Chapter 5

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 computer’s capabilities is the length of the data words it can operate on. • 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 Module’s 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 • 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 (cont’d.) • 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 • 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 (cont’d.) • 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 (cont’d.) • 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 upgradable 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. Output status Input status Processor status Binary or bits Timers Counters Integer Floating point User defined SLC 500 Data Files

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 • 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 • 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.