programmable logic controller internal operations

Programmable Logic Controller Internal Operations

Chapter PLC3 Slide 2

Learning Objectives

covers the internal operations of a PLCillustrates the concept of scan timeexplains I/O response timediscusses practical issues of input signal interface with PLCdiscusses practical issues of output signal interface with PLC

This topic


Applications of PLCs:

Sequence control

Motion Control

Process Control

Field bus

Communication

Data Management


Internal Operation of PLCs: Input / Output Addressing



INPU

TO

UTPU

T000.00 010.01

PROGRAM



INPU

TO

UTPU

T000.00 010.01

PROGRAM

Reads Input Executes Program Sets Output


Input / Output Addressing

These outputs are latched and they retain their status until they are updated by the next I/O copying routine.

Input / Output Processing

Reads Input Executes Program On Output

Most common method is mass input/output copying

Reads All Input Status

ExecutesProgram

Turns on

OutputDecides which Outputs to turn on

Updates Status at

Input RAM

read one at a time

Stores Output

Status at Output RAM

Transfer to Output Module


I/O Refresh

The process of writing from RAM to the output relays and reading input relay status to RAM is known as I/O Refresh.

Copy all inputsinto RAM

Fetch, decode and execute all instructions in sequence

Copy all outputs from O/PRAM to output unit and input all inputs to I/P RAM

Time depends on length of total programe.g. 1K program = 5 ms

Fixed length delay e.g.5 ms

START I/O Copy Program Execution End I/O Copy Program

I/O Refresh.


Scan Time

Common processes, such as watchdog timer resetting, checking the I/O bus, checking scan time, diagnostic operations and housekeeping on program memory – processes. Communication windows which include the link service of a host computer and communications with peripheral devices.

On power up, CPU goes through a series of internal processes

ReadsInput

SetsOutput

Data input and output when status of the input relays will be written into the input portion of the I/O RAM and status of the output portion will be reflected in the output relays.

Executes Program

Instruction execution which includes an executive routine in which the base intelligence of the system is used to interpret the user program containing the instructions to be executed.


Scan Time

ReadsInput

SetsOutputExecutes Program

Scan time is the total time required for PLC to perform activities in each scan cycle.

Typical scan time for 500 instruction words varies between 3 ms and 10 ms with ladder logic programming.

ReadsInput


One Scan One Scan


ReadsInput


ReadsInput


Scan Time

One Scan One Scan

INPUT SIGNAL comes in ….. And DETECTED !!

Case 1 :


ReadsInput


ReadsInput


Scan Time

One Scan One Scan

No INPUT SIGNAL comes in …..

And DETECT nothing !!

Case 2 :

Some INPUT SIGNAL comes in …..

And DETECTED !!


ReadsInput


ReadsInput


Scan Time

One Scan One Scan

INPUT SIGNAL comes in ….. And NOT DETECTED !!

Case 3 :

Because of the cyclic nature of I/O copying routine, input signals of high frequency which change state after the READ routine will not be recognised until the next copy occurs.

Only certain PLCs incorporate a subroutine to interrupt the scan to pick up such signals.


Interfacing On-Off Sensors with PLCs

Mechanical Switches

or ‘Dry Contacts’

DC proximity sensors

AC/DC 2-wire

Proximity sensors

relay output solid state output

NPN PNP1

2

3

4

5

COM

NC

NO

24V Load

sink

+24V DC

(-)

+

-

Load

source

+24V

(-)

-

+

Load 24V DC

Sensors


Interfacing On-Off Sensors with PLCs

solid state output

NPN PNP

Load

sink

+24V DC

(-)

+

-

Load

source

+24V

(-)

-

+

NPN (current sinking) sensor must be matched with a source type input card.

Input Cards can be broadly classified into two categories:

a) source type (or common positive), &

b) sink type (or common negative).

PNP (current sourcing) sensor must be matched with a Sinking type input card.


When Sensor is OFF…

Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits

NPN (current sinking) sensor must be matched with a source type input card.

When Sensor is ON…

No Signal Signal is detected

SENSOR Sinks

PLC Sources


Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits

When Sensor is ON…SENSOR Souces

PLC Sinks

PNP (current sourcing) sensor must be matched with a Sinking type input card.


Interfacing On-Off Loads with PLCs

Output Module of PLCs are used to drive loads, such as

• solenoids,

• lights,

• buzzers,

• motor contactors, etc

These loads are powered by an External Power Source not within the PLC.

Power

Supply

Output ModuleOutput Module


Types of Output Modules

npn pnp

Transisterised

Triac Relay



Load type Load current Response Time

RELAY

TRIAC

Transistorised (npn/ pnp)

Characteristics

Type of Output

AC/ DC

AC

DC

2A Typical

1A Typical

1A Typical

10ms Typical

3ms Typical

1ms Typical

Factors affecting selection of Output Module:



LOAD

Transisterised NPN Type



Transisterised PNP Type

LOAD



Triac Type

LOAD



Relay Type

LOAD


This chapter :

Summary

covers the internal operations of a Programmable Controller

illustrates the concept of scan time explains I/O response time discusses practical issues of input signal interface with

Programmable Controller discusses practical issues of output signal interface with

Programmable Controller

programmable logic controller internal operations

Documents

program slide

program sets output

scan slide

input output processing

input sets output data

reads input sets output

output module slide

input ram