Introduction to PLC’sIntroduction to PLC’s

Programmable Logic ControllerProgrammable Logic Controller

A programmable controller is a digital A programmable controller is a digital electronic apparatus with a programmable electronic apparatus with a programmable memory for storing instructions specific memory for storing instructions specific function, such as logic, sequencing, function, such as logic, sequencing, timing, counting, and arithmetic to control timing, counting, and arithmetic to control machines and processes. machines and processes.

A typical PLC can be divided into four A typical PLC can be divided into four components:components:

Processor unitProcessor unit

Power supplyPower supply

Input/output sectionInput/output section

Programming deviceProgramming device

Fixed I/OFixed I/O

PLC’s with fixed I/O typically come in a PLC’s with fixed I/O typically come in a complete unit that contains the processor, complete unit that contains the processor, I/O section, and power supply.I/O section, and power supply.

Modular I/OModular I/O

is modular in nature, more flexible than is modular in nature, more flexible than fixed I/O units. The I/O rack or chassis is fixed I/O units. The I/O rack or chassis is a framework or housing into which a framework or housing into which modules are inserted.modules are inserted.

FixedFixed

ModularModular

PLC,SLC-500 chassisPLC,SLC-500 chassis

Racks or chassis come in many shapes & sizes, Racks or chassis come in many shapes & sizes, and typically allow 4, 8, 12, or 16 modules to be and typically allow 4, 8, 12, or 16 modules to be inserted. Racks contain I/O modules and the inserted. Racks contain I/O modules and the processor are referred to as processor are referred to as locallocal I/O. Racks I/O. Racks that contain I/O modules, remote I/O that contain I/O modules, remote I/O communication cards, power supplies, and are communication cards, power supplies, and are mounted separately of away from the processor mounted separately of away from the processor are referred to as are referred to as remoteremote I/O. I/O.

Allen-Bradley uses Allen-Bradley uses the rack number, the rack number, location of a module location of a module within a rack, and the within a rack, and the terminal number of a terminal number of a module to which an module to which an input or output device input or output device is connected to is connected to determine the determine the device’s address.device’s address.

Discrete I/O ModulesDiscrete I/O Modules

Discrete I/O modules are types of modules Discrete I/O modules are types of modules that only accept digital or that only accept digital or ON-ON- and and OFF-OFF- type signals.type signals.

Discrete Input Module communicates the Discrete Input Module communicates the status of the various real-world input status of the various real-world input devices connected to the module (devices connected to the module (ONON of of OFFOFF) to the processor.) to the processor.

Discrete I/O ModulesDiscrete I/O Modules

Pilot Duty OutputsPilot Duty OutputsOutputs of this type typically are used to drive high-current electromagnetic Outputs of this type typically are used to drive high-current electromagnetic

loads such as solenoids, relays, valves, and motor starters. These loads are loads such as solenoids, relays, valves, and motor starters. These loads are highly inductive and exhibit a large inrush current. Pilot duty outputs should highly inductive and exhibit a large inrush current. Pilot duty outputs should be capable of withstanding an inrush current of 10 times the rated load for a be capable of withstanding an inrush current of 10 times the rated load for a short period of time without failure. short period of time without failure.

General - Purpose OutputsGeneral - Purpose OutputsThese are usually low- voltage and low-current and are used to drive indicating These are usually low- voltage and low-current and are used to drive indicating

lights and other non-inductive loads. Noise suppression may or may not be lights and other non-inductive loads. Noise suppression may or may not be included on this types of modules.included on this types of modules.

Discrete InputsDiscrete InputsCircuits of this type are used to sense the status of limit switches, push buttons, Circuits of this type are used to sense the status of limit switches, push buttons,

and other discrete sensors. Noise suppression is of great importance in and other discrete sensors. Noise suppression is of great importance in preventing false indication of inputs turning on or off because of noise.preventing false indication of inputs turning on or off because of noise.

Anaolog I/O ModulesAnaolog I/O Modules

Circuits of this type sense or drive analog signals. Circuits of this type sense or drive analog signals.

