Application Of Plc Controlled System

Published Date: 02 Nov 2017

Introduction

The first PLC was developed in 1969. PLCs are now widely used and extend from small,

self-contained units for use with perhaps 20 digital inputs/outputs to modular systems that

can be used for large numbers of inputs/outputs, handle digital or analog inputs/outputs, and carry out proportional-integral-derivative control modes.

Typically a PLC system has the basic functional components of processor unit, memory,

power supply unit, input/output interface section, communications interface, and the

programming device.

• The processor unit or central processing unit (CPU) is the unit containing the

microprocessor. This unit interprets the input signals and carries out the control actions

according to the program stored in its memory, communicating the decisions as action

signals to the outputs.

• The power supply unit is needed to convert the mains AC voltage to the low DC voltage

(5 V) necessary for the processor and the circuits in the input and output interface

modules.

• The programming device is used to enter the required program into the memory of the

processor. The program is developed in the device and then transferred to the memory

unit of the PLC.

• The memory unit is where the program containing the control actions to be exercised by

the microprocessor is stored and where the data is stored from the input for processing

and for the output.

The term ’programmable logic controller’ is defined as follows by EN 61131-1 (IEC 61131-1):

" A digitally operating electronic system, designed for use in an industrial environment, which uses a programmable memory for the internal storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic, to control, through digital or analogue inputs and outputs, various types of machines or processes.

Both the PC and its associated peripherals are designed so that they

can be easily integrated into an industrial control system and easily used

in all their intended functions."

PLC Design and Operation Characteristics

The operation of the PLC system is simple and straightforward. The Process or CPU completes three processes: (1) scans, or reads, from the input devices (2) executes or "solves" the program logic, and (3) updates, or writes, to the output devices.

There are different types of mechanical design for PLC systems and these are unitary, modular and rack mounted.

Design Characteristics

Unitary

A unitary PLC is the more simple type of controller, and contains all of the basic system components within a single housing, or box. These components typically include the processor, which runs the software program, in addition to ports for input and output connections. Unitary PLCs are typically attached directly to the device or application that is being controlled.

Pro: Small, lightweight, cheap, tough, simple, easy to replace.

Con: limited function, poor interfaces, limited communication and data types, limited memory and program size, fixed inputs and outputs, not expandable.

Modular

A modular PLC contains several different modules that can be coupled together to build a customized controller. Typically, as base module contains core functions such as electrical power regulation, the computer processor, and input connections. Additional modules, including analog to digital signal converters or additional outputs, can be added to this core unit as needed. This modular design allows a PLC to be customized and changed easily.

Pro: Small, fixed I/O which is expandable

Con: Dependant on CPU capabilities

Rack Mounting

The rack mounting type of PLC is similar to the modular concept, but is implemented differently. Whereas each module in a modular PLC connects to the base unit directly, a rack mounting PLC keeps each module separate. All extra modules are connected through a network, and modules are held in organized racks. This approach allows for larger systems to be built, without becoming overly cluttered and complicated. Modules are well organized on the rack and can be removed and reinserted as needed.

Pro: Expandable, powerful, full function, full communication, good interface, essentially unlimited input/output, all functions customizable.

Con: Large, expensive, fragile, complicated.

Input and Output Devices

The input/output unit provides the interface between the system and the outside world,

allowing for connections to be made through input/output channels to input devices such as sensors and output devices such as motors and solenoids. It is also through the input/output unit that programs are entered from a program panel. Every input/output point has a unique address that can be used by the CPU.

Input Devices

Mechanical switches

A mechanical switch generates an on/off signal or signals as a result of some mechanical

input causing the switch to open or close. Such a switch might be used to indicate the

presence of a workpiece on a machining table, the workpiece pressing against the switch and so closing it. The absence of the workpiece is indicated by the switch being open and its presence by it being closed.

limit switch

Applies to a switch that is used to detect the presence or passage of a moving part. It can be actuated by a cam, roller, or lever.

Liquid-level switches

To control the level of liquids in tanks. Essentially, these are vertical floats that move with the liquid level, and this movement is used to operate switch contacts.

