UNIT 4 Mechatronics

8/14/2019 UNIT 4 Mechatronics

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UNIT IV

PROGRAMMEBLE LOGIC CONTROLLERS

4.1 INTRODUCTION

v A PLC or a programmable logic controller, is a solid state digital industrial

computer, in which control devices such as limit switches, push buttons,proximity or photoelectric sensors, float switches or pressure switches, etc.,

provide incoming control signal into the unit.

v This incoming control signal is called an input.

v A formal definition is given by National Electrical Manufactures Association(NEMA):

v A PLC is a digitally operated electronic system designed for use in anindustrial environment which uses a programmable memory for the internal

storage of user oriented instructions for implementing specific functions suchas logic, sequencing, timing, counting and arithmetic to control, through

digital and analog inputs and outputs, various types of machines or processes.

v This was designed in the early 1970s to replace electromechanical relays,

mechanical timers, counters and sequencers. It is small, requires less power,has fast switching capability, and a reliable control device.

v Just like the other controller chips explained in the previous chapter PLC

follows the instructions stored in PLC s memory.

v The processor or CPU reads the input signals, initiates the processes by

prompting the PLC. PLC in turn carries out the operations on the inputs and

converts them into the proper control operations or switching operations on

the system.

Advantages

1. This is a hardened industrial computer designed to withstand the harsh factory

environment.

2. The troubleshooting is easy so also the installation.3. They are compact and are reusable.

4.2 BASIC STRUCTURE

The block diagram of a PLC is given in Fig.4. I. The six major sections of a PLC are1. Sensing inputs or controlling hardware.

2. Input section.3. CPU

4. Handheld programming device or personal computer.5. Output section.

6. Output devices


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Fig. 4.1. Programmable logic controller block diagrani

1. Sensing section: These are usually made up of sensors and switches which transmit

the signals from the input devices.2. Input section: This contains two major areas the physical terminals where the

input signals from the input devices are attached to the PLC and the internal

conversion electronics. This internal conversion electronics converts and isolates the

high voltage inpul level from field devices, into +5 V dc which is necessary for th

microprocessor and the other solid state circuitry.

3. Controller: This is the processor which processes the signals from input section and

generates controlling signals for the system.

4. Programmer: This is usually a PC which is used to enter the program to the PLC.

5. Output section: This receives the signals from the PLC which are used to control

the system to which the PLC is connected.

6. Field hardware devices: This is the system which is controlled by the PLC. As

mentioned before, it may be a motor which controls the movement of a conveyor or alift, it may be metal cutting machine whose outputs are to be precision made, etc.

4.3 PLC HARDWAREThese are two different types of physical configurations in PLC.

1. Fixed Input / Output2. Modular Input / Output

1. Fixed I/O PLCs: This contains a fixed or a built-in I/O section. The section is builtinside the PLC and cannot be changed.

2. Modular I/O PLCs: This contains I/O ports as in a microprocessor to which

removable I/O units or modules can be connected as desired. Here the user can select

separate modules for I/O or mix them according to use. Typical modules will contain

4, 8, 12, 16 or 32 1/0 points. The printed circuit board which acts as I/O port is built

into the chassis and is called backplane. Some PLCs have a simple rail called DIN rail

to which the modules simply clip together. The advantage of this type is that it

permits the easy addition and removal of modules.


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Modular PLC installation on a DIN rail is explained below.

v The power supply which is connected to the line voltage is installed either onthe right side, or left side or in any position in the modular rack or chassis.

v Some PLCs allow the power supply to be installed as either in the chassis or asa standalone device outside the chassis.

v CPU takes up the next position to the power supply. The remaining slots aretaken up by the I/O modules.

4.4 I/O SIGNALS

v A standard input modules has 16 input points. So a 16 bit input is given as

signal. To indicate the 16 inputs 16 LEDs serve as indicators.

v Eight point input modules are also available.

v In these, only the lower 8 bits are used. Fig,4.5 shows the correlation of input

signals into control signals in 8, 1 6 bit I/O modules.


