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Chapter
14
Data Handling
Instructions
Objectives
• Use PLC instructions for moving data from
one register to another.
• Create stacks for storing and retrieving data
from PLC memory.
• Move data between tables.
• Use PLC instructions for number system
conversion.
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Data Handling Instructions
• Reading, storing, retrieving, and
writing of data on ports and registers.
• Moving or copying data to another
register or file.
• Converting data to a different number
system.
• Performing a logic operation on data.
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Data Handling Instructions
(Cont.)
• Move (MOV).
• Masked move
(MVM).
• FIFO load (FIFO).
• FIFO unload (FFU).
• LIFO load (LIFO).
• LIFO unload (LFU).
• Copy file (COP).
• Fill file (FLL).
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• Convert to BCD (TOD).
• Convert from BCD
(FRD).
• Convert from radians to
degrees (DEG).
• Convert from degrees
to radians (RAD).
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Index Register
• Sometimes simply called a pointer.
• A register that holds an address of a
data.
• Used to move a word into a specific
address.
• Data is placed in another data register.
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Move (MOV) Instruction
• Transfers a number into a register.
• Transfers content of a register to another
register.
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Ladder Logic Diagram Using
Index Register
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Masked Move (MVM)
Instruction
• Hides or masks a portion of data prior
to transferring it into a register.
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Masked Move Example
• Using masked move (MVM) instruction to read
data from input module two (I:2)
• Mask the lower byte of the data prior to
transferring it into address B3:0.
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Masked Move Example
(Cont.)
• Input module two (I:2) data ANDed with mask
number FF00h to set lower bytes to zero.
• Bits in the upper byte have same state as bits
in I:2 module’s upper byte.
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FIFO and LIFO
• Like pallets, data registers can be
stored and retrieved in two different
ways:
– First-in-first-out (FIFO).
– Last-in-first-out (LIFO).
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FIFO and LIFO (Cont.)
• Concept of storing and retrieving
pallets is similar to the way data is
placed in and retrieved from a stack.
• A stack in PLC or computer terms:
– A group of consecutive data registers, or
words, placed temporarily in a memory
location, or buffer.
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FIFO and LIFO (Cont.)
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FIFO and LIFO (Cont.)
• When stacking data:
– Stack grows larger as data registers are
placed in stack.
– Stack grows smaller as data registers are
retrieved from stack.
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FIFO and LIFO (Cont.)
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FIFO and LIFO (Cont.)
• In FIFO operation, first item in stack is
first item out of stack.
• In LIFO operation, last item in stack is
first item out of stack.
• Push operation – placing an item in a
stack.
• Pop operation – retrieving an item from
a stack.
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FIFO and LIFO (Cont.)
• Allen-Bradley SLC 500 PLCs have four
instructions for LIFO and FIFO
operations:
– FIFO load (FFL).
– FIFO unload (FFU).
– LIFO load (LFL).
– LIFO unload (LFU).
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FIFO and LIFO (Cont.)
• In PLC systems, stacks are created
when stacks of integer words (i.e.,
double-byte data) are stored in the
memory buffer.
• Data can be transferred into a register
or another memory location from the
stack.
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FFL Load Instruction
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LFL Load Instruction
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Copy File and Fill File
Instructions
• Data handling instructions used to
store blocks of data in a PLC file:
– Copy file (COP).
– Fill file (FLL).
• Can use these instructions to manually
enter a file with constant numbers or
data from another file.
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Data Conversion
• Data conversion instructions used to
convert content of a PLC register from:
– One number system to another number
system.
– One unit system to another unit system.
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Data Conversion Instructions
• TOD instruction:
– Converts binary number in source
register to binary coded decimal (BCD).
– Places result in destination register.
• FRD instruction:
– Converts BCD number in source register
to binary.
– Places result in the destination register.
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Data Conversion Instructions
(Cont.)
• DEG instruction:
– Converts angle value from radians to
degrees.
– RAD instruction converts value of an
angle from degrees to radians.
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Convert to BCD (TOD) and
Convert from BCD (FRD)
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