Proximity switches

Used to detect the presence of an item without making contact with it. There are a number of forms of such switches, some being suitable only for metallic objects.

Reed Switches

Consists of two overlapping, but not touching, strips of a springy ferromagnetic material sealed in a glass or plastic envelope. When a magnet or current-carrying coil is brought close to the switch, the strips become magnetized and attract each other.

Output Devices

Relay

When a current passes through a solenoid, a magnetic field is produced; this can then attract ferrous metal components in its vicinity. With the relay, this attraction is used to operate a switch. Relays can thus be used to control a larger current or voltage and, additionally, to isolate the power used to initiate the switching action from that of the controlled power. For a relay connected to the output of a PLC, when the output switches on, the solenoid magnetic field is produced, and this pulls on the contacts and so closes a switch or switches.

Motors

A DC motor has coils of wire mounted in slots on a cylinder of ferromagnetic material, which is termed the armature. The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic field produced by permanent magnets or current passing through coils of wire, which are called the field coils. When a current passes through the armature coil, forces act on the coil and result in rotation.

Directional Control Valves

Another example of the use of a solenoid as an actuator is a solenoid operated valve. The

valve may be used to control the directions of flow of pressurized air or oil and so used to

operate other devices, such as a piston moving in a cylinder.

Transducers

A transducer is a device that is used to convert a physical quantity into its corresponding electrical signal. In most of the electrical systems, the input signal will not be an electrical signal, but a non-electrical signal. This will have to be converted into its corresponding electrical signal if its value is to be measured using electrical methods.

Passive Type Transducers

Resistance Variation Type

Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to elongation or compression is known by the measurement of torque, displacement or force.

Resistance Thermometer – The change in resistance of metal wire due to the change in temperature known by the measurement of temperature.

Capacitance Variation Type

Variable Capacitance Pressure Gauge – The change in capacitance due to the change of distance between two parallel plates caused by an external force is known by its corresponding displacement or pressure.

Dielectric Gauge – The change in capacitance due to a change in the dielectric is known by its corresponding liquid level or thickness.

Inductance Variation Type

Eddy Current Transducer – The change in inductance of a coil due to the proximity of an eddy current plate is known by its corresponding displacement or thickness.

Variable Reluctance Type – The variation in reluctance of a magnetic circuit that occurs due to the change in position of the iron core or coil is known by its corresponding displacement or pressure.

Proximity Inductance Type – The inductance change of an alternating current excited coil due to the change in the magnetic circuit is known by its corresponding pressure or displacement.

Voltage and Current Type

Photo-emissive Cell – Electron emission due to light incidence on photo-emissive surface is known by its corresponding light flux value.

Hall Effect – The voltage generated due to magnetic flux across a semi-conductor plate with a movement of current through it is known by its corresponding value of magnetic flux or current.

Active Type Transducers

Photo-voltaic Cell – The voltage change that occurs across the p-n junction due to light radiation is known by its corresponding solar cell value or light intensity.

Thermopile – The voltage change developed across a junction of two dissimilar metals is known by its corresponding value of temperature, heat or flow.

Piezoelectric Type – When an external force is applied on to a quartz crystal, there will be a change in the voltage generated across the surface. This change is measured by its corresponding value of sound or vibration.

Moving Coil Type – The change in voltage generated in a magnetic field can be measured using its corresponding value of vibration or velocity.

Communication Links

PLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Other options include various fieldbuses such as DeviceNet or Profibus. Other communications protocols that may be used are listed in the List of automation protocols. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser. PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.

Profibus System

It is a messaging format specifically designed for high-speed serial I/O in factory and building automation applications. It is an open standard and is recognized as the fastest FieldBus in operation today. It is based on RS485 and the European EN50170 Electrical Specification. The DP suffix refers to "Decentralized Periphery", which is used to describe distributed I/O devices connected via a fast serial data link with a central controller. To contrast, a programmable logic controller (PLC) normally has its input/output channels arranged centrally. By introducing a network bus between the main controller (master) and its I/O channels (slaves), we have decentralized the I/O.

