Plc Manual

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 1 of 59

PROGRAMMABLE LOGIC CONTROLLER

(PLC)

CONTENTS

1.0

Introduction

2.0

Functional Description of Hardware

2.1

Power supply

2.2

Input System

2.2.1 Opto Isolated Digital Inputs 2.2.2 High Speed Counter Inputs 2.2.3 Analog Inputs

2.2.3.1 Digital to Analog Converter (DAC) 2.2.3.2 Analog to Digital Converter (ADC) 2.2.3.3 Multiplexer

2.2.3.4 Interfacing

2.3

Output

2.3.1 Relay Outputs 2.3.2 Solid State Relay 2.3.3 Transistor Outputs

2.4

CPU

2.5

Memory

2.5.1 Memory Storage Capacity 2.5.2 Memory Map

2.6

Programmer Units

3.0

Design a System Based PLC

3.1

Project Execution

3.1.1 System Analysis

3.1.1.1 Engineering Design

3.1.1.2 Software (Program) Development 3.1.1.3 Software / Hardware Integration 3.1.1.4 Systems Checkup and Startup

3.2

Ladder Diagram

4.0

Hardware & System Sizing and Selection

4.1

I/O Quantity

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Rev. : 00 Date : 25/12/97 Page 2 of 59

4.3

Memory Quantity

4.4

Redundancy Level

4.5

Programmers

4.6

Special Design Norms for PLC

5.0

PLC Installation

5.1

Safety Considerations

5.2

Implementation

5.3

Enclosure

5.4

Temperature Considerations

5.5

Noise

5.6

Hookup

6.0

Applications

6.1

PLC Peripherals

6.1.3 Programming and Documentation Tools

7.0

Communications

7.1

Introduction

7.2

Parallel Communication

7.3

Serial Communication

7.4

Local Area Network (LAN)

7.4.1 Response Time of Network 7.4.2 Network Stations

8.0

DCS System Integration with PLCs

8.1

Man, Machine and Integration (MMI)

8.1.1 Sequencing

8.2

Integration with PLC

8.2.1 Integration with Direct I/O 8.2.2 Serial Linkages

8.2.3 Links between Networks (TCP/IP)

8.3

DCS Integration with PLCs

9.0

RIL Installations

10.0 References

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 3 of 59

1.0

Introduction

Programmable Logic Controllers (PLC), widely also called as Programmable Controllers (PC) is a major tool in today’s highly automated environment. A PLC is a modern way to perform industrial control functions (essentially Boolean Logic) that formerly required relays, solid state electronics or a microcomputer. The PLC has a number of inputs connected to it and its function is to activate outputs in accordance with a predetermined logic or program. Following figure illustrates the system operation :

Power

Control

Power

Unlike the hardwired system, the input is not physically connected to each line of the ladder logic, but the status as entered in the processors memory is used in solving the lines by digital electronic principles. Hence there is no limitations on the number of times the input or output status is used within the program for computation.

Programming Panel

Stored User Program

Scan all input and store the status in memory

Solve all logic functions as per Program

Activate all Outputs

Input Modules

Output Modules

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 4 of 59

The features and advantages of PLC are listed below : 1) Solid State Stability

2) Logic functions are easily programmed into the system which are alternatable again by program for modifications and future changes. 3) Virtually off the shelf control system resulting in quicker project

implementation.

4) They are light weight, rugged and compact and they work in industrial environment.

5) Ease of maintenance.

6) Compared to minicomputers they are a dedicated piece of equipment and hence very reliable.

7) Serial mode communication, with digital processors and can be used in the backup.

8) Competitive in cost.

9) optionally they can generate their own documentation or display the final entered program when called for.

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 5 of 59

2.0

Functional Description of Hardware

Programming Device

Power Supply CPU Memory

I / O System Module

Output Devices Input Devices

Solenoid, motor, Switches,

starters, etc. Push buttons etc.

Figure 1 : Block Diagram of a PLC

The block diagram of a PLC is shown in figure 1. The block consists of a Central Processing Unit (CPU), a main memory and connection circuitry for digital input / output devices. A communications bus (i.e. a group of parallel wires used for transmitting digital signals) forms a common link to allow each element to share information. The input image memory in the I/O system module is used to hold the on/off states of individual input ports. In the image memory, an ON state is stored as binary 1 and an OFF state is stored as binary 0.

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 6 of 59

The CPU processes the binary data stored in the input image memory and the corresponding data held in the output image memory according to the users program which is stored in the main memory. The bit values held in the output image memory determine which output ports are energized. A binary 1 sets an output port on and a binary 0 sets an output port off.

A special program called the operating systems controls the actions of the CPU and consequently the execution of the users program. The operating system is supplied by the PLC manufacturer and is permanently held in the memory. A PLC operating system is designed to scan image memory and the main memory which stores the ladder diagram program.

2.1

Power Supply :

The power supply may be separate or integrally mounted. It always provides isolation necessary to protect solid state components from most high voltage line spikes. All PLC manufacturers provide the option to specify line voltage conditions. In addition, power supply is rated for heat dissipation requirements for plant floor operation.

The power supply drives the I / O logic signals, the CPU, the memory unit and some peripheral devices. Power supplies fall into two categories : Linear and Switch mode. A linear power supply uses a simple regulator circuit to convert the mains supply to a constant DC voltage. A switch mode power supply uses a high frequency switching regulator to produce a series of pulses. Averaging the pulses provides a smooth DC voltage. The main advantage of switch mode power supply (SMPS) are :

1) It is capable of providing a wide range of supply voltages.

2) Switch action makes it highly efficient so that the amount of heat dissipated from the supply is small.

3) It is compact and light weight.

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 7 of 59

2.2

Input Systems :

Inputs are defined as real - world signals, giving the controller real time status of process variables. These signals can be digital, low or high frequency, maintained or momentary. Typically they are presented to the programmable controller as a varying voltage, current or resistance value.

