PLC ARCHITECTURE AND HARDWARE COMPONENTS

PLC ARCHITECTURE AND HARDWARE COMPONENTSPRESENTED BY:Akshay DholeMechatronics (SMBS)13mmt1013

indexIntroduction Basics of PLCPLC evolutionRelays SemiconductorsPLC PLC ArchitectureHardware ComponentsAdvantages Disadvantages

INTRODUCTION

A Programmable Logic Controller (PLC) is a specialized computing system used for control of industrial machines and processes.

A PLC is a computer designed to work in an industrial environment

Equipped with special input/output interfaces

Programmed using a control programming language.

BASICS OF PLC

EVOLUTION OF PLCSRelay controllersSemi-conductor logic controllersProgrammable Logic Controllers (PLC)

RELAY CONTROLLERSLarge dimensionsSlowNoisyRequires a lot of energy to workSensible for dirt

SEMICONDUCTORS LOGIC CONTROLLERSFastSmallReduced energySilentHard to adapt to another controlling task

PLCSAll advantages of the semi-conductor logic controllers

Easy to adapt (re-programming)

Real-time working

Minimal maintenance

Low costs

PLC ARTICTURE

Program memory

Data memory

Processor

Communicationunit

Input module

Programming unit

Output module

Systemmemory

PROCESSORThe main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the users defined control program and return signal back through output modules to the field devices.

Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

MEMORYSystem memoryROM (Read Only Memory) PROM (Programmable Read Only Memory) EPROM (Erasable Programmable Read Only Memory)EEPROM (Electric Erasable Programmable ROM)

Program/Data memoryRAM (Random Access Memory)

SYSTEM MEMORYSystem memory includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities, such as execution of the users control program, communication with peripheral devices, and other system activities.

The system memory also contains the routines that implement the PLCs instruction set, which is composed of specific control functions such as logic, sequencing, timing, counting, and arithmetic.

System memory is generally built from read-only memory devices.

ROMRead Only Memory

A non-volatile memory. It has the ability to retain stored information when power is removed, accidentally or intentionally. A ROM does not require battery back-up.

Read only indicates that the information stored in memory can be read only and cannot be changed. Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC.

PROMProgrammable ROM

Allows initial and/or additional information to be written into the chip.

PROM may be written into only once after being received from the PLC manufacturer; programming is accomplish by pulses of current.

The current melts the fusible links in the device, preventing it from being reprogrammed. This type of memory is used to prevent unauthorized program changes.

EEPROMElectrically EPROM

Also referred to as E2PROM, is a chip that can be programmed using a standard programming device and can be erased by the proper signal being applied to the erase pin.

EEPROM is used primarily as a non-volatile backup for the normal RAM memory. If the program in RAM is lost or erased, a copy of the program stored on an EEPROM chip can be down loaded into the RAM.

Program/Data memoryThe program and data memories are called together application memory.

The data memory stores any data associated with the users control program, such as system input and output status data, and any stored constants, variables, or preset values. The data memory is where data is monitored, manipulated, and changed for control purposes.

The user program area is where the programmed instructions entered by the user are stored as an application control program.

RAMRandom Access Memory (Read/Write)A volatile memory, that is one that loses its stored information when power is removed.Even momentary losses of power will erase any information stored or programmed on a volatile memory chip.Read/write indicates that the information stored in the memory can be retrieved or read, while write indicates that the user can program or write information into the memory.The words random access refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage memory in many PLCs.RAM memory must have battery backup to retain or protect the stored program.

Types of ram memoryMOS (Metal Oxide Semiconductor)HMOS (High Density, short channel MOS )CMOS (Complimentary MOS)

The CMOS-RAM is probably one of the most popular, because it has a very low current drain when not being accessed (15microamps.), and the information stored in memory can be retained by as little as 2Vdc.

Input/Output modulesThe I/O interface section of a PLC connects it to external field devices.

The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.

Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLCs processor.

Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.

DC INPUT MODULEStandard Digital Input signals: 24VCurrent limiting resistor

Opto-isolatorBuffer, filter, hysteresiscircuits

DC input

Processor

Used to drop the voltage to logic level

Prevents voltage transients from damagigng the processor.

Helps to reduce the effects of electrical noise.

AC INPUT MODULEStandard Analog Input signals: 4-20mA; 0-10VRectifier, Resistor Network

Opto-isolatorBuffer, filter, hysteresiscircuits

AC input

Processor

Converts the AC input to DC and drops the voltage to logic level

Prevents voltage transients from damagigng the processor.

