FESTO Basic PLC

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Presentation Transcript

Programmable Logic Controllers : 

Programmable Logic Controllers

OBJECTIVE : 

OBJECTIVE To enable the participants to understand the design and principle of operation of Programmable Logic Controllers (PLC), write and implement simple programs and be familiar with applications of PLC’s in industries.

Topic Outline : 

Topic Outline Advantage of a PLC compared to conventional controls such as electrical, electro-pneumatic or electro-hydraulic controls Function of the system components of a PLC Commissioning of a Programmable Logic Controller Criteria for the use of mechanical, optical, capacitive and proximity sensors Circuit development/diagram design Development of logic control system Development of sequence control system Defining appropriate control system for a given task Control using timers and counters

Slide 5: 

Pneumatic Control System

PNEUMATIC ACTUATORS : 

PNEUMATIC ACTUATORS LINEAR CYLINDERS Single-acting Cylinder Double-acting Cylinder ROTARY CYLINDERS Rotary vane Cylinder Pneumatic motors

Slide 8: 

Single acting cylinder

Slide 9: 

Piston Rod Spring Connection Seal Bearing Vent

Single Acting Cylinders : 

Single Acting Cylinders If compressed air is supplied, air hits the piston surface and the piston rod moves out. When air is released, the return spring moves the piston to its initial position. Single acting cylinders do work in one way, therefore they are ideal for tensioning, ejecting, compressing etc.

Slide 11: 

Operation of Single Acting Cylinders

Slide 12: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 13: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 14: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 15: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 16: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 17: 

Force = Pressure x Area (piston) Operation of Single Acting Cylinders

Slide 18: 

Operation of Single Acting Cylinders

Slide 19: 

Operation of Single Acting Cylinders

Slide 20: 

Operation of Single Acting Cylinders

Slide 21: 

Operation of Single Acting Cylinders

Slide 22: 

Operation of Single Acting Cylinders

Slide 23: 

Operation of Single Acting Cylinders

Slide 24: 

Operation of Single Acting Cylinders

Slide 25: 

Double acting cylinder

Double Acting Cylinder : 

Double Acting Cylinder

Slide 27: 

Piston Rod Connections Base end Rod end Seals Bearing Wiper

Double Acting Cylinders with Air Cushioning : 

Double Acting Cylinders with Air Cushioning When the piston approaches its final position, the damping piston shuts off the direct air-outlet. Excess pressure sets up an air-cushion in the remaining cylinder volume and kinetic energy is converted into pressure. At this stage, air may only leave the cylinder through a controlled cross section of stream discharge.

Slide 29: 

Cushioning sleeves Cushioning adjustment Seals Non-return valve Operation of Double Acting Cylinders with Air Cushioning

Slide 30: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 31: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 32: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 33: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 34: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 35: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 36: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 37: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 38: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 39: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 40: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 41: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 42: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 43: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 44: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 45: 

Operation of Double Acting Cylinders with Air Cushioning

Slide 46: 

Operation of Double Acting Cylinders with Air Cushioning

PNEUMATIC VALVES : 

PNEUMATIC VALVES FUNCTIONS : open and close flow paths regulate pressure directs flow to various paths adjust flow volume

Slide 48: 

Directional control valves

3/2 - way Directional Control Valve, Solenoid Actuated, Spring Returned : 

3/2 - way Directional Control Valve, Solenoid Actuated, Spring Returned When an electric current is applied to the coil, an EMF is generated which lifts the lower sealing lips of the armature and opens the passage for pilot air. Pilot air then applies pressure on the diaphragm which then causes the valve to switch its position. Upon removal of the current, the pilot air passage closes and a spring returns the valve to its normal switching position.

5/2 - way Directional Control Valve, Solenoid Actuated, Spring Returned : 

5/2 - way Directional Control Valve, Solenoid Actuated, Spring Returned When the solenoid is energized, the armature moves and the pilot air passage opens. The pilot air applies pressure to the left side of the valve piston resulting to the valve switching its position. Upon removal of the electrical signal, a spring returns the valve to its neutral switching position. Used for the control of double acting cylinders.