Analog inputs come from devices, such as thermocouples, strain Analog inputs come from devices, such as thermocouples, strain gages, or pressure sensors, that provide a signal voltage or current gages, or pressure sensors, that provide a signal voltage or current that is derived from the process variable. that is derived from the process variable.

Standard Analog Input signals: Standard Analog Input signals: 4-20mA; 0-10V4-20mA; 0-10V

Analog outputs can be used to drive devices such as voltmeters, X-Y Analog outputs can be used to drive devices such as voltmeters, X-Y recorders, servomotor drives, and valves through the use of recorders, servomotor drives, and valves through the use of transducers. transducers.

Standard Analog Output signals: Standard Analog Output signals: 4-20mA; 0-5V; 0-10V4-20mA; 0-5V; 0-10V

Special-Purpose I/O ModulesSpecial-Purpose I/O Modules

Circuits of this type are used to interface PLCs to very Circuits of this type are used to interface PLCs to very specific types of circuits such as servomotors, stepping specific types of circuits such as servomotors, stepping motors PID (proportional plus integral plus derivative) motors PID (proportional plus integral plus derivative) loops, high-speed pulse counting, resolver and decoder loops, high-speed pulse counting, resolver and decoder inputs, multiplexed displays, and keyboards. inputs, multiplexed displays, and keyboards.

This module allows for limited access to timer and counter This module allows for limited access to timer and counter presets and other PLC variables without requiring a presets and other PLC variables without requiring a program loader. program loader.

Discrete I/O ModulesDiscrete I/O Modules

OPTO-ISOLATOR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

CurrentLimitingResistor

FROM INPUTDEVICE

USE TO DROP THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, hysteresis Circuits

TOPROCESSOR

DC Discrete Input ModulesDC Discrete Input Modules

Input module wiring DiagramInput module wiring Diagram

AC Discrete Input ModuleAC Discrete Input Module

OPTO-ISOLATOR

IS NEEDED TO: Prevent voltage transients from damaging the processor.Helps reduce the effects of electrical noise

Rectifier,ResistorNetwork

FROM INPUTDEVICE

CONVERTS THE AC INPUT TO DC AND DROPS THE VOLTAGE TO LOGIC LEVEL

Buffer, Filter, Hysteresis Circuits

TOPROCESSOR

AC Discrete Input ModuleAC Discrete Input Module

Discrete Output ModulesDiscrete Output Modules

The purpose of a discrete output module is The purpose of a discrete output module is to control the current flow to real-world to control the current flow to real-world devices such as motor starter coils, pilot devices such as motor starter coils, pilot lights, control relays, and solenoid valves.lights, control relays, and solenoid valves.

Discrete DC OutputDiscrete DC Output

Discrete AC OutputDiscrete AC Output

Discrete Input ExampleDiscrete Input Example

OFFLogic 0

IN

PLC

InputModule

24 V dc

OFFLogic 1

IN

PLC

InputModule

24 V dc

Analog I/O ModulesAnalog I/O Modules

Analog input modules are used to convert Analog input modules are used to convert analog signals from analog devices that analog signals from analog devices that sense such variables as temperature, light sense such variables as temperature, light intensity, speed, pressure, and position to intensity, speed, pressure, and position to 12-bit binary or to 3-digit binary-coded 12-bit binary or to 3-digit binary-coded decimal (BCD), depending on the decimal (BCD), depending on the manufacturer, for use by the processor.manufacturer, for use by the processor.

Analog InputAnalog Input

An analog input is an input signal that has a continuous signal. Typical inputs An analog input is an input signal that has a continuous signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V. Below, a level transmitter may vary from 0 to 20mA, 4 to 20mA or 0 to10V. Below, a level transmitter monitors the level of liquid in the tank. Depending on the level Tx, the signal monitors the level of liquid in the tank. Depending on the level Tx, the signal to the PLC can either increase or decrease as the level increases or to the PLC can either increase or decrease as the level increases or decreases.decreases.