AS-Interface System

AS-Interface provides a basis for Functional Safety in machinery safety/emergency stop applications. Safety devices communicating over AS-Interface follow all the normal data rules. The required level of data verification is provided by dynamic changes in the data. This technology is called Safety at Work and allows safety devices and standard, non-safe devices to be connected to the same network. Using appropriate safe input hardware (e.g. light curtains, e-stop buttons, and door interlock switches), AS-Interface can provide safety support up to SIL (Safety Integrity Level) 3 according to EN 62061, CAT 4 according to EN954-1 as well as Performance Level e (PL e) according to EN ISO 13849-1.

AS-Interface is a system that requires four basic components:

Exactly one network master, in most cases in the form of a Gateway to a higher level industrial network or a PLC backplane card,

A number of network slaves, in most cases input and output modules,

Exactly one power supply used to power the network slaves and enabling communication with the network master, and

The wiring infrastructure, in most case accomplished using the yellow flat cable.

Master/Slave

In computer networking, master/slave is a model for a communication protocol in which one device or process (known as the master) controls one or more other devices or processes (known as slaves). Once the master/slave relationship is established, the direction of control is always from the master to the slave(s). The County of Los Angeles, saying the term master/slave may be offensive to some of its residents, has asked equipment manufacturers not to use the term. Some manufacturers prefer the term primary/secondary.

Screened Twisted Pair Cable

A cable consisting of an overall shield over four twisted-pair to minimize Electro Magnetic Interference (EMI) radiation and susceptibility to outside noise, 100 ohm Unshielded Twisted Pair (UTP), and is tested to 650 MHz that is third-party verified to Category 6. ScTP can be thought of as a shielded version of the Category 3, 4, & 5 UTP cables. It may be used in Ethernet applications in the same manner as the equivalent Category of UTP cabling. ScTP is technically a form of shielded twisted pair, the term Shielded Twisted Pair (STP) most often refers to the 150 ohm twisted pair cabling defined by the IBM Cabling System specifications for use with Token-Ring networks.

Coaxial Cable

Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable differs from other shielded cable used for carrying lower-frequency signals, such as audio signals, in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line.

Ribbon Cable

A Ribbon cable (also known as multi-wire planar cable) is a cable with many conducting wires running parallel to each other on the same flat plane. As a result the cable is wide and flat. Its name comes from the resemblance of the cable to a piece of ribbon.

Ribbon cables are usually seen for internal peripherals in computers, such as hard drives, CD drives and floppy drives. On some older computer systems (such as the BBC Micro and Apple II series) they were used for external connections as well. Unfortunately the ribbon-like shape interferes with computer cooling by disrupting airflow within the case and also makes the cables awkward to handle, especially when there are a lot of them; round cables have almost entirely replaced ribbon cables for external connections and are increasingly being used internally as well.

Internal Architecture

Central Processing Unit - CPU

Central Processing Unit (CPU) is the brain of a PLC controller. CPU itself is usually one of the microcontrollers. Aforetime these were 8-bit microcontrollers such as 8051, and now these are 16- and 32-bit microcontrollers. Unspoken rule is that you'll find mostly Hitachi and Fujicu microcontrollers in PLC controllers by Japanese makers, Siemens in European controllers, and Motorola microcontrollers in American ones. CPU also takes care of communication, interconnectedness among other parts of PLC controller, program execution, memory operation, overseeing input and setting up of an output. PLC controllers have complex routines for memory checkup in order to ensure that PLC memory was not damaged (memory checkup is done for safety reasons). Generally speaking, CPU unit makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early. You can simply look at any PLC controller and see that there are several indicators in the form of light diodes for error signalization.

Memory

All PLCs contain both RAM and ROM in varying amounts depending upon the design

of the PLC. The use of a PLC's memory is determined again by the design of the unit.

However, all PLC memories can be subdivided into at least five major areas. A typical memory utilization map for a PLC is depicted in the following figure.