5/2 - way Directional Control Valve, Double Solenoid Actuated : 

5/2 - way Directional Control Valve, Double Solenoid Actuated Because of the absence of a return spring, double solenoid actuated valves retain the last signal administered to them. They remain in their last switched position even with power removed from both solenoids. Effectively, this means that this valve has “memory characteristic”.

Compact Performance Valve : 

Compact Performance Valve CP stands for compact performance, and thus for maximum performance in the smallest of spaces. Flow rates ranging from 400 to 1600 liters per minute combined with minimal weight and installation space requirements predestine CPV valve terminals for use in direct proximity to the pneumatic drive, assuring shortest possible response times and increased productivity. A wide range of additional valve and pneumatic functions allow for individualized, custom tailored design solutions. Flexibility is also trump where mounting options (top-hat rail mounting right on up to pneumatic multi-connector plate) and electrical connection options are concerned.

Process Automation : 

Process Automation

Slide 54: 

Programmable Logic Controllers

Definition : 

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

Slide 56: 

A programmable logic controller (PLC) is a specialized computer to perform logic functions for machine control. PLCs are used to implement logic functions such as not allowing a drill press to start unless the operator has one hand on each of the two start switches. Such control functions used to be implemented using relays. PLCs revolutionized this by allowing the control logic to be implemented using software. What is a PLC?

Advantages of using PLC’s : 

Advantages of using PLC’s Highly reliable Highly versatile (universal applicability) Simple troubleshooting Simple installation Quick modification of the program (highly flexible) Capable of task not possible with relays before as indicated by the ff: calculation information exchange text and graphic display data processing networking Low space requirement Low power consumption High processing speed No moving parts, hence no wearing parts

Disadvantages of using PLC’s : 

Disadvantages of using PLC’s High initial cost ( for a simple process ) Sensitive to dust, high temperature and high humidity Repair must be made by a qualified personnel Not very widespread No uniform programming language

Selecting a PLC : 

Selecting a PLC Number of I/O’s Kind of signals : digital or analog Fieldbus system or standalone Modular or compact Operating voltage Positive or negative triggerred input Relay or transistor output Operating systems

Slide 61: 

PLC System

Slide 62: 

System Components

Slide 63: 

PLC SYSTEM

The PLC System : 

The PLC System input output process ccu Input devices send intelligible signals into the PLC Output devices receive intelligible signals from the output terminals

Slide 65: 

INPUTS Acknowledging Limit switches Sensors Pushbuttons Relays Transmitters Signal Generators

Slide 66: 

Input Module : Functions The input module of a PLC is the module to which sensors are connected to. The sensor signals are to be passed on to the central control unit. The important functions of an input module are as follows: Reliable signal detection Voltage adjustment of control voltage to logic voltage Protection of sensitive electronics from external voltages Screening of signals

Slide 67: 

PROCESSORS Central Control Unit CCU the most important component of the PLC it directs the operation of the system it contains the arithmetic logic unit it processes input signals to produce the necessary output signal it has memory to store programs and data to be processed it allows communication with external peripherals it is interfaced with sensors and actuators using the input and output modules respectively

Slide 68: 

Output modules conduct the signals of the central control unit to final control elements, which are actuated according to the task. The functions of the output module, as seen from the application of the PLC, include the following: Output Module : Functions Voltage adjustment of logic voltage to control voltage Protection of sensitive electronics from spurious voltages from the controller Power amplification sufficient for the actuation of major final control elements Short circuit and overload protection of output

Slide 69: 

OUTPUTS Final control elements Relays Electrical loads Solenoid coils Lamps Indicators Alarms / buzzers

Slide 70: 

Fixed I/O Modular FEC34 PLC in Distribution Station Construction of a PLC

Slide 73: 

IPC FEC Compact IPC FEC Standard FED – Front End Display Compact PLC FST – Festo Software Tool

Slide 74: 

FEC- FC600 32/16 digital I/Os FEC- FC620 32/16 digital I/Os + 3/1 analog I/Os FEC- FC640 32/16 digital I/Os + Ethernet FEC- FC660 32/16 digital I/Os + 3/1 analog I/Os + Ethernet IPC FEC Standard - The 600 line

