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Basics of PLC Programming
Industrial Control Systems
Fall 2006

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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PLC Architecture

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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PLC System

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Processor Memory Organization
The memory of a PLC is organized by types
...

Advanced ladder logic functions allow controllers to
perform calculations, make decisions and do other
complex tasks
...
They are more complex than
basic inputs contacts and output coils and rely
heavily upon data stored in the memory of the PLC
...

§ Input/output locations
§ Internal relay and
timer/counter locations

Data table

User program

The user program causes
the controller to operate
in a particular manner

Housekeeping memory

Used to carry out
functions needed to make
the processor operate
(no access by user)

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Program Files
The user program will
account for most of the
memory of a PLC system
...


Program file organization
for SLC-500 controller
...

MME 486 – Fall 2006

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Data Files
Data file organization
for SLC-500 controller
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Data Files
Data files are organized by the type of data they contain

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Input Table File Operation
Input module
Switch Open

Binary 0 stored
0

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Input Table File Operation
Input module
Processor continually
reads current input
status and updates
input image table file

Switch Closed

Binary 1 stored
1

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Output Table File Operation
Output module
Output
OFF

Processor continually
activates or deactivates
output status according
to output image table
file status
0
Status 0

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Output Table File Operation
Output module
Processor continually
activates or deactivates
output status according
to output image table
file status

Output
ON
1

Status 1

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Program Scan
During each operating cycle, the processor reads all
inputs, takes these values, and energizes or de-energizes
the outputs according to the user program
...

I/O scan – records status data of input
devices
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Scan Process
The scan time indicates how fast the controller can react
to changes in inputs
...
If a controller
has to react to an input signal that changes states twice
during the scan time, it is is possible that the PLC will
never be able to detect this change
...


End of ladder

In addition to the program itself, the scan time is also dependent on
the clock frequency of the processor!
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Scan Patterns
Vertical Scanning
Order
The processor examines
input and output
instructions from the
first command, vertically,
column by column and
page by page
...


End of ladder
Misunderstanding the way the PLC scans can cause programming
bugs!
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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PLC Programming Languages
The term PLC programming language refers to the method
by which the user communicates information to the PLC
...

Functional
chart
Boolean language
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Comparing Programming Language
PB1

CR1

CR2

SOL

Relay Schematic

LS1
PB1 CR1

CR2

SOL

Equivalent ladder
diagram language
LS1

Equivalent Boolean language

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Relay-Type Instructions
The ladder diagram language is basically a
symbolic set of instructions used to create the
controller program
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Examine If Closed (XIC) Instruction
Symbol

Analogous to the normally open relay
contact
...
Can be a switch
or pushbutton, a contact from a connected output,
or a contact from an internal output
...

The status bit will be either 1 (ON) or 0 (OFF)
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Examine If Closed (XIC) Instruction

I:012
I:012
04
If the status bit is 1 (ON), then the instruction is TRUE
...
For this instruction we
ask the processor to EXAMINE IF (the
contact is) OPEN (XIO)
...
Can be a switch or
pushbutton, a contact from a connected output,
or a contact from an internal output
...


The status bit will be either 1 (ON) or 0 (OFF)
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Examine If Open (XIO) Instruction

I:012
I:012
04
If the status bit is 1 (ON), then the instruction is FALSE
...
The
processor makes this instruction true
(analogous to energizing a coil) when
there is path of true XIC and XIO
instructions in the rung
...
Can be a connected
device or an internal output (internal relay)
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Output Energize (OTE) Instruction
O:013

I:012
I:012

I:012

11

15

O:013

01

OUTPUT ENERGIZE instruction - TRUE
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Output Energize (OTE) Instruction
O:013

I:012
I:012

I:012

11

15

O:013

01

OUTPUT ENERGIZE instruction - FALSE
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Status Bit Examples
Input module

A

Bit status

Button not actuated
Output

A

OFF

False
A

Output
ON

True
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Status Bit Examples
Input module

A

Bit status

Button actuated
Output

A

ON

True
Output

A

OFF

False
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Ladder Rung
A

B

C

Output
instruction

Input conditions
D

A ladder rung consists of a set of input conditions,
represented by contact instructions, and an output
instruction at the end of the rung, represented by the
coil symbol
...
A complete
path is referred to as having logic continuity
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Rung Continuity
Bit in memory

Bit in memory

1

1
LS_1

SOL_5

The Examine If Closed instruction is TRUE
making the rung TRUE
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Rung Continuity
Bit in memory