Level TransmitterLevel Transmitter

Tank

IN

PLCAnalogInputModule

Analog OutputAnalog Output

An analog output is an output signal that has a continuousAn analog output is an output signal that has a continuoussignal. Typical outputs may vary from 0 to 20mA, 4 to 0mAsignal. Typical outputs may vary from 0 to 20mA, 4 to 0mAor 0 to10V.or 0 to10V.

OUT

PLC

AnalgeOutputModule

EP Supply air

0 to 10V

Sourcing and SinkingSourcing and Sinking

This is the manner in which DC devices This is the manner in which DC devices are wired.are wired.In electronics, if a device (input module) In electronics, if a device (input module) provides current, or is the source of provides current, or is the source of current, it is said to be sourcing.current, it is said to be sourcing.If the device (input module) is receiving If the device (input module) is receiving current, it is said to be sinking.current, it is said to be sinking.

Input Module SinkingInput Module Sinking

ControlLogix IB32 DC Input ControlLogix IB32 DC Input Module Connection (Sinking)Module Connection (Sinking)

PLC-5 IBD DC Input Module PLC-5 IBD DC Input Module Connection (Sinking)Connection (Sinking)

PLC-5 OBD DC Output Module PLC-5 OBD DC Output Module Connection (Sourcing)Connection (Sourcing)

SLC-500 IB16 DC Input Module SLC-500 IB16 DC Input Module Connection (Sinking)Connection (Sinking)

SLC-500 OB16 DC Output SLC-500 OB16 DC Output Module Connection (Sourcing)Module Connection (Sourcing)

S7 SM321 DI32x24V DC Input S7 SM321 DI32x24V DC Input Module Connection (Sinking)Module Connection (Sinking)

S7 SM322 DO16x24V DC Output Module S7 SM322 DO16x24V DC Output Module Connection (Sourcing)Connection (Sourcing)

Modicon DDI 1602 24V DC Input Module Modicon DDI 1602 24V DC Input Module Connection (Sinking)Connection (Sinking)

Modicon DDO 1602 24V DC Output Module Modicon DDO 1602 24V DC Output Module Connection sourcingConnection sourcing

GE MDL 660 12/24V DC Input Module GE MDL 660 12/24V DC Input Module Connection (Sinking)Connection (Sinking)

GE MDL 754 12/24V DC Output Module GE MDL 754 12/24V DC Output Module Connection sourcingConnection sourcing

Interposing RelayInterposing Relay

When it is necessary to control loads When it is necessary to control loads larger than the rating of an individual larger than the rating of an individual output circuit, a standard control relay, output circuit, a standard control relay, which has a small inrush and sealed which has a small inrush and sealed current value, is connected to the output current value, is connected to the output module.module.

ProcessorProcessor

The processor module contains the PLC’s The processor module contains the PLC’s microprocessormicroprocessor, its , its supporting circuitrysupporting circuitry, and its , and its memory system.memory system.

The main function of the The main function of the microprocessormicroprocessor is to analyze data coming is to analyze data coming from field sensors through input modules, make decisions based on from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through the user’s defined control program and return signal back through output modules to the field devices. output modules to the field devices. Field sensorsField sensors: : switches, flow, switches, flow, level, pressure, temp. transmitters, etc. level, pressure, temp. transmitters, etc. Field output devicesField output devices: : motors, valves, solenoids, lamps, or audible devices.motors, valves, solenoids, lamps, or audible devices.

The The memory systemmemory system in the processor module has two parts: a in the processor module has two parts: a system memorysystem memory and an and an application memoryapplication memory..

MemoryMemory

VOLATILEVOLATILE. . A volatile memory is one that loses its stored information when power A volatile memory is one that loses its stored information when power

is removed. is removed.

Even momentary losses of power will erase any information stored or Even momentary losses of power will erase any information stored or programmed on a volatile memory chip.programmed on a volatile memory chip.

Common Type of Volatile Memory Common Type of Volatile Memory

RAM.RAM. RRandom andom AAccess ccess MMemory(Read/Write) emory(Read/Write) Read/write indicates that the information stored in the memory can be Read/write indicates that the information stored in the memory can be

retrieved or read, while write indicates that the user can program or retrieved or read, while write indicates that the user can program or write information into the memory. write information into the memory.