Slide 75: 

FEC Standard – systematically more intelligent

Slide 76: 

FEC + Ethernet = FC34 12 digital Inputs (PNP/NPN) 24 V DC Run / Stop Switch 2 Relay / 6 Transistor Outputs COM and EXT Interfaces Ethernet Twisted Pair RJ45 connector 10 MBit/s 256 timers, 256 counters 256 registers 160,000 flag Multitasking 186 Micro Processor FC34 - Hardware Features PROGRAMMING SOFTWARE FST4, FST IPC 3.21 MULTIPROG

PLC & Mobile Phone : 

PLC & Mobile Phone Remote control and diagnostic of test stations Hardware is FEC FC38 (FEC with GSM modem) Software written in FST

SMS PLC : 

SMS PLC

Slide 79: 

24 VDC Connection Serial Interface AS-Interface – CPU and AS-i Master IPC-CPU AS-i Master IPC – I/O Card

Slide 80: 

AS-Interface Power Supply IPC-CPU IPC-AS-interface Master AS-interface I/O Module AS-i cable AS-Interface - Hardware

FESTO PLC : Optimum Control : 

FESTO PLC : Optimum Control Festo FPC 400 - the multi-functional one Festo IPC - the allrounder

Slide 82: 

PLC Programming

Slide 83: 

Sample setup

Slide 84: 

FESTO SOFTWARE TOOL ( FST ) PLC Programming

Slide 85: 

Ladder Diagram

Slide 86: 

Ladder Diagram The ladder diagram describes the program in graphic form. It was developed from the relay ladder but is structured in diagrammatic form. It is made up of two vertical lines of which the left one is connected to the voltage source and the right is zero voltage source. The various current paths(rungs) run horizontally between the two (from left to right).

Slide 87: 

Program processing by PLC

Slide 88: 

Ladder diagram is a drawing showing all the switches, sensors, motors, valves, and relays that are used in the control circuit. Today, many PLCs are programmed using ladder logic programming. Ladder Diagram

Slide 89: 

Sample setup

Slide 90: 

Sample setup

Slide 91: 

Executive Part Conditional Part Ladder Diagram

Coil Operations : 

Coil Operations

Allocation List Sample : 

Allocation List Sample

Slide 95: 

Allocation List The items of equipment associated with the FEC are addressed via operands in the control programs. An operand consists of an abbreviation identifying the equipment group and an address specifying a level within the group. Output 3 in output word 5, for example, is identified as O5.3

Slide 96: 

Symbolic operands The FST software also allows you to use symbolic designations for the equipment: O5.3 may switch a motor on and off. For Example, in which case you can refer to it as ‘MotorOn’ in your programs. In this way you can make the programs more easily comprehensible, and also retain a clear overview even when there are a large number of operands. You are largely free to decide on the names of the symbolic operands yourself.

Slide 97: 

Absolute operands In the following the direct entry of an operand (such as O5.3) is referred to as an absolute operand, whereas entry in symbolic form (MotorOn) is referred to as symbolic operand. The assignment of symbolic operands to absolute operands is defined in the allocation list. It is advisable to create the allocation list before program entry and then only to use the symbolic operand designations in the control programs.

Ladder Diagram Contacts : 

Ladder Diagram Contacts

Slide 100: 

Logic Operations

Slide 101: 

Logic Operations

Slide 102: 

Statement list (STL) language is another language used in PLC programming besides the ladder programming. STL allows the programmer to solve control tasks using simple English statements to describe the desired operation of the controller. SENTENCE 1 Conditional part THEN Executive part   SENTENCE 2 Conditional part THEN Executive part SENTENCE 1 Conditional part THEN …. 2.6 PLC Programming using STL

Commonly used commands in STL : 

Commonly used commands in STL IF - always introduces a conditional part. Operands can be interrogated and linked by logical associations within the statement. THEN - introduces the executive part. This part is executed if the condition is true. The statement may contain commands to modify outputs or flags etc., to carry out arithmetic operations, to activate timers or counters, or to call other programs or program modules. OTHRW - introduces a second, alternative executive part. This part is executed if the result of the conditional part of the step is not true and hence the THEN clause cannot be executed. STEP - The STEP statement is very important in sequence programs as it determines the structure of the program or, if branches are used, the sequence of processing.

