Chapter 5
Introduction to PLC Operation

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.

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.

PLC Block Diagram

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.

Common Industrial Hardware
Representing the Binary Concept

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.

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.

PLC Words
• One measure of a computer’s 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.

Information Represented as Combinations of Bits

Parts of a 16-Bit Word

Bytes, Nibbles, and Bits

16Point Module’s
I/O Points Represented in a Word

Physical Input Conditions and the
Corresponding Input Data Word

8-Point Input Module
Represented in a Word

24-Point I/O Module
Represented in Two Words

Two Words Representing
Inputs for a 32-Bit Module

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).

Words Arranged in a Data Table

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.

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

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.

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

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

Output Status File
Correlation To Module

Modular PLC and
Output and Input Status Tables

Fixed PLC and Output and Input Status Table
Image courtesy of Allen-Bradley, a Rockwell Automation business

Fixed I/O PLC Interaction
With Input Status File

I/O Address Structure

I/O Address Format for
SLC 500 Family of PLCs

RSLogix 500 Software
Input Status Table Screen View

RSLogix 500 Software
Output Status Table Screen View

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

Two 8-bit
Unsigned Bytes of Data

16-bit Unsigned Integer

16-bit Signed Integer

Binary Coded Decimal

Hexadecimal

PLC Memory Categories
• PLC memory is divided into two categories.
– System memory
– Application memory

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

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

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

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

SLC 500 Data Files
• Output status
• Input status
• Processor status
• Binary or bits
• Timers
• Counters
• Integer
• Floating point
• User-defined

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.

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

• B3
• T4
• C5
• R6
• N7
• F8
Data File Identification
(2 of 2)
Binary or bit file
Timers
Counters
Control
Integer
Floating point

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.