Executive Memory

The operating system or executive memory for the PLC is always in ROM since, once

programmed and developed by the manufacturer, it rarely needs changing. It is the one that actually does the scanning in a PLC. The operating system is a special machine language program that runs the PLC. It instructs the microprocessor to read each user instruction, helps the microprocessor to interpret user programmed symbols and instructions, keeps track of all the I/O status, and is responsible for maintaining/monitoring the current status of the health of the system and all its components.

System memory

In order for the operating system to function, a section of the memory is allotted for system administration. As the executive program performs its duties, it often requires a place to store intermediate results and information. A section of RAM is installed for this purpose. Normally this area is allotted for use of the operating system only and is not available to the user for programming. It might be thought of as a scratch pad for the operating system to doodle on as necessary. Some PLCs use this area for storing the information which passes between programmer and operating system, e.g. the operating system generates certain error codes store in the specific address in this area during the execution of user program which can be read by user program; or the user may also give additional information to the operating system before execution of user program by writing some codes in the specific address in this area, etc.

I/O Status Memory - I/O Image Table

Another portion of RAM is allocated for the storage of current I/O status. Every single

input/output module has been assigned to it a particular location within the input/output image table. The location within the input and output image tables are identified by addresses, each location has its own unique address. During the execution of user program, the microprocessor scans the user program and interpret the user commands, the status of input modules used are read from the input image table (not directly from the input module itself). Various output device status generated during the execution of user program are stored in the output image table (not directly to output modules

Data Memory

Whenever timers, counters, mathematics and process parameters are required, an area of

memory must be set aside for data storage. The data storage portion of memory is allocated for the storage of such items as timers or counter preset/accumulated values, mathematics instruction data and results, and other miscellaneous data and information which will be used by any data manipulation functions in the user program. Some manufacturers subdivide the data memory area into two sub-memories, one for fixed data and other for variable data. The fixed data portion can only be programmed via the programming device. The CPU is not permitted to place data values in this area. The variable portion of the data memory is available to the CPU for data storage.

User Program Memory

The final area of memory in a PLC is allocated to the storage of the user program. It is this memory area that the executive program instructs the microprocessor to examine or 'scan' to find the user instructions. The user program area may be subdivided if the CPU allocates a portion of this memory area for the storage of ASCII messages, subroutine programs, or other special programming functions or routines. In the majority PLCs, the internal data storage and user program areas are located in RAM. Several systems do offer an option that places both the user program and the fixed data storage areas in EPROM type memory. The user can develop program in RAM and run the system to ensure correct operation. Once the user is satisfied that the programming is correct, a set of EPROMs is then duplicated from the RAM. Then the user can shut down the CPU and replaces the RAM with the newly programmed EPROM. Any future change would require that the EPROMs be reprogrammed.

Opto Isolator

A Opto isolator is a light emited diode and a photo transistor that are separated one from other by a distance or a very,very thin piece of glass on a Integrated chip. The led is connected to a output of some output device. The photo transistor is connect to the input of a computer or controller or device. What is used for: is to send signal or digital information in high electromagnetic noise area or vibration to isolate it ( a device) from one another. In some cases; to keep the inputting device; which as tendency to burn out ; that allow a lot of voltage through. So if they were connected electrically together they would burn both the contoller and the inputting device up. So to save the computer or the controller from being damage. It easier to replace this chip than the computer or controller.

Input and Output Units

The input/output unit provides the interface between the system and the outside world,

allowing for connections to be made through input/output channels to input devices such as sensors and output devices such as motors and solenoids. It is also through the input/output unit that programs are entered from a program panel. Every input/output point has a unique address that can be used by the CPU. It is like a row of houses along a road; number 10 might be the "house" used for an input from a particular sensor, whereas number 45 might be the "house" used for the output to a particular motor. The input/output channels provide isolation and signal conditioning functions so that sensors

and actuators can often be directly connected to them without the need for other circuitry.

Shift registers

A 'shift register' is a means of providing information, concerning an item, determined at one point in a process to another point later in the process. The 'information' may be as sparse as 'an item exists or doesn't exist' (in which case the 'information can be a '1' or a '0') to 'this item has the following complex set of information' (in which the information could be a variety of information like size, color, weight or any other parameters relevant to the process).