STL conditional part : 

STL conditional part AND - This is the logical operator AND. It allows logical “AND”-ing of a number of input conditions. The condition is fulfilled when all “AND”-ed input conditions are true. OR - This is the logical operator OR. This operator allows you to create a logical expression with several input conditions. The condition is fulfilled when at least one of the conditions is true. NOP - NOP means “No operation”. You can use this statement when you want to carry out an executive part without an input condition. N - This is a negation. It allows you to invert an input condition.

STL executive part : 

STL executive part SET - activates a one-bit operand. You can use this for setting outputs, flags, timers, counters and programs to logical one. RESET - is the opposite of SET. It deactivates one-bit operands, used, for example, for setting an output to logical zero. LOAD - A LOAD statement reads a register or a multibit operand, i.e. its value is written to a multibit accumulator. Normally this statement is followed by the keyword TO, indicating the destination of the operation. TO - A TO statement assigns a value to a word operand. TO specifies the destination of the operation. JMP TO - A JMP TO statement causes the program to branch to a specific program step.

Slide 108: 

Sensors

What are sensors? : 

What are sensors? A sensor is a technical converter, which converts a physical value such as temperature, distance or pressure, into a different value which is easier to evaluate. This is usually an electrical signal such as voltage, current, resistance or frequency of oscillation.

Basic Functions of Sensors : 

Basic Functions of Sensors Detect presence of object Determine type of material Recognize appearance of material Measure dimension of material Measurement of distance, force, pressure, temperature, speed and acceleration

Fields of application for Proximity Sensors : 

Fields of application for Proximity Sensors Automotive industry Mechanical industry Packaging industry Woodworking industry Printing and paper industry Drinks industry Ceramics and bricks industries Plastics industry Electronics industry

Sensor Classifications : 

Sensor Classifications

Slide 113: 

Sensors Devices which convert physical variables into form of electrical signals to gather data, monitor or control a process. TYPES: Contact Sensors – mechanical in nature, subject to mechanical wear and with predictable failure rate. Contact sensors include limit switches, roller switches, and pressure sensors. Contactless Sensors – Proximity sensors (reed switch, inductive, capacitive, and optical sensors).

Switching Distance : 

Switching Distance It is defined as the actuating distance between the object and the sensor. The switching distance is dependent upon the actuating object.

Slide 116: 

Inductive Proximity Sensor

Sensing Distance of Inductive Sensors : 

Sensing Distance of Inductive Sensors

Slide 118: 

Applications

Detecting of aluminium caps : 

Detecting of aluminium caps At the final control of yoghurt tubs, the presence of aluminium foils is to be detected. This is accomplished by means of a small, powerful proximity switch with a long switching distance.

Check for gripped product : 

Check for gripped product A particularly small inductive sensor with a diameter of just 4 mm is implemented in a gripper. From the picking up of the work piece to its depositing, the sensor controls whether the work piece is clamped in the gripper.

Sorting of pallets : 

Sorting of pallets At a bottling plant for drink a synchronous belt is controlled as to the presence of empty pallets. Three inductive sensors detect each one row of bottles with aluminium caps. Empty pallets are sorted out by a pusher.

Positioning for punching : 

Positioning for punching A round metal sheet with a hub is turned in the punching press until it reaches its exact punching position. Positioning is achieved by means of an inductive sensor of small design.

Detection of loose screws : 

Detection of loose screws An inductive sensor serves to detect loose screws.

Slide 124: 

Capacitive Proximity Sensor

Sensing Distance of Capacitive Sensor : 

Sensing Distance of Capacitive Sensor

Detection of boxes : 

Detection of boxes

Detection of liquid level inside a plastic container : 

Detection of liquid level inside a plastic container

Slide 128: 

Optical Proximity Sensor

Slide 129: 

Level control of powder A diffuse sensor is fitted above a container filled with a light-coloured powder and is adjusted in such a way as to emit a signal if the container is filled and interrupt further feed

Sorting of cans : 

Sorting of cans On a conveyor belt steel cans with an inserted ring only may be processed. A diffuse sensor is adjusted in such a manner that it emits an OK signal whenever a ring is present. Cans without rings are sorted out.