2.2.1 Opto Isolated Digital Inputs :

A base unit input interface circuit will use an Opto isolator arrangement such as that shown in figure 2. An Opto isolator is a device which uses light to couple signals from one system to another; in this case the input device and the image memory circuit. It incorporates a light emitting diode (LED) and photo transmitter for this purpose. The device provides a very large degree of isolation between two circuits.

Figure 2 : Opto Isolated Input Interface

Figure 2 shows a typical Opto isolator input interface. When 24 VDC is applied to the input port, a current of approximately 10 mA flows through the Opto isolator LED causing it to emit light and so turn ON the photo transistor. If the supply is accidentally reversed, the protection diode protects the reverse voltage breakdown of the Opto isolator LED. When the photo transistor is turned ON current flows through the status LED which lights up. Thus the status LED tells the user the current logic state of the input point.

Protection Diode 560  24 VDC Inpu t Status LED Opto Isolator 5 V Internal Circuits 2.2 K 0V V

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Rev. : 00 Date : 25/12/97 Page 8 of 59

Input interfaces can be divided into those which require the input device to source current and those which require the input device to sink current. The interface of figure 2 requires that the input device sources current from the power supply. Other types of interface require that the input device sinks current to the 0V terminal.

2.2.2 High Speed Counter Inputs :

A PLC is often required to read high speed pulses from an input device such as a shaft encoder to produce pulses to drive a stepper motor. PLC ports cannot be used to generate or read high speed pulses as the scan time WHICH depends on the program length, etc. is a limiting factor. Instead, use is made of interfaces which operate independently of the scan but are able to interpret it when some action is required.

2.2.3 Analog Inputs :

Many of the transducers produce analog signals. Consequently PLC manufacturers provide ports for handling analog signals. To handle analog signals special interface devices based on analog to digital converters (ADC’s), digital to analog converters (DAC’s), multiplexers and demultiplexers are required. These are discussed below.

2.2.3.1 Digital to Analog Converter (DAC) :

Figure 3 : Eight Bit DAC

Vref DAC Binary Input Vout Analog Output (MSB) B7 B0 (LSB)

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 9 of 59

A Digital to Analog Converter (DAC) produces an analog output from a digital input (see figure 3). In all types of DAC, the analog voltage is produced from a reference voltage (Vref). Binary code is input to the DAC and determines what fraction of the Vref is presented at the output. The output from a DAC is not truly continuous but rather a series of discrete voltage levels. For example, the 8 bit DAC shown in figure 3 has an output given as : Vout = Vref (B7/2+B6/4+B5/8+B4/16+B3/32+B2/64+B1/128+B0/256)

where bits B7 to B0 can take values 0 or 1 and are the binary inputs. B7 is the most significant bit (MSB), while B0 is the least significant bit (LSB).

Consider an 8 bit DAC with a reference voltage Vref as 10 V. The binary input of 00000001 generates a smallest discrete output i.e. 10 / 256 volts. The next discrete output is 10 / 128 volts, generated from binary code 00000010. Clearly 256 discrete analog levels (referred to as quantization levels) can be produced from the binary input. The voltage resolution of an N bit DAC is calculated by dividing the maximum operating voltage by 2N - 1. The factor 2N - 1 represents the number of steps between quantization levels. An 8 bit DAC with a reference voltage of 10 V has a resolution of 10 / 255.

The speed of a DAC is determined by how long it takes to settle to a stable value after a change in the input. This is specified as the setting time. The other main parameters of a DAC are linearity and accuracy. Linearity is the measure of the deviation from a straight line of output voltage plotted against binary input. Accuracy is the variation between the DAC’s actual output and the intended one.

2.2.3.2 Analog to Digital Converter (ADC) :

Figure 4 : Eight Bit ADC

Start Converter (SC)

End of Converter (EOC)

ADC

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 10 of 59

The analog to digital converter (ADC) produces an digital output from analog input (see figure 4). ADC’s incorporate start convert (SC) and end convert (EOC) connections. When the start convert signal is pulsed, the ADC converts the analog input at that time into an equivalent digital value. The ADC then produces an end of convert signal (EOC) to indicate that the conversion has finished.

The main parameters of ADC’s are again resolution, accuracy, linearity and speed. Comments about resolution, accuracy and linearity have already been made in 2.2.3.1. Concerning operating speed, ADC’s are generally slower than DAC’s because the process involves comparing one signal with another. Successive approximation of the input value rather than ramping the DAC from a counter speeds up the conversion process. For high speed, so called ‘flash’ converters are used.

2.2.3.3 Multiplexer : C B A Channel Selected 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 2 3 4 5 6 7 Figure 5 : Multiplexer Output A B C 0 Input Channel s 1 2 3 4 5 6 7

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Rev. : 00 Date : 25/12/97 Page 11 of 59

Multiplexer allows several signal carrying channels to share a signal line. The block diagram of a multiplexer is illustrated in figure 5. This shows that each input channel may be connected to the output line when one of a bank of switches inside the multiplexer is turned on. In practice, the bank of switches shown in figure 5 is a bank of transistor switches controlled by lines A, B, and C. A binary code placed on the lines A, B, and C determines which of the channels of switched through the output.

Demultiplexers are multiplexers which work in reverse.

2.2.3.4 Interfacing :

The general rule when interfacing the analog signals is to match voltage levels and to ensure that the impedance of the sourcing circuit is less than or equal to that of its load circuit. Impedance matching is essential for optimum power transfer to the load circuit. To match the voltage levels it may be required to amplify the voltage level or perhaps convert a bipolar voltage into a unipolar voltage. A circuit for changing the impedance may be used for impedance matching.

2.3

Output :

Common types of outputs are discrete and analog. Discrete outputs can be pilot lights, solenoid valves or annunciator windows; analog outputs can drive signals to variable speed drives or to I/P converter and thus to control valve. Generally I/O systems are modular in nature. A system can be arranged by the use of modules that contain multiples of I/O points. These modules can be composed of 1, 4, 8 or 16 points and plug into the existing bus structure. The bus structure is a high speed multiplexer that carries information back and forth between the I/O modules and the CPU.

