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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.
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
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
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…
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
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).
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.
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
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.