Wire break detection : 

Wire break detection A wire to be winded is controlled for a break. A diffuse sensor or a through-beam sensor (depending on the thickness of the wire and the distance) is adjusted in such a manner that the sensor receives the emitter signal when wire is present. If no wire is to be detected, the sensor indicates wire break. The same applies to presence detection of a cotton, wool or vinyl string.

Detecting IC orientation : 

Detecting IC orientation Before being packed, IC are checked for their correct position. A diffuse type sensor with fibre optic cable detects the hub of the housing of 2000 IC per minute. Wrongly positioned IC are pushed out.

Monitoring of injection nozzle : 

Monitoring of injection nozzle Monitoring of an injection nozzle using diffuse sensor. The continuous stream of adhesive material from a dosage nozzle reflects the light of the diffuse sensor. If the adhesive stream breaks away, there is no reflection and the sensor responds. The above arrangement also allows detection of single drops.

Presence of electronic component : 

Presence of electronic component A fibre optic cable and diffuse sensor connected are used to control the surface of circuit boards that are difficult to access for the presence of small components. The positions that are hard to access can only be attained by means of highly flexible polymeric fibre optic cables.

Monitoring of liquid level : 

Monitoring of liquid level Control of filling level through gauge glass, making use of the laws of refraction (simplified representation): The liquid, the optically denser medium, breaks the light beam Without liquid, the receiver is activated.

Checking of punched holes : 

Checking of punched holes Before a work piece is fitted after having punched, a through beam sensor checks whether the dipping have been removed.

Fast counting of resistors : 

Fast counting of resistors On a production line for resistors 10,000 units per minute are produced. In order to achieve exact counting of the output of small components, a through-beam sensor connected with a fibre optic cable is aligned so as to point towards the resistors.

Checking of drill bit breakage : 

Checking of drill bit breakage A through-beam sensor in a machining centre controls a drill for breaks. Before every machining cycle, the drill is moved to a test position. If the drill point is present, it breaks the light beam and an OK signal is given to continue operation

Filling control : 

Filling control At the bottling plant with a production rate of 16,000 bottles per hour a through-beam sensor is adjusted in such a manner that the light beam penetrates the bottle. If in any bottle the liquid attains the predetermined level, the light beam is broken and the sensor responds.

Checking of drill holes : 

Checking of drill holes Two through-beam sensors connected with a fibre optic cable check the presence and the distance of 2 drill holes in a passing work piece. Only if both through-beam sensors simultaneously emit a signal, have the holes been drilled at the correct distance

Slide 141: 

Sensor Function Description of the object to be detected Installation conditions Environmental conditions Application area Things to consider in Selecting a Sensor

Slide 143: 

Flags

Slide 144: 

FLAGS One bit memories May be set or reset and interrogated for logic 1 or 0.

Sequence Control System : 

Sequence Control System

Slide 151: 

Representations Chronological Order Cylinder 1.0 extends and lifts the box Cylinder 2.0 extends and pushes the box Cylinder 1.0 retracts, then Cylinder 2.0 retracts Tabular Form Work Step Motion of Cylinder 1.0 Motion of Cylinder 2.0 1 out - 2 - out 3 in - 4 - in

Slide 152: 

Abbreviated Notation Extension represented by : + 1.0 + Retraction represented by : - 2.0 + 1.0 - 2.0 - Representations

Slide 153: 

Representations

Slide 159: 

TIMERS

Slide 160: 

Every binary signal has a positive and negative edge POSITIVE EDGE (RISING EDGE) marks the moment at which a change from 0 to 1 takes place ( or from OFF to ON. NEGATIVE EDGE (TRAILING EDGE) marks the moment at which a change from 1 to 0 takes place (or from ON to OFF) SIGNAL EDGES

Slide 161: 

Representations Tnn for the timer status (active or inactive) - shows whether the timer is active or not. It is a one- bit operand than can be set, reset or interrogated. 1 = active, 0 = inactive TPnn for the timer preset (preset run-time) - defines the run-time of the timer given in hundredths of a second and can be in the range 0.00s to 655.35s. The operand for the timer preset is a permanent multi-bit operand and remains stored until a new preset is defined. TWnn for the timer word (current run-time) - is a multi-bit operand and represents the current run-time of the timer.