The information is passed from one place in memory to another as different areas of the process take responsibility for the item. (Note: a 'shift register' can be achieved by moving the information or by moving pointers to information. The result is the same.)

The information may be moved based on time or on an event like a change in position. In general a 'shift register' moves all the pieces of information at the same time indicating a synchronous transference of the information from one point to another.

A shift register could be constructed which moves information in a sequential mode, that is, information 'n-1' moves to space 'n' then some time later information 'n-2' moves to space 'n-1' etc. With the shift point moving backwards through the shift register avoiding an overlap and overwriting of information. That is generally beyond this discussion.

A practical example is the 'pass/fail' information from an inspection device being passed downstream toward a rejection station. In this case the 'information' is typically a '1' or '0' though the 'reason for rejection' could be passed as a more complex set of data.

Controlling outputs is the PLC's way of getting its inputs to change.

Operational Characteristics

Scanning

A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.

Step 1

CHECK INPUT STATUS - First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step.

Step 2

EXECUTE PROGRAM - Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.

Step 3

UPDATE OUTPUT STATUS - Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.

PLC Information and Communication Techniques

Forms of Signal

There are a lot of different types of signals that PLCs can read and write. The two most basic signals are discrete (digital) and analogue.

Discrete (digital) means on or off, 1 or zero, high or low, etc. Two possible states. Using discrete signals you could have a switch that when pressed starts a motor running. Both the switch "input" and the motor start "output" would be discrete signals.

Analogue signals are continuous signals varying between two limits. Analogue signals can have a multitude of different values between those two limits such as pressure, temperature, level, etc.

Digital is a sequence of pulses

Number Systems

Decimal

The decimal number system is the number system we use everyday, from counting to simple math like checking store recepts.

The word 'deci' means 10, therefore there are ten numbers (digits) in the decimal decimal number system. The valid numbers in a base 10 number system are:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9

All number systems have a 'base'. The base is the same as the number of valid numbers in the system. Since the decimal number system has 10 valid numbers it is a 'base 10' number system. The base is also referred to as a 'radix' and in the PLC/PAC software the user can change the radix (base) of the numbers for display purposes.

A decimal number showing its base would be written:

25710 or 257d

Where the small letter 'd' designates that it it a decimal number. We normally do not write the base on decimal numbers. If the base is not displayed the number is assumed to be a decimal number.

Digit Weighting

Each digit in a number system has a weight value assigned. Decimal numbers are 'base-10' numbers, therefore the weight of each digit is a power of 10.

Each digit also has a position number. The first digit on the right is position zero. The next digit to the left is position one. To the left again is position two, etc.

The weight of the digit in each position is the base of the number system raised to the power of the position number. The value of the number is than calculated by multiplying the value of the digit in each position by the base raised to the power of that position and then summing the products.

The following figure represents a decimal number with the position and weight of each digit shown and how the value of the number is derived.

Binary

The binary number system is used by all computers, PLC/PAC's and digital device.

The binary number system is a base-2 number system, therefore there are two valid digits:

0 and 1

One digit of binary is called a bit. Bits are used in groups to represent all other numbers. Bit grouping nomiclature is as follows:

1 binary digit is a bit

4-bits is a nibble (term is not used often)

8-bits is a byte

16-bits is a word

32-bits is a double word or DWORD

64-bits is a quad word or QWORD

The binary number system is used in computers and PLC/PAC's because it is easy to use a bit to represent voltage levels within the computer or PLC/PAC. A '0' = 0Vdc or Gnd and a '1' = +Vdc.

Each bit in the binary system as a position and a weight value assigned to it. Binary is a base-2 number system, therefore the weight of each bit is 2 raised to the power of the bit position.

Binary numbers can be written in two formats:

10110012 or 1011001b

The figure below depicts a 16-bit binary word showing terminology and the weight values of each bit location. Bit location zero is the bit on the far right and bit 15 is the bit on the far left.

Hexadecimal

Hexadecimal, simply referred to as Hex, is a base-16 number system.

Most modern day PLC/PAC's use the hexadecimal number system some where within their architecture. Some PLC/PAC require that some instruction parameters be entered in Hex.