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Fig. 4.5, (b) Input conditions reflected in the input module (8 bit)

v A 24 bit I/O module is represented with 2 words of 16 bit.

v The first word gets all the 16 bit signals while the second word gets only thelower 8 Bits and higher order 8 bits are ignored. For a 32 bit I/O module, all

the 16 bits of both words are used.

v In a modular PLC, let us assume 4 I/O modules are fixed. So there will be 4

input words corresponding to them.

v These input words at different instants are grouped together to form the data

file stored in the memory as shown in Table 4.1.


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v Here the word 0 indicates the input status of input module 0, word I

corresponds to input module 1 and so on.

v This data file representing a data word for each input module in the system is

called the input status file.

v This data is processed by the CPU when the program is executed. This is alsocalled I/O mapping.

v Similar to this there is an output status file which stores the ON or OFF statusof each output module.

v This output then is used to control the load connected to the output module.This is shown in Fig.4.6.


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1. Fig.4.7 shows how more than one module is connected in a PLC.2. The main advantage of modular PLC is that after connecting the power

supply and CPU in the first two slots we are free to fix up any module

whether input or output in any of the slots as shown in Fig.4.7.


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1. Power supply

2. Processor and memory

3. 1/0 interfaces (modules)

v Usually a separate programming device like a handhold unit for small PL( s or

a PC for larger PLCs is separately added.

v Whatever the size of the PLC, the processor or controller and memory are in

the same unit called the CPU.

v Since the memory has already been explained, let us carry on with the

microprocessor part.

vWe have already seen an 8 bit microprocessor 8085 in the previous chapter.PLCs can operate with any one of the Intel microprocessors as its heart.

v Intel has developed 8 bit, 16 bit, 32 bit microprocessors with the clock speeds

ever increasing.

v Although some high end, larger PLCs use the 80486 chip, some smaller PLCs

use 8 bit iP like 8085.


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4.8 POWER SUPPLIES

v Typically a bridge rectifier with a filter and a regulator constitute a PLC power

supply.

v In addition a battery back up system is also provided.

v The battery back up switch is provided for emergencies where the powersupply fails. If the line input to the power supply ceases the switch switches

the output from power supply to battery back up power quickly and

automatically so that the power input to the PLC is not affected.

v

4.9 INPUT I OUTPUT PROCESSING

v PLC is capable of handling discrete as well as analog I/O sigrals.

v Discrete input module is the most common input interface used with

programmable controllers.

v Discrete input signals from field devices can be either AC or DC.

v The most common module types are listed below.

AC input modules

The block diagram of a typical AC input circuit is shown below.


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v A 120 V AC input module will accept signals between 80 and 135 V AC.

v This module is considered the load for the field input device.

v The module s job is to convert the 1 20 V AC high voltage signal to the 5 V dc

level, with which the PL(. can work. It verifies the input as a valid signal,

isolate the high voltage field device signal from the lower voltage CPU signaland send the appropriate ON or OFF signal to the CPU for placement in the

input status file.

v As shown in Fig.4.l0 the module consists of three parts.

1. Power file conversion,2. Isolation and

3 Logic.v Rectifier and filter constitutes the power conversion section.

v The DC output thus got is passed on to a threshold detector.v This detects if the incoming signal has reached or exceeded a predetermined

value for a predetermined time and whether it should be classified as a validON OFF signal.

v A typical valid OFF signal is between 0 and 20 or 30 V AC and ON signal isbetween 80 and 132 V AC depending on the manufacturer.

v The optical isolation circuit is usually made up of an opto coupler which is a

combination of a photoemissive device (LED) and a photodetective device

(photodiode).

v The input signal energises the LED which transmits a signal of light energy to

the receiver i.e., photodiode.

v There is no actual electrical or physical coupling between LED and

photodiode. So this provides the necessary isolation between power

conversion circuitry and the logic circuitry.

v This is necessary because the input is 120 V AC but CPU runs from 5 to 18 V

DC and any electrical short between these two will prove fatal for the system.

v There are two types of input devices commonly interfaced to an input module.One type includes the mechanical limit switch, toggle switch, selector switch,

push button and contacts from an electromechanical relay.v All these mechanical contacts which require no electrical power. Circuit

continuity is made or broken by physically opening a set of contacts.

v The second type is a solid state proximity device which requires electricalpower to operate.

v A small amount of current called leakage current must continuously flowthrough the device, even in the OFF state to keep the internal electronics

working so that the switch will be able to sense the presence of an object.

vDC input modules: Low voltage, 24 V DC inputs are commonly used for start/ stop control circuitry and sensor interface to the PLC.

v DC sensors can drive electromechanical relays, counters and solenoids and

other solid state devices.

v A DC sensor with a proper DC input module does not need any interfacing

device. The sensors, are commonly solid state sensors like inductive proximity

sensors, capacitive proximity sensors or photoelectric sensors. Typical sensors

use IOV 3OVDC.