MemoryMemory

The words The words random accessrandom access refer to the ability of any location refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage used for both the user memory (ladder diagrams) and storage memory in many PLC’s.memory in many PLC’s.

RAM memory must have RAM memory must have battery backupbattery backup to retain or protect the stored to retain or protect the stored program.program.

MemoryMemory

NON-VOLATILENON-VOLATILEHas the ability to retain stored information when power is removed, Has the ability to retain stored information when power is removed,

accidentally or intentionally. These memories do not require battery accidentally or intentionally. These memories do not require battery back-up.back-up.

Common Type of Non-Volatile Memory Common Type of Non-Volatile Memory

ROM, RROM, Read ead OOnly nly MMemoryemoryRead only indicates that the information stored in memory can be read Read only indicates that the information stored in memory can be read

only and cannot be changed. Information in ROM is placed there by only and cannot be changed. Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC. the manufacturer for the internal use and operation of the PLC.

MemoryMemory

Other Types of Non-Volatile Memory Other Types of Non-Volatile Memory

PROM,PROM, PProgrammable rogrammable RRead ead OOnly nly MMemoryemoryAllows initial and/or additional information to be written into the chip. 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 PROM may be written into only once after being received from the PLC manufacturer; programming is accomplish by pulses of current. manufacturer; programming is accomplish by pulses of current.

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

MemoryMemory

EPROM,EPROM, EErasable rasable PProgrammable rogrammable RRead ead OOnly nly MMemory emory

Ideally suited when program storage is to be semi-permanent or Ideally suited when program storage is to be semi-permanent or additional security is needed to prevent unauthorized program additional security is needed to prevent unauthorized program changes. changes.

The EPROM chip has a quartz window over a silicon material that The EPROM chip has a quartz window over a silicon material that contains the electronic integrated circuits. This window normally is contains the electronic integrated circuits. This window normally is covered by an opaque material, but when the opaque material is covered by an opaque material, but when the opaque material is removed and the circuitry exposed to ultra violet light, the memory removed and the circuitry exposed to ultra violet light, the memory content can be erased. content can be erased.

The EPROM chip is also referred to as The EPROM chip is also referred to as UVPROM.UVPROM.

MemoryMemory

EEPROMEEPROM, , EElectrically lectrically EErasable rasable PProgrammable rogrammable RRead ead OOnly nly MMemoryemory

Also referred to as EAlso referred to as E22PROM, is a chip that can be programmed using a PROM, is a chip that can be programmed using a standard programming device and can be erased by the proper standard programming device and can be erased by the proper signal being applied to the erase pin. signal being applied to the erase pin.

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

ScanningScanning

While the PLC is running, the scanning process includes the following While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of four phases, which are repeated continuously as individual cycles of operation:operation:

PHASE 2Program

Execution

PHASE 3Diagnostics/

Comm

PHASE 4Output

Scan

PHASE 1Read Inputs

Scan

ScanningScanning

The time it takes to implement a scan cycle is called The time it takes to implement a scan cycle is called SCAN TIMESCAN TIME. The . The scan time composed of the scan time composed of the program scan timeprogram scan time, which is the time , which is the time required for solving the control program, and the required for solving the control program, and the I/O update timeI/O update time, or , or time required to read inputs and update outputs. The program scan time required to read inputs and update outputs. The program scan time generally depends on the amount of memory taken by the control time generally depends on the amount of memory taken by the control program and type of instructions used in the program. The time to program and type of instructions used in the program. The time to make a single scan can vary from 1 ms to 100 ms. make a single scan can vary from 1 ms to 100 ms.

As part of the processor’s internal self-diagnostic system, a watchdog As part of the processor’s internal self-diagnostic system, a watchdog timer is used. The watchdog timer is preset to an amount of time that timer is used. The watchdog timer is preset to an amount of time that is slightly longer than the scan time would be under normal conditions. is slightly longer than the scan time would be under normal conditions.