One of the most important function of the I/O is its ability to isolate real world signals from the low signal levels in the I/O bus. This is accomplished by the use of optical isolators.

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 12 of 59

2.3.1 Relay Outputs :

Figure 6 : Relay Output Stage

A traditional relay is a switch controlled by an electromagnet. Relays are used in PLC’s because they can handle large current and offer high degree of isolation between the PLC and the load circuits. A typical relay will be capable of switching a few amperes. However, the relays have the following disadvantages :

a) They are slow to operate

b) When closed, their contacts can bounce before settling.

c) Relay coils can generate large inductive currents when energized. A typical electromagnetic relay based output circuit is shown on figure 6. A NPN transistor switches current through the relay coil to close its contacts. The transistor is controlled by the image memory circuit of the PLC. The diode is connected across the to protect it from its back emf. On a practical side, it is important not to exceed the maximum current that the output relay contacts can handle. For DC loads, the current rating is given in amperes. For AC loads, the maximum current could be given as a VA (volt amperes) power rating.

For example, if the specifications of the output relays is given as 35 VA, then the maximum current the contacts can handle at 240 Vrms is calculated as

I = (35/240) = 146 x 10-3 = 146 mA

If the load current is likely to exceed the rated maximum current of the internal relay, then a external secondary heavy duty relay should be used.

Protection Diode

270 

+V

Input Status LED

Switching Transistor Relay Output Port 0 V

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 13 of 59

2.3.2 Solid State Relay :

Figure 7 : Solid State Relay which switches at a Zero Crossing Point

A solid state relay performs the same functions as a traditional relay but no moving parts. It is basically a optically isolated triac. As shown in figure 7, a triac is a two back to back silicon diodes which are switched into conduction by a third electrode called the gate. As the solid state relay is optically isolated the gate may be thought of as being triggered by an LED.

24 V Input 0V ZCC Gate Output N L Triac

Zero Crossing Point

Off Delay

Output

Zero Crossing Point

On Delay

Prepared By : AAB Reviewed By : RB Approved By : P. N. Thakre

Rev. : 00 Date : 25/12/97 Page 14 of 59

Solid state relays are often useful in switching mains and often incorporate a zero crossing circuit (ZCC). This monitors the mains cycle so that the relay can be turned on at a zero crossing point. Switching at the zero crossing point prevents high frequency noise being generated. However, on and off delays do occur as a result of waiting for the next zero crossing point (see figure 7).

2.3.3 Transistor Outputs :

Transistor may be used for switching current through a load. The switching speed of a transistor is faster than that of an electromagnetic relay. Consequently, the use of transistor output ports can reduce response time. Switching capabilities of PLC transistor output ports are usually quoted in terms of the maximum voltage and current that can be used. For example 24 VDC at 0.5 A.

2.4

CPU :

CPU performs the tasks necessary to fulfill the PLC functions like scanning, I/O bus traffic control, program execution, peripheral and external device communications, special functions or data handling execution and self diagnostic.

The central processing unit of a PLC is built-up around a microprocessor which is an integrated circuit which performs the computing operations. The function of the CPU is to accept data in the form of groups of binary digits and perform arithmetic and logical operations on the data in accordance with instructions stored in the memory.

The internal structure of CPU comprises input and output interfaces, a memory in the form of registers, and the control element called the arithmetic and logic unit (ALU). The input and output interfaces allow the CPU to read the data from the memory and write data into the memory via the communication bus. The ALU performs the arithmetic and logical operations on the data stored in the CPU registers.

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Rev. : 00 Date : 25/12/97 Page 15 of 59

2.4.1 Registers :

Most CPU operations involve the use of register, which is a memory element used to store a group of bits on a temporary basis. CPU registers are located inside the microprocessor. So called data registers are located in the RAM and are used for storing flags, counter and timer constants and other types of data. Registers are having different storage capacities. A 4 bit register stores a

nibble, which is 4 bits of data. An 8 bit register stores a byte which is 8 bits of

data. A 16 bit register stores a word, which is 16 bits of data.

Figure 8 : Registers

2.4.2 Flag Registers :

If a bit state (0 or 1) is used to indicate that some condition has occurred, than it is called a flag. A register which stores a group of flag bits is called a flag register. The CPU has an internal flag register which contains information about the result of the latest arithmetic and logical operations. The PLC image memory is effectively a flag register, as it contains the current status of inputs and outputs.

2.4.3 Auxillary Relays :

Auxillary relays are single bit memory elements located in RAM that may be manipulated by the users program. They are called auxillary relays because they may be likened to imaginary internal relays. A battery backed auxillary relay is called a retentive holding relay and can be used for storing data during power failure. A number of auxillary relays may be grouped together to form a register.

4 bit

8 bit

16 bit

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Rev. : 00 Date : 25/12/97 Page 16 of 59

It is important to remember that because auxillary relays are bit values stored in the memory output, loads cannot be connected directly to them. However, output loads can be used to control output loads indirectly.

2.4.4 Shift Registers :

Some registers are arranged so that bits stored in them can be moved one position to the left or to the right with the application of a shift command or pulse. Such registers are called shift registers and can be used for sequence control applications.

2.4.5 Binary Counter :

The CPU can function as a binary counter since it is able to increment and decrement binary data stored in a register and compare binary data stored in two separate registers. Counters are used to count, for example, digital pulses generated from a switching device connected to an input port. An output is usually generated after a predetermined number of input pulses have been counted. The count value required is stored in a data register.

2.4.6 Timers :

The CPU has a built in clock oscillator which controls the rate at which it operates. The CPU uses the clock signal to generate delay times. A delay time could be used, for example, to keep an output relay energized for a fixed period.

2.5

Memory :

It is the storage place in which both application program and executive programs are stored. An executive program functions as the operating system of the PLC. It is the program that interprets, manages and executes the users application program.