Base-16 numbers can be written in two formats:

2416 or 24h

Base-16 also means that there are 16 valid numbers. Starting with zero they are:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9

A, B, C, D, E, F

Where:

A = 10, B = 11, C = 12, D = 13, E = 14, F = 15

Question to Ponder and Research

Why the switch to letters? Why not simply use the numbers 10 through 15?

How many bits of binary does it take to represent all 16 digits of a hex number?

The answer is that it takes 4-bits of binary to represent any hex digit. The following table shows the hex digits and the corrisponding 4-bit binary number.

Hex Number

4-bit Binary

0

0000

1

0001

2

0010

3

0011

4

0100

5

0101

6

0110

7

0111

8

1000

9

1001

A (10)

1010

B (11)

1011

C (12)

1100

D (13)

1101

E (14)

1110

F (15)

1111

Each digit in a hexadecimal number has a weight Value. The weight of a hex digit is the base raised to the power of the digit position. The figure below depicts an example.

Hexadecimal Weighting

Octal

Octal is not as common as it's cousin hexadecimal it is still used in various PLCs so it's important to grasp the concept. For instance, when programming an AutomationDirect PLC the memory addresses are in octal. Octal, like an octopus' eight legs, means eight and therefore there are eight numbers to use from zero to seven. The column weights are 1, 8, 64, 512, etc. The weights are derived by taking the base number to the power of the column, 80=1, 81=8, 82=64, 83=512, etc. Now we can do the same exercise as in the last chapter to convert an octal number to decimal.

I know this isn't helpful so far. Where it really comes in handy is coverting from binary to octal because all you have to do is break down the binary number into chunks of three. This is because 8 is 23.

Most programmable controllers have inputs and output cards grouped in 8 or 16 (and high density of 32 and 64). The reason for this is the way computers like to have things in powers of 2, 4, 8, 16 and so on. So if it is not in octal it is typically in hexadecimal.

BCD

Binary Coded Decimal (BCD) numbers use four binary bits (a nibble) for each digit. (Note: this is not a base number system, but it only represents decimal digits.) This means that one byte can hold two digits from 00 to 99, whereas in binary it could hold from 0 to 255. A separate bit must be assigned for negative numbers. This method is very popular when numbers are to be output or input to the computer. An example of a BCD number is shown below. In the example there are four digits, therefore 16 bits are required. Note that the most significant digit and bits are both on the left hand side. The BCD number is the binary equivalent of each digit.

Most PLCs store BCD numbers in words, allowing values between 0000 and 9999. They also provide functions to convert to and from BCD. It is also possible to calculations with BCD numbers, but this is uncommon, and when necessary most PLCs have functions to do the calculations. But, when doing calculations you should probably avoid BCD and use integer mathematics instead. Try to be aware when your numbers are BCD values and convert them to integer or binary value before doing any calculations.

Protocols

RS232

RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997.

IEE488

IEEE-488 is a short-range digital communications bus specification. It was created in the late 1960s for use with automated test equipment, and is still in use for that purpose. IEEE-488 was created as HP-IB (Hewlett-Packard Interface Bus), and is commonly called GPIB (General Purpose Interface Bus). It has been the subject of several standards.

RS422

RS-422 is a technical standard that specifies electrical characteristics of a digital signalling circuit. Differential signaling can transmit data at rates as high as 10 million bits per second, or may be sent on cables as long as 1500 metres. Some systems directly interconnect using RS-422 signals, or RS-422 converters may be used to extend the range of RS-232 connections. The standard only defines signal levels; other properties of a serial interface are set by other standards.

Universal Serial Bus (USB)

Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices.

USB was designed to standardize the connection of computer peripherals, such as keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.

PLC Programming Techniques

Method of Programming

Ladder and Logic Diagrams

Ladder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. It is primarily used to develop software for programmable logic controllers (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble ladders, with two vertical rails and a series of horizontal rungs between them.

