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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

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PLC System

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Processor Memory Organization
The memory of a PLC is organized by types.
The memory space can be divided into two broad categories: program and data memory.
Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than basic inputs contacts and output coils and rely heavily upon data stored in the memory of the PLC.
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Memory Map
A memory map can be used to show how memory is organized in a 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)

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

• Program files contain the logic controlling machine operation.

Program file organization for SLC-500 controller.
Lecture – PLC Programming Basics

• This logic consists of instructions that are programmed in a ladder logic format.
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Data Files
Data file organization for SLC-500 controller.

The data file portion of memory stores input and output status, processor status, the status of various bits and numerical data.

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

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Input Table File Operation
Input module
Switch Open

Binary 0 stored
0

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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

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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

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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

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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. This process is known as a scan.
I/O scan – records status data of input devices. Energizes output devices that have their associated status bits set to
ON (1)

Program scan – instructions are executed sequentially

Because the inputs can change at any time, the PLC must carry on this process continuously.
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Scan Process
The scan time indicates how fast the controller can react to changes in inputs. Scan times vary with computer model and program content, and length. 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.

Scan time may be a concern in high speed operations

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Scan Process
Read inputs

The scan is a a continuous and sequential process Adjusts outputs Lecture – PLC Programming Basics

Run program MME 486 – Fall 2006

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Data Flow Overview

Input modules Input data Input image table file

Output image table file

Examine data

Output data Output modules Return results
Program

Check/compare/examine specific conditions
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Take some action 16 of 62

Scan Process
Output
Module

Input
Module
Input file I:3/6

Input device Output file Output device O:4/7
O:4/7

I:3/6
Program
When the input is closed, the input module senses a voltage and an ON condition (1) is entered into the input table bit I:3/6

I:3/6

O:4/7

During the program scan the processor sets instructions
I:3/6 and O:4/7 to ON (1)

Lecture – PLC Programming Basics

MME 486 – Fall 2006

The processor turns light output O:4/7
ON during the next
I/O scan

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Scan Patterns
Horizontal Scanning
Order
The processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. End of ladder

In addition to the program itself, the scan time is also dependent on the clock frequency of the processor!
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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. Pages are executed in sequence.

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

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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.

The three most common language structures are: ladder diagram language, Ladder diagram language
Boolean language, and functional chart.
Functional
chart
Boolean language
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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

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Relay-Type Instructions
The ladder diagram language is basically a symbolic set of instructions used to create the controller program.
These ladder instructions symbols are arranged to obtain the desired control logic.

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Examine If Closed (XIC) Instruction
Symbol

Analogous to the normally open relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) CLOSED (XIC)

Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output.
Has a bit-level address which is examined for an
ON condition.
The status bit will be either 1 (ON) or 0 (OFF).
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Examine If Closed (XIC) Instruction

I:012
I:012
04

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Examine If Closed (XIC) Instruction

I:012
I:012
04
If the status bit is 0 (OFF), then the instruction is FALSE.
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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.

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Examine If Open (XIO) Instruction
Symbol

Analogous to the normally closed relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) OPEN (XIO).

Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output.
Has a bit-level address which is examined for an OFF condition. The status bit will be either 1 (ON) or 0 (OFF).
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Examine If Open (XIO) Instruction

I:012
I:012
04
If the status bit is 0 (OFF), then the instruction is TRUE.

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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.

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Output Energize (OTE) Instruction
Symbol

Analogous to the relay coil. The processor makes this instruction true
(analogous to energizing a coil) when there is path of true XIC and XIO instructions in the rung.

Typically represents any output that is controlled by some combination of input logic. Can be a connected device or an internal output (internal relay).
If any left-to-right path of input conditions is TRUE, the output is energized (turned ON).
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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
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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
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Status Bit Examples
Input module

A

Bit status

Button not actuated
Output

A

OFF

False
A

Output
ON

True
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Status Bit Examples
Input module

A

Bit status

Button actuated
Output

A

ON

True
Output

A

OFF

False
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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.
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Ladder Rung
A

B

C

Output instruction Input conditions
D

For an output to be activated or energized, at least one left-to-right path of contacts most be closed. A complete path is referred to as having logic continuity. When logic exists the rung condition is said to be TRUE.
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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
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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
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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
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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

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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.
If at least one of these parallel branches forms a true logic path, the logic is enabled.

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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.
With output branching, you can program parallel outputs on a rung to allow a true logic path to control multiple outputs. Lecture – PLC Programming Basics

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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.
A nested branch starts or ends within another branch.
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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
A

B

C

D

Y

C

Contact instruction C repeated

Reprogrammed to obtain the required logic. E
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PLC Matrix Limitation Diagram
Max series contacts No. outputs per rung and location of the output in the rung

Max parallel lines There may be limitations to the number of series contacts instructions, number of parallel lines, and the number of outputs and their location on the rung.
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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

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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

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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 internal output does not directly control an output device.
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. Their use in this type of instance can minimize output card requirements.

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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)

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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. This is because the normal state of an input (NO or NC) does not matter!
What does matter is that if contacts need to close to energize the output, then the XIC instruction is used. Since both PB1 and PB2 must close to energize the PL, the XIC instruction is used for both.

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Programming The XIO Instruction
PB1

PB1
CR

PL

CR1
PL

User program providing the same results

Hardwired Circuit
When the pushbutton is open in the hardwired circuit, relay coil CR is deenergized and contacts CR1 close to switch the PL on. When the pushbutton is closed, relay coil CR is energized and contacts CR1 open to switch the PL off. The pushbutton is represented in the user program by an XIO instruction. This is because the rung must be true when the external pushbutton is open, and false when the pushbutton is closed.

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Operation of The XIC and XIO Instructions
Summary of status conditions
The status of the instruction is:
XIC
Examine If Closed

XIO
Examine If Open

Logic 0

False

True

False

Logic 1

True

False

True

If the data table bit is:

Lecture – PLC Programming Basics

MME 486 – Fall 2006

OTE
Output Energize

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Operation of The XIC and XIO Instructions
State of the output as determined by the changing state of the inputs in the rung
Inputs

Time

XIC

XIO

OTE

0

0

0

Goes true

1

0

1

False

Goes false

1

1

0

False

Remains false

0

1

0

XIC

XIO

t1 (initial)

False

True

t2

True

True

t3

True
False

t4

Input instructions
XIC

XIO

Bit status

Output
OTE
False

Output instruction
OTE

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Entering the Ladder Diagram
A personal computer is most often used to enter the ladder diagram.

The computer is adapted to the particular PLC model using the relevant programmable controller software. Lecture – PLC Programming Basics

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RSLogix Main Screen
Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment

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Bit Instructions Tool Bar

To place an instruction on a rung, click its icon on the toolbar and simply drag the instruction straight off the toolbar onto the rung of the ladder. Lecture – PLC Programming Basics

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Select Processor Type Screen
The programming software needs to know what processor is being used in conjunction with the program.
1747-L40E

You simply scroll down the list until you find the processor you are using and select it.

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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. Lecture – PLC Programming Basics

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Data File Screen
Data file screens contain data that is used in conjunction with ladder program instructions. These include: Input
Output
Timer
Counter
Integer
Bit

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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. Highlighted rungs indicate the instruction is true
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Modes of Operation
A processor has basically two modes of operation: the program mode or some variation of the run mode.
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.

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Variations of the Run Mode
Run Mode – is used to execute the user program. Input devices are monitored and output devices are energized accordingly. Test Mode – is used to operate, or monitor, the user program without energizing any outputs. 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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