Memory is characterized by its volatility. A memory is volatile if it looses its data when the power to it is switched off and non-volatile otherwise. Common types of memory include semi-conductor memory and magnetic disk. The various types of semi-conductor memory are :

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Rev. : 00 Date : 25/12/97 Page 17 of 59

1) RAM :

Random Access Memory is a flexible type of read / write memory. All PLC’s have some amount of RAM, which is used to store ladder programs being developed by the user, program data which needs to be modified and image data.

RAM is volatile. This means that RAM cannot be used to store data while the PLC is turned off unless the RAM is battery backed. A type of RAM called CMOS RAM (complementary metal oxide semiconductor RAM) is suitable for use with batteries because it consumes very little power and operates over a wide range of supply voltages.

2) ROM :

Read Only Memory is programmed during the manufacture using a mask. It is a non-volatile memory and provides a permanent storage for the operating system and fixed data.

3) EPROM :

Erasable Programmable Read Only Memory is a type of ROM which can be programmed by electrical pulses and erased by exposing the transparent quartz window found in the top of each device to ultraviolet light. EPROM is a non-volatile memory and provides permanent storage for ladder programs.

4) EEPROM :

Electrically Erasable Programmable Read Only Memory is similar to EPROM but is erased by using electrical pulses rather than by using ultraviolet light. It has the flexibility of battery backed CMOS RAM. However, writing data into a EPROM takes much longer than into a RAM.

2.5.1 Memory Storage Capacity :

The storage capacity of a memory device is determined by the number of binary digits i.e. on / off states, it can hold. Clearly, the storage capacity of the user memory will determine the maximum program size. 1 K byte memory will hold 1024 program instructions and data if these are stored as groups of 8 bits.

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Rev. : 00 Date : 25/12/97 Page 18 of 59

2.5.2 Memory Map :

Memory mapping is a term to describe the situation in which input / output ports are controlled by writing data into the image memory. A diagram which shows the allocation of memory addresses of RAM, ROM and I/O is called a memory map. Figure 9 illustrates the memory map of a typical PLC. In this image bits are stored in RAM above the users program and data for flags, counters and timers. With most PLC’s memory map is already configured by the manufacturer. This means that the program capacity, the number of input / output ports and the number of internal flags, counters and timers are fixed.

Figure 9 : Memory Map

OPERATING SYSTEM INPUT / OUTPUT IMAGE BITS DATA USER’S PROGRAM SPACE MEMORY LOCATION 0002 MEMORY ADDRESSES 0001 BOTTOM OF MEMORY 0000 ROM RAM

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Rev. : 00 Date : 25/12/97 Page 19 of 59

2.6

Programmer Units :

This unit provides an interface between the PLC and the user during program development, start up and trouble shooting. The instructions to be performed during each scan are coded and inserted into memory with the programmer. Programmers vary from small hand held units the size of a small calculator, to desktop standalone intelligent CRT based units. These units come complete with documentation, reproduction, I/O status and on-line and off-line programming ability. Many PLC manufacturers now offer controller models that can use personal computer as a programming tool. Under these circumstances, the manufacturer will sell a program for the personal computer that usually allows the computer to interface with a serial input module installed in the programmable controller.

Programming units are the liaison between what the PLC understands (words) and what the engineer desires to occur during the control sequence. Some programmers have the ability to store programs on other media, including cassette tapes and magnetic disks. Another important feature is the automatic documentation of the existing program. This is accomplished by a printer attached with the programmer. With off-line programming, the user can write a program on the programming unit, then take the unit to the PLC in the field and load the memory with the new program, all without removing the PLC. Selection of these features depends on the user requirements and budget. On-line programming requires the cautious modification of the program while the PLC is controlling the process or the machine.

2.7

Peripheral Devices :

Peripheral devices are grouped into several categories : programming aids, operational aids, I/O enhancements and computer interface devices are most common.

Programming aids provide documentation and program recording capabilities. Although some devices can program many models of different manufacturers PLC’s, most are dedicated to single supplier and specific models. The definite trend in programming aids is PC compatible software that allows the programmable is sold by the PLC manufacturer or a license and is often model specific. If the software also offers online programming and trouble-shooting characteristics it may infact be used on a single specific programmable controller. This isolation is achieved by means of a software or hardware keys that come with each copy of the software purchased.

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Rev. : 00 Date : 25/12/97 Page 20 of 59

Operational aids include a variety of resources that range from colour graphics CRT to equipment or support programs that can give the operator specific access to process parameters. In this situation the operator is allowed to read and modify timer, counter and loop parameters but not have access to the program itself. Some aids facilitate the interaction between the programmable controller and the dumb terminals such as printers, to deliver process information in a desired format. Some devices have the ability to setup an entire panel and plug into the PLC through an external RS232C ports, thereby saving enormous panel and wiring costs.

I/O enhancement group is a large category of PLC peripheral equipment. It includes all types of modules, from dry contact modules to intelligent I/O to remote I/O capabilities. Some I/O simulators used to develop and debug programs can be categorized in the I/O enhancement group. These specific devices are typically hardware modules that can be plugged into the PLC. The computer interface device group is a rapidly expanding section of programmable controller peripheral devices. These devices offer peer to peer communications (i.e. one programmable controller connected to another), as well as network interaction with various computer systems. Infact, this group of devices will certainly expand in number as communication standards become more openly accepted and more and more products are provided to facilitate such network interactions.

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Rev. : 00 Date : 25/12/97 Page 21 of 59

3.0

Design a System Based PLC

3.1

Project Execution :

The PLC project must take into account the important considerations of schedule and budget as it is true for any major undertaking. The PLC can facilitate the transition, however by simultaneously pursuing several activities, thereby condensing the overall project schedule.