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v The sensor s switch which controls the input into the module is made of either

NPN or PNP transistor.

v The input device with NPN transistors are called sinking input devices and

those with PNP transistors are called sourcing input devices.

Sourcing and Sinking:

v Conventional flow of current is defined from positive to negative of the

battery. So if a switch is connected to the positive of the battery it is said to besourcing the current and if it is connected to the negative of the battery it is

said to be sinking the current. Similarly a load may also be sinking or sourcing

the current in the same way.

v Fig.4.11 shows the different combinations.

The block diagram of a typical DC input module is shown below.

Typical PLC input and output devices with indications to show how they are indicatedin the circuit diagrams in the following topics.


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4.10. PROGRAMMING EQUIPMENT AND OPERATIONS PROCEDURE

v To design any PLC based system, just like any other system, block diagrams

are to be drawn.

v In addition, one more diagram indicating the flow of control over the different

subsystems, the conditions to be satisfied, also has to be drawn. It is the ladder

logic diagram.

A ladder logic diagram is one where all the different inputs and outputs are shown intheir order in different branches called rungs of ladder.


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v The first rung contains the first input and output and the second one the second

pair and so on.

v Example 4.1 I Let us take an example system to explain the actual operation /

process of I/O with PLC : A relay coil is to be activated when two toggle

switches and one limit switch are operated.

v The first step is to assign individual PLC identification numbers to the inputs

and outputs.v Inputs are indicated by 1 N and outputs by CR (Control Relay). Therefi)re, the

following members can be assigned.

Switch I for relay IN 001

Switch 2 for relay IN 002Limit switch for relay IN 003

Relay output CR 001Next, sketch a ladder logic diagram to represent the operational circuit as shown in

Fig.4.15.

The next step is the actual hardware connection of input and output modules.

Assuming an eight terminal input and output, the connections are shown in Fig.4. 16.

Finally the ladder program must be entered into the CPU by means of a keyboard. For

this any one of the programming equipments may he used.

The general procedure to enter the program in ladder format is1. Clear the PLC program memory with the CPU on stop.

2. Go into the EDIT mode and start entering the relay control line as follows.(a) Select the input function and enter the number 001. The contact should appear on

the monitor.(b) Moving the cursor one space to right repeat the same procedure for the 002 input

and 003 input.(c) Select the coil / output key and enter 001.

(d) If the arrangement looks correct, select the ladder key and enter. The ladderdiagram for this arrangement will appear on the monitor as below.


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Though the procedure explained above is a general procedure, most of the

programming equipments are the same to operate.Three types of programming monitors or PMs are in use.

1. Hand held, palm size units with dual function keypads and a Liquid Crystal Display(LCD) or Light Emitting Diode (LED) window.

2. Full size keyboards accompanied by a large LCD display or Cathode Ray Tube(CRT) screen.

3. PCs with software for developing the PLC program.

Advantages of using handheld programming terminal

1. Easy editing and debugging.

2. Compact in size, low cost and easy to use.

3. Easy transport to the field.

Disadvantages

1. Not compatible with all PLC CPUs.

2. Limited memory and so limited number of programs.

3. Small screen size and so limited capability to display ladder rungs.

4. Documentation not displayed.5. Volatile memory, so battery back up is required.

Advantages of Software Programming Using PC

1. Larger screen so possible to display multiple rungs of logic, easy to troubleshoot.

2. More non-volatile memory, so storage possible.

3. Easy to transport the program through a floppy or CD, also useful for back up.

4. Rung comments, instruction comments, symbols, etc., are easy to be displayed or

added for editing.

