Introduction to PLC Operation

1
Chapter 5

• Introduction to PLC Operation

2
Objectives

• Explain what binary information is and how it is
  used in PLCs.
• Describe how computer and PLC data are
  represented.
• Examine PLC memory and how it is used.
• Explain how data gets into a PLC.
• Identify what addresses are and how they are used
  in PLCs.

3
The PLC Is a Digital Computer

• The PLC is a computer similar to a desktop or
  notebook computer.
• A PLC is an industrially hardened computer.

4
PLC Block Diagram
5
Binary Concept

• Binary is based on two states ? on or off.
• Two-state devices are described as either
  discrete or digital devices.
• Discrete or digital devices are simply either on
  or off.

6
Common Industrial Hardware Representing the
Binary Concept
7
Binary Data Representation

• We communicate to others using groups of letters
  arranged into words.
• The PLC uses groups of bits called words.
• Different bit patterns represent different
  information.

8
Bits

• Unlike English, computers have only two
  characters available ? 1 or 0.
• Each 1 or 0 is called a binary digit or bit.
• Binary is base or radix 2.
• A single bit is the smallest unit of computer
  data.

9
PLC Words

• One measure of a computers capabilities is the
  length of the data words on which it can operate.
• Current PLCs use 16-bit words.
• Newer PLCs use 32-bit words.
• SLC 500 family PLCs are 16-bit computers.

10
Information Represented as Combinations of Bits
11
Parts of a 16-Bit Word
12
Bytes, Nibbles, and Bits
13
16-Point Modules I/O Points Represented in a
Word
14
Physical Input Conditions and the Corresponding
Input Data Word
15
8-Point Input Module Represented in a Word
16
24-Point I/O Module Represented in Two Words
17
Two Words Representing Inputs for a 32-Bit Module
18
Data Table Format

• Words are 16 bits.
• Bits 0 through bit 15
• First word or bit is always 0.
• SLC 500 data tables can contain up to 256 words
  (0 to 255).

19
Words Arranged in a Data Table
20
Input Data File (1 of 2)

• Each input screw terminal has one memory location
  to store on or off status.
• Input data is stored in the input data file.
• Also called the input status file
• Input status file holds input status information,
  which is used to solve ladder program.

21
Input Data File (2 of 2)

• Identified as an I-type data file
• Only one input status file allowed per project
• Only has words created for actual modules in
  system

22
Output Data File (1 of 3)

• Each output screw terminal has one memory
  location to store on or off status.
• Output data is stored in the output data file,
  also called the output status file.

23
Output Data File (2 of 3)

• Output status file holds output status
  information to update outputs
• The result of solving the ladder program
• Output data sent to modules during output update
  portion of scan

24
Output Data File (3 of 3)

• Identified as an O-type data file
• Only one output status file allowed per project
• Only has words created for actual modules in
  system

25
Output Status File Correlation To Module
26
Modular PLC and Output and Input Status Tables
27
Fixed PLC and Output and Input Status Table
Image courtesy of Allen-Bradley, a Rockwell
Automation business
28
Fixed I/O PLC Interaction With Input Status File
29
I/O Address Structure
30
I/O Address Format for SLC 500 Family of PLCs
31
RSLogix 500 Software Input Status Table Screen
View
32
RSLogix 500 Software Output Status Table Screen
View
33
PLC Data Formats

• Two 8-bit unsigned bytes of data
• 16-bit unsigned integer
• 16-bit signed integer
• Binary coded decimal
• Hexadecimal

34
Two 8-bit Unsigned Bytes of Data
35
16-bit Unsigned Integer
36
16-bit Signed Integer
37
Binary Coded Decimal
38
Hexadecimal
39
PLC Memory Categories

• PLC memory is divided into two categories.
• System memory
• Application memory

40
System Memory

• Differentiates a PLC from another type of
  computer device
• Gives PLC its personality
• Programmed into PLC at factory
• Also called its operating system

41
SLC 500 Operating System

• SLC 5/03, 5/04, and 5/05 processors have
  field-upgradeable operating systems.
• Add new features
• Add new instructions
• Fix problems

42
Application Memory

• Stores user program
• Stores data associated with user program
• Input status file
• Output status file
• Timers and counters
• Numerical data such as recipes
• Results of math operations

43
Ladder Files

• Ladder files contain ladder programs.
• Ladder file 2 must be main ladder program.
• Ladder files 3 through 255 are subroutines.

44
SLC 500 Data Files

• Output status
• Input status
• Processor status
• Binary or bits
• Timers

• Counters
• Integer
• Floating point
• User-defined

45
Default Data Files

• Data files O through 8 are created by the
  processor with new project.
• Floating point file is available on SLC 500
  modular processors 5/03, 5/04, and 5/05.
• 5/03 processor must have operating system OS 301
  and above.

46
Data File Identification (1 of 2)

• O Output Status File
• I Input Status File
• S Processor Status File
• Cannot create additional O, I, or S data files

47
Data File Identification (2 of 2)

• B3 Binary or bit file
• T4 Timers
• C5 Counters
• R6 Control
• N7 Integer
• F8 Floating point

48
User Configurable Files

• Data files greater than file 8 up to file 255 can
  be created by the user.
• These are user-defined files.
• B, T, C, N, F file types
• Each file can contain up to 255 elements with
  adequate processor memory.