Storing and Organizing Data

Why Do I Need to Understand How Data Is Represented?

• In order to install, program,maintain, and troubleshoot today’s PLCs, you must understand the different methods by which internal data is represented.

Objectives

• Identify the commonly used number systems used in PLCs.

• Convert binary data to decimal.

• Examine how data is stored in PLC data tables.

• Describe how BCD field devices interface.

PLC Words

• One measure of a computer’s capabilities is the length of the data words on which it can operate.

• Many current PLCs use 16-bit words.• Many newer PLCs use 32-bit words.• SLC 500 and PLC 5 family PLCs are 16-bit

computers.• Control Logix is a 32-bit computer.

Number Systems Typically Used with PLCs

We Use Words to Represent Information

• Our words are groups of characters grouped together to represent something.

• The words we use are of different lengths.– Controller– The– Monday

We Use Symbols Called Numbers to Represent Data

• Everyday numbers are decimal. 12,345

Computers Do Not Understand

• Computers do not understand the words and numbers humans use.

• Computers have their own language called binary.

Binary Concept

• Two-state devices are described as either discrete or digital devices.– Discrete or digital devices are simply either on

or off.– Binary is based on two states, on or off.

Binary Language

• Binary information is also represented in groups of characters.

• A group of binary digits called bits can be organized into words.

• Binary bits consist of only two characters– 1 and 0

Binary Words

• 16 bits grouped together is called a word.

• A binary word might look like:

1010 1010 1010 1010 1010

Information Represented as Combinations of Bits

Decimal Numbers

• Ten digits– 0,1,2,3,4,5,6,7,8,9

• Base or radix– 10

• Weights– 1, 10, 100, 1000

Decimal Number System

The Binary Number System Has the Following Characteristics

• Two digits– 0 or 1

• Base or radix– 2

• Weights– 1, 2, 4, 8, 16, 32, 64…

16-bit Binary Word Bit Weighting

MSB LSB

• LSB = least significant bit• MSB = most significant bit

Binary Numbers Place Values

Binary Number Weighting

Decimal 0 through 7 Represented with Binary Bits

Comparison of Decimal to Binary Numbers

Parts of a 16-bit Word

Bytes, Nibbles, and Bits

PLC Data Formats

• Two 8-bit unsigned bytes of data

• 16-bit unsigned integer

• 16-bit signed integer

• 32-bit signed integer

• Binary coded decimal

• Hexadecimal

Two 8-bit Unsigned Bytes of Data

Hexadecimal

16-bit Signed Integer

16-bit Unsigned Integer

32-bit Signed Integer (1 of 2)

0 1 1 1 0 0 0 1 1 1 1 1 1 0 0 0

0 1 1 1 0 0 0 1 1 1 1 1 1 0 0 0

Double Word

Sign bit

Lower 31 bits contain the value

32 – bit signed integer data range:

-2,147,483,648 to +2,147,483,647

32-bit Signed Integer (2 of 2)

0 1 1 1 0 0 0 1 1 1 1 1 1 0 0 0

0 1 1 1 0 0 0 1 1 1 1 1 1 0 0 0

Double Word

WordWord

Byte Byte Byte Byte

Basic PLC Data TypesData Type Description Size Range

Bool Single bit Bit 1 = ON

0 = OFF

Sint 8 bits Byte -128 to

+127

Int 16 bits Word -32,768 to +32,767

Dint 32 bits Double

Word

-2,147,483,648 +2,147,483,647

Real Floating point Real or

floating Point

Larger than Dint or with Decimal

point

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).

• PLC 5 data tables can contain up to 1,000 words (0 to 999).

Words Arranged in a Data Table

Status Table Words Assigned by Module Requirements

• The number of status table words assigned depend on what a specific module needs.– 16-point module equals 16 bits.– 32-point module equals 32 bits.– Four-channel analog equals four words.– Eight-channel analog needs eight.

SLC 500 I/O Configuration

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

SLC 500 Input Status Table

24-point I/O Module Represented in Two Words

Two Words Representing Inputs for a 32-bit Module

Output Status File Correlation to Module

SLC 500 Digital Outputs

Four Words Representing a Four-channel Analog Module

Channel 0

Channel 1

0 1 1 1 1 1 0 1 0 1 0 1 1 1 1 1

0 0 0 0 0 0 1 1 1 1 1 1 1 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 0 0 1 1 1 1 1 1 1 0 0 0 0 0 0

Channel 2

Channel 3

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

SLC 500 Analog Input Channels

SLC 500 Output Analog Channels

So, What’s in It for ME??

• I need to be able to work with different number systems.– PLC configuration– PLC troubleshooting– Program interpretation– Error code

interpretation– Data conversion to

different hardware

SLC 500 Analog Input Module Configuration

• You need to configure a 1747sc-INI4i analog input module.

• Module configuration specifications are listed below.

Channel Configuration Word

SLC 500Processor

AnalogInput

ModuleAnalog Input Signal

Channel Data Word

Channel Status Word

Channel Configuration

Specifications

• Enable the input channel

• 4- to 20-mA input range

• Work in engineering units

• Pump to run maximum if open input

• 60-Hz input filter

• Auto calibration disabled

What Do You Need to Do?

• Determine the 16-bit data word for the configuration.

• Convert the binary word into decimal.

• Program a move instruction on your SLC 500 ladder to move the configuration data to each analog channel.

Input Channel Configuration Word

• What value will you enter into the move instruction’s source?

Understanding Binary Coded Decimal (BCD)

Single-digit BCD Thumb-wheel Interfaced to a PLC

Binary Coded Decimal Number Bit Patterns

Binary Coded Decimal

Two-digit BCD Thumb-wheel Interface

Four-digit BCD Thumb-wheel

Output Module Connected to Seven-segment Display

Comparison of BCD to Decimal and Binary Numbers

BCD Invalid Codes

Decimal, Hexadecimal, and BCD Comparisons

SLC 500 Conversion from BCD

SLC 500 Converting to BCD

PLC 5 Converting from BCD

PLC 5 Converting to BCD