Safety CircuitSafety Circuit

The National Electrical Manufacturing The National Electrical Manufacturing Association (NEMA) standards for Association (NEMA) standards for programmable controllers recommends programmable controllers recommends that consideration be given to the use of that consideration be given to the use of emergency-stop functions that are emergency-stop functions that are independent of the programmable independent of the programmable controller.controller.

Rack InstallationRack Installation

Before installing a rack or chassis, consideration must be Before installing a rack or chassis, consideration must be given to the following:given to the following:TemperatureTemperatureDustDustVibrationVibrationHumidity Humidity Field wiring distancesField wiring distancesTroubleshooting accessibilityTroubleshooting accessibilityThe ambient temperature of the proposed location The ambient temperature of the proposed location should not be lower than 32F or higher than 140F (0C should not be lower than 32F or higher than 140F (0C and 60C).and 60C).

Dust can also cause a problem in the I/O Dust can also cause a problem in the I/O rack when it accumulates on the electronic rack when it accumulates on the electronic components of the modules, power components of the modules, power supply, or processor. Accumulated dust supply, or processor. Accumulated dust prevents the components from dissipating prevents the components from dissipating heat effectively.heat effectively.Excessive vibration can also lead to early Excessive vibration can also lead to early component failure.component failure.

Exposing electronic equipment to extremely high Exposing electronic equipment to extremely high humidity environments over an extended period humidity environments over an extended period of time can reduce component life and affect of time can reduce component life and affect operation.operation.

By mounting the I/O rack close to the actual By mounting the I/O rack close to the actual equipment, the amount of conduit, cable, and equipment, the amount of conduit, cable, and other associated wiring and labor costs will be other associated wiring and labor costs will be decreased. decreased.

Safety CircuitSafety Circuit

The National Electrical Manufacturing The National Electrical Manufacturing Association (NEMA) standards for Association (NEMA) standards for programmable controllers recommends programmable controllers recommends that consideration be given to the use of that consideration be given to the use of emergency-stop functions that are emergency-stop functions that are independent of the programmable independent of the programmable controller.controller.

Electrical Noise Electrical Noise

Electrical noise is generated whenever inductive loads such as relays, solenoids, motor starters, and motors are operated by “hard contacts” such as push buttons, selector switches, and relay contacts. When the circuit to an inductive load is opened, much of the energy stored in the load must be dissipated as arcing at the contacts unless some alternative means of energy absorption is provided.

Electrical Noise Electrical Noise

Isolation of the electrical noise is Isolation of the electrical noise is accomplished by installing and isolation accomplished by installing and isolation transformer for the PLC system to supply transformer for the PLC system to supply the power for the controller and the input the power for the controller and the input circuits.circuits.A second method in reducing EMI is to A second method in reducing EMI is to install surge suppression networks or install surge suppression networks or devices on the individual motor starters, devices on the individual motor starters, motors, and soleniods.motors, and soleniods.

Electrical NoiseElectrical Noise

A A surge protector surge protector limits the voltage limits the voltage supplied to an electric device by either supplied to an electric device by either blocking or by shorting to ground any blocking or by shorting to ground any unwanted voltages above a safe threshold.unwanted voltages above a safe threshold.

With solid-state control systems, proper With solid-state control systems, proper grounding helps eliminate the effects of grounding helps eliminate the effects of electromagnetic induction.electromagnetic induction.

Electrical NoiseElectrical Noise

Certain I/O modules such as TTL, analog, Certain I/O modules such as TTL, analog, and thermocouples require shielded cable and thermocouples require shielded cable to reduce the effects of electrical noise.to reduce the effects of electrical noise.

When installing shielded cable, it is When installing shielded cable, it is important that the shield only be grounded important that the shield only be grounded at one end.at one end.

Guarding Against Electrostatic Guarding Against Electrostatic Discharge (ESD)Discharge (ESD)

1) use nonstatic floor coverings1) use nonstatic floor coverings

2) Handle chips correctly2) Handle chips correctly

3) Ground the work surface3) Ground the work surface

4) wear a wrist strap4) wear a wrist strap

A programming device is needed to enter, A programming device is needed to enter, modify, and troubleshoot the PLC program, modify, and troubleshoot the PLC program,

or to check the condition.or to check the condition.