3.1.1 System Analysis :

The control system should be analyzed as a whole to determine plant control requirements. The PLC plays an integral part in these analyses and its capabilities should be thoroughly understood by the control engineer. Vital to the system analysis are process instrument diagrams (P & ID), the descriptive operational sequence and the logic diagram or electrical schematic. Part of this evaluation will be system sizing and selection. Once the appropriate PLC is selected and purchase orders are placed, two activities should begin immediately : engineering design and software development.

3.1.1.1 Engineering Design :

The first step is the development of input - output (I/O) list. This detailed document will be used extensively and should be developed with great care. Once I/O numbers are assigned, it becomes very difficult to change all references to these numbers. The I/O list is followed by configuration drawing. The configuration drawing shows the arrangement of the I/O and support hardware. The point to point wiring diagram will be used by the panel shop and the installation contractor to make the I/O devices interconnect.

Panel, or enclosure, deign should now be coordinated with the additional panel for instrumentation, such as light switches, meters and recorders. Once these steps are completed, panel fabrication and assembly can begin.

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3.1.1.2 Software (Program) Development :

The I/O list mentioned previously will be used to begin the program development. Basic control philosophy decisions need to be made at this point. Should valves fail to open or close? What fail safe provisions are necessary for analog control? These decisions should be documented and included with the process operational descriptions. Quite often this document is referred to as the software functional specification. Its purpose is to define as precisely as possible, the operation of controls. It has several other functions :

1) It communicates the functional requirement of the control system to those wiring the PLC code.

2) It records the thought process (regarding control) of the system designer to be used in the event of a personnel change. Such information can be invaluable.

3) It provides a review document for personnel working in other disciplines (mechanical, process, etc.) to ensure that they understand the operations of the controls.

4) It provides the guide for developing the operational description for the operator’s manual.

After the functional specifications have been reviewed and approved, a detailed operational sequence chart, timing diagram, logic diagram, flow chart or electrical schematic is developed from it. This schematic is translated or coded into the appropriate PLC language. The piping and instrumentation diagram is also cross referenced. In this way, future cross referencing of system drawings and PLC codes is facilitated.

As the code is entered, a memory map or register index is kept by the programmer. This map is useful in organizing the program data in logical arrangements and will prove invaluable during start-up, when the programmer may need to located the available blocks of memory quickly for program revisions.

Once the program is entered, a simulation is recommended and the program check out process is begun ‘on the bench’. This process uses the functional specification to prove that the software is compatible. A large percentage of the program can be proved in this manner. Program debugging can be completed before field installation.

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Using the PLC and programming aids, the panel wiring can be ‘rung out’ (i.e. checked point by point) in through the PLC. Each I/O point should be activated separately to the terminal block or from the panel controls (buttons, lights, switches). In this manner, the electrical integrity of the panel from the terminal blocks inward is ensured. If any continuity problem exists hereafter, they will be located in the field wiring.

Some organizations prefer to perform a simulated operation check out at this time. This is highly successful approach and can be implemented if the simulators and the I/O point arrangement are organized to simulate the process outside the panel.

3.1.1.4 System Checkup and Startup :

After the electrical connections have been made and point to point wiring has been completed (mechanical completion), the system is ready for startup. The ability of the PLC to operate step by step through the startup becomes very useful at this stage.

Experienced PLC personnel may provide temporary STOP, CONTINUE and STEP switches in the back of the panel in order to facilitate the startup procedures. These switches can be key locked, software locked, or disconnected for normal operation. They are also useful as future maintenance and trouble shooting tools to diagnose future problems as being either hardware or software based.

3.2

Ladder Logic :

With a majority of PLCs, writing a program is equivalent to drawing a switching circuit. The switching circuit is drawn in a ladder diagram format. This format requires that :

1) Circuits are arranged as a series of horizontal lines containing inputs (refered to as contacts) and outputs (refered to as coils). Typical circuit lines are shown in figure 14 below.

2) Inputs must precede outputs and are in the form of normally open and normally closed contacts. Ladder symbol for a normally open contact is | |. The symbol for normally closed contact is | | .

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Rev. : 00 Date : 25/12/97 Page 24 of 59

3) There must always be atleast one output on each line. An output is, for example, a PLC output relay. The ladder symbol for a PLC output is drawn either as two parenthesis close together, i.e. ( ), or as a circle. 4) Circuits in the form of vertical lines are not used.

5) Numerical assignments for inputs (contacts) and outputs (coils) form part of the ladder diagram.

6) Other elements such timers, counters and shift registers can be implemented in ladder diagrams.

The term ladder is used because the lines of the completed diagram resemble the rungs of a ladder. The two vertical lines are called the bus lines and represent the power connections, in this case 24 V and 0 V. Each horizontal line represents a program line. The output on a program line is energized (turned on) when the input contact(s) to it are made (i.e., when the contacts connect the 24 V supply to the coil).

A ladder diagram can be translated into a program consisting of instructions and data. Table 1 describes the Boolean instructions that are used by the PLC manufacturers. Ladder programs are entered into memory in an address instruction data format. An address is a number which activates a memory location. Instructions and data are entered into sequential memory locations usually starting from address zero.

Figure 1 : Ladder Format

Table 1 : Ladder Instructions

24 V Inputs Outputs 0 V Program Line 1 Program Line 2 24V Bus Line Program Line 3 Program Line 4 Inputs 0 V Bus Line

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Rev. : 00 Date : 25/12/97 Page 25 of 59

INSTRUCTIONS DESCRIPTION

LOAD Load logical state of start input

LOAD NOT Load logical state of start input and invert

AND Logical AND operation

AND NOT Logical AND NOT operation

OR Logical OR operation

OR NOT Logical OR NOT operation

OUT Output

It is always the ladder logic which determines how the outputs are energized. Consider figure 2, which shows two controllers both having a normally open switch connected to their input ports IN1. The controller shown in figure 2(a) is ladder programmed so that when the switch connected to IN1 closes the output connected to CR1 is energized (turned on). The controller shown in figure 2(b) is ladder programmed so that when the switch connected to CR1 is de-energized (turned off). The control action of figure 2(b) is opposite to that of figure 2(a) because NOT function is used in the ladder.