Ladder Diagram

This is the most common of the PLC Methods. The diagram looks like a wiring schematic for a relay circuit with the power line on the left and the outputs on the right. This is the main programming method for PLCs in industrial controls. It is referred to as a ladder diagram because when you look at it, it looks like a ladder with the inputs and outputs of the program contained on each rung. As an example, you have a proximity sensor that when triggered, sends 24VDC to the PLC. In the program, you want that trigger to power on a motor. The rung for that sequence will look like this: --||-----( )--, where --||-- represents the input from the proximity sensor and --( )--represents the motor output.

Function Block Diagram

The function block diagram method is also a pictorial method of programming. It consists of blocks for each function that show the inputs and outputs for more complex sequences and lines drawn between each block illustrating what each output will do and what will affect each input. For example, you may have a scale in your process and if you want an alarm to sound if the weight measured on the scale is too high or too low, then the scale will have a box with the line drawn from the weight output to the variable input of the alarm box. The output of the alarm box for either the too high or too low alarm will go to an alarm horn and/or light.

Sequential Function Chart

The sequential function chart method is another pictorial method. It most closely resembles a flow chart, only it's more complex. There are three primary elements in a sequential function chart: steps, actions and transitions. Each step contains the logic for a particular portion of the process. As an example: weighing an item, checking for alarms and sounding the alarm if the weight is out of limits. The actions are the individual activities of performing the steps. Transitions move the process from one step to the next.

Structured Text

This is a text language and is not used often with PLCs, though many manufacturers do allow for this within their PLCs' programming software. It is very similar to Pascal or BASIC, and for people trained with computer programming, it can be the easiest. Complex math or decision making processes are often easier to accomplish with structured text as it can be done on one page versus many rungs of a ladder diagram.

Instruction List

The instruction list method is probably the most complicated method, as it most closely resembles Assembly language. This can be useful for processes that repeat a small function often. Though it is a powerful method, it is often easier to just program the process in a ladder diagram than it is to learn how to program with an instruction list.

Associated Elements

Timers

In many control tasks there is a need to control time. For example, a motor or a pump 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 count seconds or fractions of seconds using the internal CPU clock.

Counters

Counters are provided as built-in elements in PLCs and allow the number of occurrences of

input signals to be counted. Some uses might include where items have to be counted as they

pass along a conveyor belt, the number of revolutions of a shaft, or perhaps the number of people passing through a door.

Flip Flop

A flip-flop or latch is a circuit that has two stable states and can be used to store state information. Flip Flop is a bistable multivibrator. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic. Flip-flops and latches are a fundamental building block of digital electronics systems used in computers, communications, and many other types of systems.

Shift Register

The term register is used for an electronic device in which data can be stored. An internal

relay is such a device. The shift register is a number of internal relays grouped together that allow stored bits to be shifted from one relay to another. This chapter is about shift registers and how they can be used when a sequence of operations is required or to keep track of particular items in a production system.

Self Holding Circuit

Drilling Rig

Start Sequence of Circuit

Cylinder 1 clamp in closed position

Cylinder 2 drill in closed position

Cylinder 3 eject in open position

Cylinder 1 clamp in open position

Cylinder 2 drill in closed position

Cylinder 3 eject in open position

When the start switch is operated the circuit is energised and S1, S5, S3 is made and COIL 1 which then energises SOL 1 which moves cylinder 1 to the clamp position.

Cylinder 1 clamp in open position

Cylinder 2 drill in open position

Cylinder 3 eject in open position

S2, S3 and Lockout 1 is made and Sol 6 is energised which operates cylinder 2 drill to the down position.

Cylinder 1 clamp in open position

Cylinder 2 drill in closed position

Cylinder 3 eject in open position

S2, S6, S3 is made and energises Sol 5 which then operates Cylinder 2 drill to the UP postion.

Cylinder 1 clamp in open position

Cylinder 2 drill in closed position

Cylinder 3 eject in closed position

Cylinder 1 clamp in closed position

Cylinder 2 drill in closed position

Cylinder 3 eject in closed position

Drill back to normal position at end of cycle

Cylinder 1 clamp in closed position

Cylinder 2 drill in closed position

Cylinder 3 eject in open position

Circuit when Reset button is Operated