Bit in memory

0

0
LS_1

SOL_5

The Examine If Closed instruction is FALSE
making the rung False
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Allen-Bradley SLC-500 Controller Addressing
Output image table
file 0

Address
output
terminal
O0:4/6

O:0:4/6
Bit address
Address
input
terminal
I1:3/12

Input image table
file 1

Energized
output

I:3/12
Bit address
Closed input

I1:3

O:0:4

User-programmed rung

12
Lecture – PLC Programming Basics

MME 486 – Fall 2006

6
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Structure of A 16-Bit Word
Bit

OFF ON

Word
(16 bits)

0 1
15 14 13 12 11 10 9

8

7

6

5

4

3

2

1

0

I/O Connection Diagram
L1

L2

L1

L2

PB1

SOL 1
O:2/3

I:4/5

PL 1
LS1

O:3/6

I:4/6

Lecture – PLC Programming Basics

MME 486 – Fall 2006

R

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Parallel Input Branch Instructions
A

C

B

Lecture – PLC Programming Basics

Branch instructions are used
to create parallel paths of
input condition instructions
...


MME 486 – Fall 2006

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Parallel Output Branching
A

C
D

B

E
On most PLC models, branches can be established at
both the input and output portion of the rung
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Nested Input and Output Branches

Input and output branches can be nested to avoid
redundant instructions and to speed up the processor
scan time
...

Lecture – PLC Programming Basics

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Nested Contact Program
A

B

C

Y

Nested
contact

D

On some PLC models,
the programming of a
nested branch circuit
cannot be done directly
...


E
Lecture – PLC Programming Basics

MME 486 – Fall 2006

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PLC Matrix Limitation Diagram
Max series
contacts

No
...

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Programming of Vertical Contacts
A

D

Y

Original program

C
E
D

B
A
B

C

A

C

Y

D
E

B

Y = (AD) + (BCD) + (BE) + (ACE)

Reprogrammed to obtain the
required logic

E

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Programming for Different Scan Patterns
B

A

D

C

Y

Original program

E

F

Y = (ABC) + (ADE) + (FE) + (FDBC)

A

B

C

A

D

E

F

D

Reprogrammed to obtain the
required logic

E

F

Y

B C

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Internal Control Relay
The internal output operates just as any other output
that is controlled by programmed logic; however, the
output is used strictly for internal purposes
...

The advantage of using internal outputs is that there
are many situations where an output instruction is
required in a program, but no physical connection to
a field device is needed
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Extending the Number of Series Contacts Using
an Internal Control Relay
Internal
relay coil
Rung 1

Rung 2
Internal
relay
contact

Lecture – PLC Programming Basics

Discrete output (requires
one physical connection
on the output module)

MME 486 – Fall 2006

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Programming The XIC Instruction
PB1

PB2

Hardwired Circuit
PL

PB1

PB2

PL

User program
providing the
same results

Note that both pushbuttons are represented by the XIC symbol
...
Since both PB1 and PB2
must close to energize the PL, the XIC instruction is used for both
...
When the
pushbutton is closed, relay coil CR is energized and contacts CR1 open
to switch the PL off
...
This is because the rung must be true when the
external pushbutton is open, and false when the pushbutton is closed
...


The computer is adapted
to the particular PLC
model using the relevant
programmable controller
software
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Select Processor Type Screen
The programming software needs to know what
processor is being used in conjunction with the program
...


Lecture – PLC Programming Basics

MME 486 – Fall 2006

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I/O Configuration Screen
The I/O screen
lets you click or
drag-and-drop a
module from an
all inclusive list to
assign it to a slot
in your
configuration
...
These
include:
Input
Output
Timer
Counter
Integer
Bit

Lecture – PLC Programming Basics

MME 486 – Fall 2006

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Monitoring a Ladder Logic Program
Operation of the logic is apparent from the highlighting
of rungs of the various instructions on screen, which
identifies the logic state in real time and has logic
continuity
...

Program Mode – may be used to
Ø enter a new program
Ø edit or update an existing program
Ø upload files
Ø download files
Ø document programs
Ø change software configurations
When the PLC is switched into the
program mode, all outputs from the PLC
are forced off regardless of their rung
logic status, and the ladder I/O scan
sequence is halted
...
Input devices are monitored
and output devices are energized
accordingly
...


Remote Mode – allows the PLC to be
remotely changed between program and run
mode by a personnel computer connected
to the PLC processor

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