In figure 2(a), the logical state of the input is loaded and that is used to control the output. In figure 2(b) the logical state of the input is read, its value is inverted and then it is used to control the output.

2(a)

2(b)

Figure 2 : Ladder Control Action

4.0

Hardware & System Sizing and Selection

Ladder Program CR1 IN1 Ladder Program N0 IN1 Controller CR1 Load N0 IN1 Controller CR1 Load

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Rev. : 00 Date : 25/12/97 Page 26 of 59

Hardware and system sizing of the PLC can be determined by an analysis of the following system characteristics :

a) I/O quantity

b) I/O remoting requirements c) Memory quantity d) Redundancy level e) Programming requirements f) Programmers g) Peripheral requirements

4.1

I/O quantity :

In most PLC’s, plug in modules are used to convert the I/O signal level to one that is compatible with the bus architecture. These models can be composed of 1, 4, 8, or 16 points, depends on the manufacturers standard design.

The I/O base (rack or housing) is used to hold the I/O module in place and to provide a termination point for the wiring. The bases may be mounted anywhere in the control enclosure, however, there are cable length requirements which must be met. The majority of the bases mount horizontally to allow proper module cooling. A terminal strip is built into the mounting base for field connections so that no wiring need be distributed in order to remove or replace a module. These bases typically hold various quantities of I/O anywhere from 1 to 128 I/O points. Whereas in most of the systems, the modules have the intelligence to communicate with CPU, some systems require the use of the serial interface modules.

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A unique feature of the PLC is the muliplexed nature of the I/O bus. This can be used to great advantage to reduce overall wiring cost. If I/O racks are centralized in logical clusters, plant wiring requirements can be greatly reduced.

Remote I/O can be broken down into two distinct types : the integral type, which allows a limited transmission distance (upto 15,000 feet or 4500 meters) and transmitter / receiver type, which allows virtually unlimited transmission capability.

It is important to remember the major weaknesses of remote I/O systems. If the bus is cut or interrupted, the effects of I/O failure will be relatively unpredictable. One must consider the effect of system failure on each step in the sequence. For this reason, duplication of smaller CPU’s at each remote location is often considered preferable to a large central CPU. This is actually an extension of distributed control within the network of the PLC itself. This approach can be very cost effective, since requirement for the central unit size can be reduced.

4.3

Memory Quantity :

The type and quantity of PLC memory used depends on the controllers size and the company that manufactured it. Most small PLC’s come with a fixed quantity of RAM. Although it is 2K or 4K of memory, the actual number of memory locations is not as important as the average size application program the PLC can be expected to handle. Size refers to the number of I/O points that are to be controlled and the average number of logic, timer, counter and math operations that are to be performed. Some manufacturers may provide an extra expense option of PROM or ROM memory with their small PLC’s. Midsize and large PLC’s provide users an option for almost any type of memory desired.

Total memory, as stated in the manufacturers literature, does not necessarily mean that the entire content is available to the user. Some manufacturers reserve large blocks for the system executive. A system with 4K of 16 bit words of user memory may comfortably accommodate a program, whereas, another system with 8K of 8 bit words may have too small a memory for the same program.

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Rev. : 00 Date : 25/12/97 Page 28 of 59

Special programming language features are an important aspect of memory sizing, especially in process control. For example, one manufacturer requires 33 words of user available memory, whereas another may need in excess of 1000 words. Obviously the memory sizing for a loop control program would vary in these two systems. The closer the language is to machine code (binary based), the more user memory is required to perform the more complex functions.

The best way to determine program memory prerequisites is to write a representative sample program reflecting some actual project requirements and to request information about user memory size from various manufacturers. If the manufacturers recommendations are followed, the user can be reasonably assured that the memory will not be undersized.

The final area of caution about memory size concerns the consideration if data storage. Data tables, scratch pads and historical data retrieval requirements can inflate the size of the PLC memory. It should be remembered that the primary task of the PLC is control of the process. If the data requirements are large, connections to auxillary devices such as mini and micro computers should be given serious consideration. It is not good engineering practice to degrade control capabilities by burdening the PLC with excessive data acquisition functions.

4.4

Redundancy Level :

The availability of the PLC system depends upon the safety aspects involved in the plant process.

The Risk graph R (according to standard DIN V 19250), a process unit can be classified into various safety classes like class 1, 2, 3, 4, 5 and 6. Degree of risk factor increases from class 1 to class 6. The graph gives a better idea. Hence the availability level of the system, depends upon the safety classes as mentioned below :

i) Upto class 4 : Normal availability is required. One CPU is connected to single (non-redundant) I/O modules via an I/O bus.

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Rev. : 00 Date : 25/12/97 Page 29 of 59

ii) Upto class 5 : A redundant CPU modules are connected to non-redundant I/O modules and via a common I/O bus. In turn the CPU, the same program is processed simultaneously via a fast data link the processor exchange the state of the inputs and outputs during a cycle and thus check each other. The defective module can be easily exchanged during operation. If the safe PLC is used in requirement class 5 - for which the standard requires a redundant central unit - single channel operation in the case described is permitted for upto 72 hours.

iii) Upto class 6 : In this case the redundancy level is extended to I/O points, I/O buses and CPU level. The failure of one CPU, usually leads to complete shut down of the system. If according organizational steps are taken, a single channel operation upto one hour is possible before the plant automatically shuts down.

Damage Duration of Hazard Relative Low Very

Stay Prevention High Low

Injury 1

-

-

Value

1

1

Not Possible

2

1

Possible

3

2

Not Possible

4

3

5

4

6

5

8

7 6

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Rev. : 00 Date : 25/12/97 Page 30 of 59

4.5

Programmers :

Three basic programming tools generally provided by manufacturers are : - Hand held programmers

- CRT Programmers

- CRT programmer simulators (that run on PC)

The hand held programmers enables the operator to enter a program one contact at a time. These units are widely used because they are rugged, portable and easy to operate. They are very cost effective and give an engineer the capability to enter a program and to diagnose trouble in logic and field devices.

