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Programmable Logic Controllers

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executive or operating system. program controlling PLC ... also a register. 8 or 16 binary bits. example: one contact closure may be stored as one bit ... – PowerPoint PPT presentation

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Title: Programmable Logic Controllers


1
Programmable Logic Controllers
  • Supplement to Digital Logic

2
Programmable Controller Defined
  • National Electrical Manufacturers Association
    (NEMA) Definition
  • A digital operated electronic apparatus that
    uses a programmable memory for the the internal
    storage of instructons for implementing specific
    functions, such as logic, sequencing, timing,
    counting, and arithmetic, to control machines or
    processes through digital or analog input or
    output modules.

3
PLC Parts

External Inputs
Central Processing Unit
External Outputs
User Program
From computer through serial data
4
Central Processing Unit
  • CPU
  • memory
  • decision making
  • power supply (separate module in rack)

5
executive or operating system
  • program controlling PLC
  • gives structure to generalized microprocessor
  • permanent memory so cannot be altered.

6
Basic Operations
  • Input and Output operations
  • Arithmetic ( - x / )
  • and Logic (compare)
  • Reading and changing contents of memory locations
  • Jump Operations. Bypass portions of user program
    when requested.

7
Microprocessor Power
  • Defined in terms of number and variety of
    instructions possible
  • 8 bit register holds 28 or 256 combinations
  • 16 bit register holds 216 or 65536 combinations.
  • Bit size defines speed and precision of
    mathematical operations

8
Control Application Program
  • Entered by the user
  • ladder logic drawing
  • Executive program translates program into
    instructions for PLC.

9
Processing
Control Application Program
Executive Program - I/O interface -
diagnostics -execution of applications program
Central Processing Unit
10
Self Check Diagnostics
  • memory okay
  • processor okay
  • battery okay
  • for storage of programs when off
  • power supply okay

11
Writing
  • storing data to memory
  • write from computer to PLC to load edited
    program into PLC memory
  • write from PLC to computer to make modifications
    to program
  • Reading
  • retrieving data from memory

12
Bytes
  • memory expressed in groups of 8 bits (1 or 0)
  • a byte is a group of 8 bits

13
Types of Memory
  • Control Application Program
  • temporary memory as it must be changed
  • Executive Program
  • permanent memory so cannot be altered.

14
Address
  • memory location in the CPU where a binary word
    can be stored

15
Word
  • also a register
  • 8 or 16 binary bits
  • example one contact closure may be stored as
    one bit
  • it is either a one or a zero

16
input tables
  • Example a PLC has 32 inputs
  • requires input table of 32 bits
  • each contact closure is a bit
  • a closed contact is a 1
  • an open contact is a 0

17
output table
  • outputs such as coils
  • De-energized coil is 0
  • energized coil is 1
  • program examines memory location to see if
    contact or coil has changed state

18
Internal storage bits
  • software coils and contacts for internal program
    interlocks
  • portion of memory allocated to internal storage

19
Other memory
  • timer data
  • counter data
  • analog modules
  • called storage registers

20
Memory organization
  • data table
  • factory configured data
  • additional preset and accumulated data
  • file or bit storage
  • User program
  • main program
  • subroutines
  • message storage

21
Input Registers
  • analog inputs
  • electrical measurements of
  • temperature
  • speed
  • force
  • word inputs
  • thumbwheel switch

22
Output Registers
  • analog output
  • signals for motor drives
  • digital displays

23
Subroutines
  • storage of small programs periodically accessed
    but not scanned by executive program unless told
    to do so by application program.

24
Messages
  • user programmed messages for CRT or printer

25
Memory Configurable
  • can vary size of different memory areas as
    required

26
Good News ?
  • You can use a PLC without knowing much about its
    internal operation just as you can use a computer
    without knowing much about what goes on inside.

27
Scan function
  • Key idea for programming. The order of operations
    does make a difference.
  • across (left to right, down)

Must have an END statement
28
Input/Output System
  • Interface between real world and CPU.
  • consists of
  • rack with power supply
  • modules
  • CPU module
  • input module
  • output module
  • analog module

29
I/O Addressing
  • various methods
  • Allen-Bradley example I01/11
  • first digit input or output device
  • I for input device
  • O for output
  • second digit rack number
  • third digit bit number
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