Helps to redice the effects of electrical noise.

DC/AC OUTPUT MODULEStandard Analog Output signals: 4-20mA; 0-5V; 0-10V

TTL circuits

Opto-isolator

Aplifier

Processor

Output

COMMUNICAITON UNITManages the communication between the PLC &the controlled process,an other PLC,a PC,different peripheries.

Used ports:Serial (RS 232, RS 422, RS 485)Ethernet

SERIAL PORTSRS 232Used in short-distance computer communications, with the majority of computer hardware and peripherals.Has a maximum effective distance of approx. 30 m at 9600 baud.

RS 422 / RS 485Used for longer-distance links, often between several PCs in a distributed system. RS 485 can have a maximum distance of about 1000 meters.

Ethernet: Local Area Network (LAN)Local Area Network provides a physical link between all devices plus providing overall data exchange management or protocol, ensuring that each device can talk to other machines and understand data received from them.

LANs provide the common, high-speed data communications bus which interconnects any or all devices within the local area.

HARDWARE COMPONENTS

CONTENTS I/O Section I/O Modules Discrete Analog Special Specifications Memory Design Types

I/O SECTIONPower Supplyunaddressed (not typically part of rack) Processor in slot 0I/O in non-zero slots

LOGICAL RACKImage Table or File

File I for input

File O for output

PLC-5 -- Octal

Addressing SLC-500 Decimal Addressing

Remote I/O Rack

ADDRESS ELEMENTType File Type I for Input File O for Output File Others covered latter Slot Rack Number Slot Number Word or Bit Identifies the actual

terminal connection Discrete module use

one word and thus number specifies bit

COMMON DISCRETE I/O RATINGS

INPUT MODULES PERFORMSenses when a signal is received from a sensor on the machineConvert the input signal to correct logic level Isolates PLC from fluctuations in input signal

voltage (logic level noise) Sends a logic signal to processor indicating which sensor originated the signal

AC INPUT CIRCUIT

OUTPUT MODULE PERFORMConvert the logic level signal to correct

external signal Isolates PLC from fluctuations in output

signal voltage (logic level noise) Indicates output is active via lamp

AC OUTPUT CIRCUIT

Sinking & Sourcing

SPECIAL I/O MODULESHigh-Speed Counter Module Thumb-Wheel ModuleTTL Module Encoder-Counter Module BASIC or ASCII Module Stepper-Motor Module BCD-Output Module PID Module Servo Module Communications Module Language Module Speech Module

I/O SPECIFICATIONSNominal InputVoltage On-State Input Voltage Range Nominal Current per Input Ambient Temperature Rating Input Delay Nominal Output Voltage Output Voltage range Maximum Output

Current Rating perOutput and ModuleMaximum SurgeCurrent per Output Off-State Leakage Current per Output Electrical Isolation Number of Inputs and Outputs per Card Backplane Current Draw Resolution Input Impedance and Capacitance Common ModeRejection Ratio

PROCESSOR MODE (STATE)RUN Position Places processor in RUN mode Executes the ladder program and energizes output devices Prevents you from performing online program editing in this position Prevents you from using a programmer/operator interface device to change processor mode

PROG PositionPlaces processor in program mode Prevents processor from scanning or executing the ladder program Controller outputs de-energized Allows program entry and edit Prevents you from usingprogrammer/operator interface tochanging modeREM Position Places processor in remote mode: Remote Run, Remote Program, or Remote Test Allows you to change the processor mode with programmer/operator interface device Allows you to perform online program editing

Watch Electro-Static DischargeGround yourself by touching groundconductor Use wrist strap that is grounded Handle Cards/Modules by edge Dont touch circuit components Use conductive (static-safe) work bench top

MEMORY TYPESROM (Read-Only Memory)RAM (Random Access Memory) PROM (Programmable Read-Only Memory) EPROM (Erasable PROM) EEPROM (Electrical Erasable PROM) or Flash

ADVANTAGESIncreased Reliability More Flexibility Lower Cost Faster Response Easier to troubleshoot Remote control capability Communication Capability

disadvantages

In contrast to microcontroller systems that have what is called an open architecture, most PLCs manufacturers offer only closed architectures for their products

PLC devices are proprietary, which means that parts and software from one manufacturer can t easily be used in combination with parts of another manufacturer, which limits the design and cost options cant

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