5. Data tables can be easily monitored.

The different products follow different formats for programming the PLC. But a

general procedure for this as explained in Example 4.1 can be devised along with itsladder diagram for any system.

Limitations: There are some limitations to be observed when programming a PLCladder diagram. When incorrectly formatted ladder diagrams are given, they will not

be received by the PLC CPU. Such limitations are to be followed when drawing aladder diagram.

1. A contact must always be inserted in slot I in upper left.2. A coil must be inserted at the end of a rung.

3. All contacts must run horizontally. No vertically oriented contacts are allowed.

4. The number of contacts matrix is limited.

5. Only one output may be connected to a group of contacts.

6. Contacts must be nested properly or in some PLCs not at all.

7. Flow must be from left to right.

8. Contact progress should be right across.


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4.11 PLC LOGIC

v The devices in an electrical schematic diagram are described as being open or

closed. Each of the PLC ladder rungs indicate a program statement.

v A program statement consists of a condition or conditions, along with some

type of action.

v Inputs are the conditions and the action or output is the result of the

conditions.v The PLC combines the ladder program instructions, similar to the physical

wiring hardware devices, in series or parallel. For this, it uses logical operators

AND, OR and NOT. They are combined for the instructions on a PLC rung

so as to make the outcome of each rung either true or false, which is the result.AND Logic: This represents a series circuit. For example, switch I AND switch 2

must be closed in Fig.4. 18 to energise the light.

v Combinational Logic: Most of the PLC ladder rungs will include some

combination of AND, OR and NOT logic.

v For each of the rung, there must at least be one logic combination which givesan output of 1.

Priority of Logic:

v This is very important in a program. Priority is the method of fixing sequence

of the portion of the problem to be solved first and then rest in subsequent

steps.v If this sequence is not precisely followed then there is every possibility of an

error occurring.

v Some of the handheld programmers allow us to enter rungs of logic rather thana list of instructions.

v So by entering the rungs of logic, we can see that they are placed in the proper

position and so the priority of logic problem does not arise.

v Similarly when PC is used for programming, the same procedure is carried

out.

v The programmer physically places the instruction on the rung and in the

correct position in relation to surrounding instructions, so concerns about logic

element priority are eliminated.

v For example, the evaluation of following program shows how the PLC rungs

are built.

Program: LOAD 11

AND 12OR 13

AND 14OUT 05


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v Both (a) and (b) seem to be correct for the given set of instructions, though

they are not equivalent.

v This is where logic priority becomes important. So to solve this problem,

while programming, the following rules are to be adopted.1. Each rung begins at the left power rail.

2. Start each rung with the appropriate beginning instruction either Store orLoad instruction.

3. Program the next logic element closest to the one already programmed. In case aseries and a parallel logic element are equidistant, always program the parallel logic

function first.4. For connecting two or more instructions in a parallel branch, special instructions

are used to connect or group, parallel instructions on that branch.If no grouping instructions are included in the program, they will be programmed as

one instruction per parallel branch.

4.12 LADDER DIAGRAMS

v Ladder diagrams are the elementary or line diagrams used for non-electronic

control circuits.

v There are two types of ladder diagrams used in control systems, the controlladder diagram and the power ladder diagrams.

v Fig.4.23 shows two basic control ladder diagrams.

v The first one A is for a single switch that turns a relay output CR on and off.

v The second, B is a single function diagram with parallel lines for control and

parallel lines for output.

v The two outputs can be turned on using any one or both of the two switches.

v The general rules to be followed while drawing the basic control ladder

diagrams are


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1. All coils, pilot lights and outputs are on the right.

2. An input line can feed more than one output, which are connected in parallel.

3. Switches, contacts and so on may be connected in multiple contacts of series,

parallel or series parallel starting from left to right.

4. Every connection node should be given a unique identification number, along with

the relays, switches, lights, etc.

Let us draw a t function control diagram as below.

Explaining this diagram, we can see that the sequence of instructions should be as

follows.1. All switches are open to start; both coils are off.

