right away. Instead, it stays on for

about 5 more minutes after the motor is turned off already and then it is turned down

automatically by the external circuit.

This is a five-minute off delay timer. The 5 minute timing cycle begins when the

blower motor is turned off.

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Above figure displays an Allen-Bradley TIMER OFF-DELAY (TOF) instruction symbol.

The timer consists of a timing block containing TIMER address T4:0, TIME BASE

which is set to 1.0 second, the PRESET value which is 20 seconds and finally ACCUM

which is = 0.

TIMER Off-Delay (TOF) Example

Develop a program that will turn on when Start button I0 is pushed and can be stopped byI1. When O0 goes on initially the TON timer is used to sound the horn (O1) for the first

5 seconds to warn that the oven will start, and then the horn (O1) stops and the heating

coil (O2) starts. When the oven is turned off (by pressing I1), fan continues to blow for10 seconds.

Figure 88 ladder logic program developed Using TON & TOF timers

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AB Retentive On- Delay Timer (ladder logic symbol and function)

Figure 89 A-B Retentive On Delay Timer functions similar to TON, except that it is

retentive. The most significant point about this timer is that when the input I0, turned off,

the accumulator value does not reset to zero. And since the ACC value does not reset to

initial value. A reset instruction can be used to reset the content of the accumulator tozero.

The EN, TT and DN bits function the same as with the TON instruction. When rung 0goes true, and stays true, true, timer continues to time until the ACC value equals to

preset value. In this case, DN >1, EN > 1 and TT > 0 (timing stops).And the transition of input from 1 > 0, causes EN > 0 TT > 0 and DN > 1.And in this case, bit 13 (DN bit) is set to 1 by the processor and remains ON as long as

the accumulated value is equal to the preset value.

Notice that RES instruction must be given the same address as the retentive timer used inthe rung we intend to rest it.

Figure 90 ladder logic program developed Using RTO timers

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SIEMENS SIMATIC S7-200 TIMERS

The Siemens S7-200 PLC uses also three types of timers: On-Delay (TON),

Retentive On-Delay (TONR), and Off-Delay (TOF). They are provided with resolutions

of 1 millisecond, 10 milliseconds, and 100 milliseconds, making the maximum counts32.767 seconds, and 327.67 seconds, and 3276.7 respectively.

The total number of these Timers is 256 with different resolution of 1 ms (4 timers), 10

ms (16 Timers), and 100 ms (236 Timers).

Figure 91

According to Timer table illustrated in above in figure 91, should we need a TON timer

with 10 ms resolution, we can use Timers with numbers T33 to T36, or T97 to T100.

S7-200 On-Delay Timer (ladder logic symbol and function)

Figure 92 illustrates ladder logic symbol for On-Delay timer

According to the above figure, S7-200 timers are controlled with a single enabling input

and have a current value that maintains the elapsed time from the time that the timer wasenabled. The timers also have a preset time value (PT) that is compared to the current

value. A timer bit is set/reset based upon the result of this comparison. When the current

value is greater than or equal to the preset time value, the timer bit (T-bit) is turned on.

Otherwise, the T-bit is turned off. Timing stops when the current value reaches a

maximum value. When a timer is reset, its current value is set to zero, and its T-bit isturned off.

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On-Delay Application Example

Write a program that when S1 is activated, after elapse of 5 seconds, outputQ0.0 = 1. S1 is a simple on-off switch.

According to Timer table, we wish to choose T33, TON timer with resolution of 100 ms.

How to calculate PT?

To calculate PT value, we must know our Timer’s resolution and the time delay after

which we want the Timer output to be activated. We need to calculate PT for a timer based on condition that its resolution is 100 ms and timer time is 5 S.

Figure 93

S7-200 Off-Delay Timer (ladder logic symbol and function)

Figure 94 illustrates ladder logic symbol for Off-Delay timer

According to the above figure, when the enabling input (I0.0) turns on, the timer bit turns on immediately, and the current value is set to 0. When I0.0 turns OFF, the timer

counts until the elapsed time reaches the preset time (PT). When the preset value =

current value, the timer bit turns OFF and the current value stops counting.

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In the following example, T33 with 1 ms resolution is chosen. Hence PT is calculated toPT = 5000.

Figure 95

S7-200 Retentive On- Delay Timer (ladder logic symbol and function)

Figure 96

TONR timer functions similar to TON timer except in the cases when an On-

Delay timer current value is cleared when the enabling input is OFF, while the currentvalue of the Retentive On-Delay Timer is maintained when the input is OFF. Hence one

can use a TONR timer to accumulate time for multiple periods of the input ON. A Reset

instruction “R” is used to clear the current value of the TONR timer.

Exercise 1

Write a program that when S1 is activated, after elapse of 5 seconds, output of the TONR

timer Q0.0 = 1. I0.7 is a simple on-off switch.

According to Timer Table, T0 is a Retentive on delay timer with resolution = 1 ms.

Solution

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Figure 97

Counters

A counter allows a number of occurrences of input signals to be counted. Thismight be in a situation where items pass along a conveyor belt and a specified number

have to be diverted into a box. It might be counting the number of revolutions of a shaft,

or perhaps the number of people passing through a door.

A-B SLC 500 and Siemens S7-200 COUNTERS

Depending on the Counter application, we have two types of counters designed to

serve the same function as the mechanical counters.

Generally speaking, 2 types of counters are:

1- Up-counter (CTU)

2- Down-counter (CTD)

AB counters are programmed almost exactly like AB timers discussed previously.There is counter number, a preset, and an accumulated value. The counters are numbered

similar to timers except it begins with a “C” instead of “T”. Allen – Bradley SLC 500and MicroLogix counter addressing is outlined as follows:

C5:3

C identifies the instruction as a counter file

5 the default file number (any unused file number from 10 to 255 can be assigned).

MicroLogix is limited to one counter file, which is default file 5. Hence, it is limited to40 counters (0 to 39). SLC 500 can use files 0 to 255.

:3 “:” and number 3. Colon separates file number (5) from counter number which is # 3

this can be any number. There are 256 counters in each file number. In our case, C5:3 isthe forth counter from counter file 5. Each counter is an element and like timer, each

counter element is consists of 3 words:

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AB Up-counter (CTU) symbol

1- Up-counter (CTU)

Figure 98

Count Up Done Bit (DN)

DN bit is set to 1 and stays on when accumulated value is equal or greater than preset

value.

Count up Enable Bit (CU) Activating input I0, causes a positive edge signal to the counter which (rung 0 becomestrue) causes CU status bit to be ON and remains true as long as counter rung is true (rung

0). Clearly, CU is false when the counter rung is false or I1 (reset input) is activated.

In next figure, any time should input device I0 (false to true transition) to be activated,

causes ACC to clear and DN to be set to off.

Count up sample example

Develop a ladder logic program that will turn on a light after switch I0 has been closed

10 times. Switch I1 will be used to reset the counter.

Figure 99

2- Down-counter (CTD)

Count Down Enable Bit (CD)

CD >1 as long as counter rung is true. Hence with positive going edge of I0 (falseto true transition), notice that CD is also set to 1 and stays ON as long as the counter rung

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is true. Leading edge of I0 sets CD > 0 or false. Also notice that anytime rest instruction

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