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Jack H.Integration and automation of manufacturing systems.2001.pdf
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page 138

e.g. for an Omron PLC

00000

LDI

00001

AND

00002

LD

00003

AND

00004

ORB

00005

OUT

00006

END

A

 

B

the mnemonic code is equivalent to

C

the ladder logic below

D

 

E

 

 

A

B

E

C

D

 

 

 

END

There are other methods that are not as common,

-sequential function charts/petri nets

-state space diagrams

-etc.

7.2 WHAT DOES LADDER LOGIC DO?

page 139

7.2.1 Connecting A PLC To A Process

 

 

PROCESS

 

 

Feedback from

 

 

 

Connections to

sensors/switches

 

 

 

Actuators

 

 

 

 

 

 

PLC

 

 

 

 

 

 

The PLC continuously scans the inputs and changes the outputs.

The process can be anything - a large press, a car, a security door, a blast furnace, etc.

As inputs change (e.g. a start button), the outputs will be changed. This will cause the process to change and new inputs to the PLC will be received.

PLC program changes outputs by examining inputs

THE

CONTROL

LOOP

read inputs

Set new outputs

process changes and PLC pauses while it checks its own operation

7.2.2 PLC Operation

• Remember: The PLC is a computer. Computers have basic components, as shown below:

page 140

Keyboard

Input

80586

Serial CPU

Mouse

Input

32MB Memory

Storage

• In fact the computer above looks more like the one below:

SVGA Screen

Output

133/200 MHz Light output

2.1GB Disk

Storage

inputs

 

input memory

 

computer

 

 

 

 

output memory

 

 

outputs

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Keyboard

 

 

 

Input Chip

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

monitor

 

 

 

 

 

 

 

 

 

 

CPU ‘586

 

 

 

 

Screen memory

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

chips

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mouse

 

 

 

Serial Input Chip

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

digital output chip

 

 

LED display

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flow of Information

Disk Controller

Memory Chips Disk

storage

Notice that in this computer, outputs aren’t connected to the CPU directly.

A PLC will scan a copy of all inputs into memory. After this, the ladder logic program is run once and it creates a temporary table of all outputs in memory. This table is then written to the