2. Closing SWI or SW2 or both energises both R and L3. Closing the contact C enables line 3 but does not energise R

4. Closing C and SW3 energises RPower Ladder Diagrams

v For applications like motor control, power ladder diagrams are drawn.

v The operation of these power ladder diagrams is straight forward.

v In Fig.4.25, when the power contactor coil is energised, the power contactsclose and power is applied to the motor or the load device.

v The main difference between the power ladder diagram and ordinary ladder

diagrams is that the connecting lines are thick.

For a large process, creating a ladder diagram is a little complicated. Hence thefollowing steps are used for planning a program for a large process.

1. Define the process to be controlled.2. If possible, do a pictorial explanation of the process, either using a block diagram

or detailed diagram.

3. Create an algorithm with all the steps of process in sequence.4. Add the necessary control relays, inputs and outputs, sensors according to the

process sequence, in the above diagram.

5. Add manual controls as required for the process or testing.

6. Add or adjust controls keeping the safety of the operating personnel in mind.

7. Add master stop switches, as required, for safe shutdown.

8. Create the ladder logic.


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4.13 PLC PROCESSOR

v This is a microprocessor with memory and circuits to store and retrieve

information from and also communication circuits. Just like a microprocessor

based control system, the operations are carried out as per the instructions

given.

v So the explanations are the same as in the systems explained in the previous

chapter. The process of reading inputs, solving logic and updating outputs iscalled the processor scan or processor sweep or operating cycle.

v As in any other program, PLC program also will be executed from the top.

v So the first rung of the ladder will be executed and subsequent rungs will

follow, unless altered by an instruction specifically designed to alter the flowof the program. Program flow instructions direct this flow of instructions and

their execution within a ladder. As in microprocessor, jump and branch alterthe course of program.

Steps

1. The inputs are read and stored in the input status file.2. The ladder program is solved starting from rung zero and it goes on to the

subsequent instructions.3. While evaluating the rung, the PLC proceeds from instruction to instruction till

output instruction is reached.

4. Executing the output instruction, the logical one or zero output status is placed in

the output status file. The output status is the logical resultant of the solved input logic

for that rung.

5. After the last rung is executed, there is one additional rung which is automatically

inserted by the software and is called end rung This rung alerts the CPU that it has

reached the end of the ladder program.

6. After this the CPU services communications which is really the updating of

handheld or personal computers monitor screen or sending this through to other PLCs

in the network etc.

Housekeeping and Overhead: These are part of the scan cycle in which the CPU takescare of memory management, updating timers and counters, internal time base, the

processor status file and other internal registers.

4.14 PLC INSTRUCTIONSv There are many different PLCs in the market, and though they all operate in

basically the same way, there are many differences in features and instructionsets.

v Basic instructions are the same but they may be programmed differently.v Before going into the actual instructions, it will be better if the registers in the

PLC are explained along with how the data and program files are built in a

PLC.v There are 10 types of data files which are created automatically by the

processor and are assigned a file number and alphabetical file identifier

They are 1. Output status file 2. Input status file

3. Processor status file 4. Bit file

5. Timer file 6. Counter file

7. Control file 8. Integer file

9. Floating point file 10. Common interface file


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1. Output status file:

v This is, by default, file 0. This is made up of single bits grouped into 16 bit

words.

v Each bit refers to the ON or OFF state of one output point.

v Status file is created only if the processor finds an output module in that

particular slot.

2. Input status file:v File I is the input status file.

v Similar to the output status file, this is also made up of 16 bit words each bit

for one input point.

3. Processor status file:v This is file 2. Information in this file includes that on PLC s operating system.

v The information falls into 3 classifications.v First, status information that cannot be modified or monitored.

v Second, dynamic configuration status words, bytes or bits used to selectprocessor options while in run mode.

v Third, static configuration status words which select processor options beforeentering run mode.

4. The bit file :v The bit file is file 3.

v A bit file is used to store single bits in a 16 bit word format.

v There can be many bit files for a single processor file. Any data file greater

than file 10 can be assigned as an additional bit file.

v Each file will have 255 sixteen bit words.

5. The Timer file :

v This file, file 4 is used to store timer data.

v Each timer has three 16 bit words called a Timer element.

v As in a bit file, this has 256 timer elements and additional files may be

created.

v The timer element with three 16 bit words consists of three parts.

v Word zero is for status bits.v Word one is for preset value.

