- Slides: 48
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
16 -Point 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
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
Data File Identification (2 of 2) • • • B 3 T 4 C 5 R 6 N 7 F 8 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.