Slide 162: 

Switch-ON Delay Timer (TONnn) - allows outputs to be activated with a 1-signal after a delay time has expired. The timer preset represents the delay period. When the timer is started, the timer status TON does not become 1 until after the delay time has expired. A rising edge in the conditional part starts the timer. It starts running until the timer has expired or the condition changes to a 0-signal.

Slide 163: 

- allows outputs to be deactivated with a 0-signal after a delay time has expired. When a rising edge is recognized for the condition, the timer preset is loaded into the timer word. A trailing edge starts the timer, which runs until the timer has expired or the timer is reinitialized by a rising edge for the condition. Switch-OFF Delay Timer (TOFFnn)

Slide 164: 

- allows an output to be activated for a specified time when an input signal is present. It only reacts to the rising edge of the condition. This pulse starts the timer (Tnn=1). The timer preset is loaded into the timer and the timer starts to decrement until it reaches a value of zero, or a further edge (pulse) is detected at the conditional part, restarting the timer, or the timer is reset (Tnn=0). Pulse Timer (Tnn)

Slide 166: 

COUNTERS

Slide 167: 

Representations Counter status - Cnn - indicates whether the counter is activated or deactivated. It is a one-bit operand that can be set, reset, or interrogated. Counter preset - CPnn - represents the end value for incremental counters and the start value for decremental counters. Ranges from 0 to 65535. Counter Word - CWnn - is a non-permanent multibit operand and indicates the current counter status.

Slide 169: 

Spindle Drive

Spindle Drive : 

GEAR MOTOR REED SWITCH INDUCTIVE SENSOR CARRIAGE SHAFT Spindle Drive

Vacuum Generator : 

Vacuum Generator Workpieces with smooth and impervious surfaces can be picked up and held (for transporting and assembly) with this vacuum generator This produces vacuum in accordance with the venturi principle, using the compressed air from P to R. Suction cups are connected to the vacuum port U

Relays : 

Relays Relays are electro-magnetically actuated switches. They consist of a housing with electromagnet and movable contacts. An electromagnetic field is created when a voltage is applied to the coil of the electromagnet. This results in attraction of the movable armature to the coil core. The armature actuates the contact assembly.This contact assembly can open or close a specific number of contacts by mechanical means. If the flow of current through the coil is interrupted, a spring returns the armature to its original position.

Relays : 

Relays

Controlling The Motor : 

Controlling The Motor

REGISTERS : 

REGISTERS Used to store binary, octal or decimal values Can be used to perform mathematical functions Not addressable on a bit by bit basis Used to simplify controlling multiple sequential processes They are retentive. It stores its current value even with power loss

Slide 176: 

Loading a value to a register ( executive part ) Interrogating a register ( conditional part ) Incrementing ( executive part ) Decrementing ( executive part ) R4 R50 TO V0 R1 = V20 R10

Slide 177: 

MULTI TASKING

MULTI-TASKING : 

The FESTO PLC can execute the processing cycles of a number of programs one after the other (referred to as tasks). While in a task, one program is being processed, the other currently active programs are not processed. However, execution of the program segments and switching to the next program (task change) takes place so quickly that the programs appear to run in parallel. This is known as pseudo-parallel program execution. The idea is that a PLC can consist of several parts each of which is a program in its own right. MULTI-TASKING

MULTI-TASKING : 

MULTI-TASKING

MULTI-TASKING : 

A program can be activated (started) and deactivated (stopped) from within another program or module. The program is activated by setting the operand Pn, and deactivated by resetting it. Note: The LDR symbols --( )-- (assignment) and --( / )-- (negated assignment) cannot be used to activate deactivate a program. They are not permissible for the operand P MULTI-TASKING PB2 PB1 P1 P2 P1 P2 S S R R P0 = main program

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