The CRT programmer provides the engineer with the visual picture of the program in the PLC. Ladder diagrams are drawn on the screen, just as they could be drawn on paper. Design and trouble shooting is reduced with the use of the CRT. With menu driven software, programmer training time is decreased. The CRT is designed for desk top or factory floor orientation. These units can be ordered with memory storage capabilities. Some CRT programmers provide complete documentation capability, including ladder diagrams, cross reference listing, and I/O listing. The CRT programmer also includes an external RS232C port for connection to a printer.

The CRT programmer simulator operates on a personal computer. Cost is an important factor to be considered. The second factor is the program version because of which the simulator may not as versatile as the CRT programmer. This versatility mismatch is in the functions it provides and the number of programmable controllers that can use it. Since the CRT was initially the primary programming tool sold by the PLC manufacturer and the CRT programmer simulator may have been developed by a software house licensed by the PLC manufacturer, there is no guarantee that the program will have all of the functions or operate the same way that the CRT programmer does.

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Rev. : 00 Date : 25/12/97 Page 31 of 59

4.6

Special Design Norms for PLC :

a) The PLC program should be protected from unauthorized changes by the use of security measures such as passwords or key lock switches. b) The system should incorporate comprehensive self diagnostic so that

all permanent and transient faults are identified. located. alarmed and reported. All diagnostics should be performed automatically on-line without disturbing the process or reducing the reliability of PLC. c) PLC on-line diagnostics should do the following :

- Test all the spare board in the system.

- Test board ID and status at a minimum frequency of once per minute.

- Check the I/O board configuration and set the main chassis alarm if boards are missing or faulted.

- Check I/O boards for faults, including fuse failures where applicable and if detected, turn on fault LEDs on the board. - Perform diagnostics on the communication processor and

cables which handle I/O board communications.

d) PLC must perform diagnostics on its main processor as follows : - Diagnostics on the processor and the floating point unit are

performed continuously in the back ground.

- Random Access Memory (RAM) diagnostics are also performed continuously.

- The micro processors on the main processor board are checked for proper response very minute.

- The control program checksum is verified.

- Universal Asynchronous Receiver Transmitter (VART) diagnostics are run continuously.

- The checksum for all program read only memories (ROMs) on the main processor are checked continuously.

- Redundant process and programs are verified as good and current.

- The PLC should perform extensive and power-up diagnostics on the main processor.

e) The processor should be modular and electrically isolated from associated I/O components.

In the event of power loss, the processor should retain its memory for minimum six months.

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Rev. : 00 Date : 25/12/97 Page 32 of 59

A read time clock with a minimum 10 ms resolution should be provided for time tagging events, rate calculations and other time dependent functions.

The processor should be capable of scanning and updating the I/O and of executing user defined discrete logic a minimum of ten times per second (100 ms) and analog functions a minimum of four times per second (250 ms).

The processor should be able to execute commands using the following functions and parameters :

- Math functionality using both integers and real numbers. - Logic including transition inputs and latching outputs. - Time delays, counters and timers.

- Arithmetic, algebraic and trigonometric functions. - PID and process control functions.

- If-then-else statement programming.

- Median select and mediation deviation function for analog input voltage.

k) The information to be transferred to from the DCS via PLC interface should include, but not limited to,

- System alarms and status - Discrete I/O status. - Analog I/O status.

The speed of transmission shall not increase 4 seconds.

l) Power supplies should be redundant for critical PLC applications with each capable of supplying complete system power. The system should accept power from two different power sources.

System power supplies should have over temperature protection, integral fuse protection and status LEDs to indicate power supply faults. In addition, each power supply should have an alarm contact to indicate presence of a fault.

m) At least 20% spare capacity should be available within each system. This includes marshaling cabinets, terminations, monitor switches. User program memory should have at least 40% spare capacity.

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Rev. : 00 Date : 25/12/97 Page 33 of 59

5.0

PLC Installation

Installation of programmable controller systems is not a difficult or mysterious procedure, but the following general rules will save time and trouble for the systems designer or installer. The basic principles of PLC installations are the same as those for installation of relay or other control systems. Safety rules and practices governing proper use of electrical control equipment in general should be observed. These include correct grounding techniques, placement of disconnect devices, proper selection of wire gauge, fusing, and logical layout of the device. PLCs can often be retrofitted into existing hardwired relay enclosures because they are designed to withstand the typical plant environment. PLC vendors provide installation manuals upon request.

5.1

Safety Considerations :

Perhaps the most important safety feature, which is often neglected in PLC safety design, is emergency stop and master control relays. This feature must be included whenever a hardwired devices is used in order to ensure operator protection against the unwanted application of power. Emergency stop functions should be completely hardwired. In no way should any software functions be relied upon to shut off the process or the machine. Disconnect switches and master relays should be hardwired to cut off power to the output supply of the PLC. This is necessary because most PLC manufacturers use triacs for their output switching devices, and triacs are just as likely to fail on as off.

5.2

Implementation :

Planning ahead is every bit as important in designing a complete PLC system as in laying out a relay logic panel. Care in counting I/O points in the beginning and leaving a safety factor will save headache in the panel fabrication stage. Panels should always have plenty of expansion room left over, since I/O is invariably added as the job progresses and the operators see the advantages of the PLCs. The designer should refer to the layout considerations provided by the manufacturer. Extra space should be left to provide access to the boards and connectors of the PLC. The diagnostic and status indictors should all be visible. The designer should leave room between I/O racks for wire ways and large hands.

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Rev. : 00 Date : 25/12/97 Page 34 of 59

One good technique for ensuring efficient panel layouts is to involve maintenance personnel in the design procedure. This not only optimizes the layout but also introduces the staff to the hardware (figure 1).

