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)
Double Word
0 1 1 1 0 0 0 1 1
0 1 1 1 0 0 0 1 1 1 1 1 1 0 0 0
1 1 1 1 0 0 0
Lower 31 bits contain the value
Sign bit
32 – bit signed integer data range:
-2,147,483,648 to +2,147,483,647
32-bit Signed Integer (2 of 2)
Double Word
0
0 1 1 1 0 0 0 1 1 1 1 1 1 0 0
Byte
Byte
Word
1 1 1 0 0 0 1 1 1 1 1 1
0
0
Byte
Byte
Word
0
0
Basic PLC Data Types
Data 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
15
14
13
12
11
10
9
8
7
6
0
1
1
1
1
1
0
1
0
0
0
0
0
0
0
1
1
0
0
0
0
0
0
0
1
0
0
1
1
1
1
5
4
3
2
1
0
1
0
1
1
1
1
1
Channel 0
1
1
1
1
1
0
0
0
Channel 1
0
0
0
0
0
0
0
0
0
Channel 2
1
1
1
0
0
0
0
0
0
Channel 3
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
Channel Data Word
Analog Input Signal
Analog
Input
Module
Channel Status Word
SLC 500
Processor
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