6. Counter file:v This is file 5 and is used to store counter data.

v Here also as in timer file, there are three 16 bit words, called counterelements.

v The details of the three words are as in timer file.v The counter element format is shown below.

7. Control file:v This is file 6 and is used to store status information for bit shift, first in and

first out stacks (FIFO), last in and first out stacks (LIFO), sequencer

instructions and certain ASCII instructions.v This is similar to counter and bit files in that this also has three word element

with three 16 bit words.

v Word zero Status word

v Word one Length of the bit array or file. Word two Current position in

the bit array


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8. Integer file:

v This is file 7.

v The integer file element a 16 bit word representing one whole number, i.e., it

stores the binary equivalent of one whole number.

9. Floating point file:

v This is file 8.

v This stores the floating point data in two word elements.v One word to store the integer and the second word is to store the exponent.

10. Common interface file:

v This is file 9.

v This is used as the target file in a PLC when the data is transferred from onesystem to another when connected in a network.

v Each system will have a unique identifier called a node address.

v Apart from these default files, the programmer can create an ASCII file in any

unused data file numbered from 10 to 255.

v ASCII data file information is comprised of one word elements.

v Two hex characters are typically used to represent an ASCII character.

v String data is another type of data used in PLC. The difference between an

ASCII file and a string file is that the string file strings together a number ofASCII characters rather than treat them separately.

v This can contain upto 255 elements, with each element made up of 42 words.

Any unused data file or files other than the default data files can be designated

as a string file.

v There are a total of 256 files available in a PLC.

v Files beyond the automatically created data files (default files) are available as

user defined files

The user can define these files for any specific application. No two files will share the

same number.

4.15. PLC REGISTERS

v In PLC, the registers are found in two locations.v The microprocessor has internal registers, most of which are not directly

accessible by the user.v These are used by the microprocessor for the different arithmetic and logic

operations. These include accumulators, data registers, index registers,condition code registers, scratch pad registers and instruction registers.

v In addition to these internal registers, the CPUs RAM also contains slots thatare the external registers.

v These are 16 bits wide.v Usually these registers are designated using prefixes followed by numbers like

OG1 (output group register 1) or HR 55 (Holding register 55), etc.

vIt can be seen that a certain numerical series of addresses may be assigned to aspecific task or a function.

v There are 5 key registers in a PLC.

(a) Holding registers.

(b) Single input registers

(c) Group input registers

(d) Single output registers

(e) Group output registers

Holding register:


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v This holds the contents of a calculation, arithmetic or logic.

v In many PLCs, this register is not directly accessible to inputs or outputs.

Separate input and output register will be used for the purpose.

v The use of this holding register is shown in the Fig.4.27.

v Processor manipulates the data in the holding register which has been

transferred from input registers. This data will be moved to the output register

next.v The number of holding registers will depend on the length of the data and the

operation. It varies from 16 for small PLCs to hundreds in large machines.

v For a timer function, the holding register is the register in which the count

takes place.

v Similarly for the counter function, the preset count value is placed in a

constant or designated register and the count takes place in the holding

register.

Input Registers:

v This has the same characteristics as a holding register but it is the intermediate

register between an input device and a holding register.

v The number of input registers in a PLC is normally one tenth of holdingregisters.

v These may also be grouped together so that the data may be received fromconsecutive input ports. If we have 16 input ports, with 8 bits each, the first bit

of each of the 16 ports is connected to one register, the second bit of each portis connected to the second register and so on.

v So at least one input point of one port will be connected to a register.v This is illustrated in Fig.4.28. The advantage of this group register system is

that only one register is required to service 16 inputs. Without this, 16 input

registers are required.

Output registers:

v These are similar to input registers except that the data flow is from the

holding registers.

v This also has the same classification of single and group registers.

4.16. PLC INSTRUCTION SET

v As in the case of microprocessors, each manufacturer s PLC processors havetheir own vocabulary of instructions.

v But the basic instructions called the relay instructions are shared by all PLCs.

v The relay instructions and their illustrations with rungs of ladder are given

below.

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UNIT 4 Mechatronics