In general, the best defense against creating a tangled mess when designing a PLC system is to follow proper documentation techniques. A little more time spent documenting panel layout, I/O counts, and wiring diagrams results in a lot less time spent starting up the system. PLCs can handle large amounts of I/O points with varying electrical characteristics, so things can get pretty confusing in a hurry. Cable requirements between hardware boxes vary from one type of PLC to another, so this is an important consideration in panel layout.

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Rev. : 00 Date : 25/12/97 Page 35 of 59

5.3

Enclosure :

Enclosures should nearly always be provided for the PLCs themselves. This protects the electronics from moisture, oil, dust particles, and unwanted tampering. Most manufacturers recommend a NEMA 12 enclosure for the standard industrial environment. This type of environment is readily available in a variety of sizes and, infact, may be already included with a new system. Programmable controllers are designed to be located close to the machine or process under control. This keeps the wiring runs short and aids the troubleshooting procedure. At times, however, mounting the PLC directly on the machine or too close to the process is not advisable, such as in case of vibration inherent in the machine, electrical noise interference, or excessive heat problems. In these situations, the PLC must either be moved away or successfully protected against these environmental conditions.

5.4

Temperature Considerations :

Installing any solid state device requires paying attention to ambient temperatures, radiant heat bombardment and the heat generated by the device itself. PLCs are typically designed for operation over a broad range of temperatures, usually from 0 to 60°C. When analyzing the proposed PLC environment, however, one should remember that enclosure temperatures usually run a few degrees higher than ambient temperatures. Radiant heat on an enclosure from surrounding tanks can raise the internal temperature beyond that specified by the manufacturer.

Heat generated by the PLC is a key issue when the device is placed in ambient temperatures close to the extremes mentioned in the specifications. The temperature rise caused by the power consumption of the PLC itself is not hard to estimate. In addition, most manufacturers will provide a notation of power consumption of the triacs driving field loads. When designing the hardware layout within the panel, one should adhere to the manufacturer’s suggestions regarding ways to minimize heating problems. Most PLCs use convection over fins to take heat away from particular areas within the hardware. Care must be taken to ensure that no obstruction to air flow over these fins is introduced by placement of the PLC in the enclosure. Wire ways are typically provided with holes to allow air to pass through. Generally, one can avoid problems with PLC enclosures by simply leaving plenty of air space around the heat producers.

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Rev. : 00 Date : 25/12/97 Page 36 of 59

Should all of these factors combine to cause a temperature problem, the panel can be vented, air conditioned, or moved to another location. Usually, simply blowing filtered air through the enclosure will resolve minor difficulties. If air conditioning is required, small units that are designed for cooling electronic enclosures are readily available.

5.5

Noise :

Noise or unwanted electrical signals can generate problems for all solid state circuits, particularly microprocessors. Each PLC manufacturer suggests methods for designing a noise immune system. These guidelines should be strictly followed in design and installation phases, since noise problems can be very difficult to isolate after the system is up and running. I/O systems are isolated from the field, but voltage spikes can still appear within the low voltage environment of the PLC if proper grounding practices are not followed.

A well grounded enclosure can provide a barrier to noise bombardment from outside. Metal-to-metal contact between the PLC and the panel is a must, as is a good connection from the panel to the ground. Noise producers within the panel should be noted during the panel design phase, and the PLC must not be located too close to these devices. Wiring within the panel should also be diverted around noise producers so as not to pickup any stray signals. Often it is necessary to keep AC and DC wiring bundles apart, particularly when high voltage AC is used at the same time that low level analog signals are present. Line voltage variations can cause hard-to-trace problems in the operation of any computer based system. PLCs are no exception, even though they are designed to operate over a much larger variation in supply voltage. Large spikes or brownout conditions can cause errors in program execution. Most manufacturers protect against this, enabling the controller to come up running after a brownout, but these measures may not be acceptable in all applications. The designer may wish to add an isolation transformer to a proposed PLC system, sized for twice the anticipated load. This is cheap insurance, and PLC manufacturers will help determine the required load.

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Rev. : 00 Date : 25/12/97 Page 37 of 59

5.6

Hookup :

PLC can be very neat and orderly if all the terminals are arranged in a logical fashion. The actual result is a direct function of the time spent during the design process. Interposing terminal blocks between the PLC I/O structure and the field is suggested, since the terminations provided by PLC manufacturers are shrinking in the race to provide higher density I/O. This also gives the panel designer the ability to place the field termination points where they are easily accessed. Wiring ducts keep the panel neat and protect the wire from mishap.

Many noise problems can be averted by following good wiring practices. Low voltage signal wiring should be kept away from noise sources. Analog signals should be shielded, with the shield terminated at an isolated ground in the panel only (to prevent shield grounding loops). Again, these analog signals should be separated from power wiring.

Figure 2 : Dummy Load for Leakage

Figure 3 : Mechanical Contact in Series, RC suppresser

L1 N L1 Common Input R L1 N C Common Output (Triac) R

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Rev. : 00 Date : 25/12/97 Page 38 of 59

Figure 4 : Mechanical Contact in Parallel, MOV suppresser

Triac outputs require some special attention that will be new to relay users. Triacs used for AC loads typically leak a small amount of current. In the case of triacs outputs from a PLC, this leakage may be enough to keep panel lamps glowing or small relays energized. When a triac is used to switch the input on a PLC, the leakage may be enough to make the PLCs ‘think’ the input is on. A dummy load (shown in figure 2) can be used to drain this leakage when the input should be off. Whenever a mechanical contact is used in series with a load energized by a triac (as shown in figure 3), a resistance-capacitance (RC) network should be used as shown to protect the triac from inductive kickback. A varistor should be provided in parallel with a load whenever the load can be ‘hot-wired’ around the triac (figure 4). The user should check with the PLC manufacturer for the suggested RC and MOV (metal oxide varistor) types for the particular application. Triacs cannot directly drive large motor starters and similar devices. PLC manufacturers provide surge specifications for the various I/O cards. Sometimes an interposing relay or dry contacts will be required for large loads.

L1 N Common Output (Triac) MOV