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Move/Logical Instructions (MOV, MVM, BTD, MVMT, BTDT, CLR, SWPB, AND, OR, XOR, NOT, BAND, BOR, BXOR, BNOT) Chapter 7
Swap Byte (SWPB)
Operands:
The SWPB instruction rearranges the bytes of a value.
Relay Ladder
Operand
Source
Type
INT
DINT
REAL
Format
tag
Enter
tag that contains the bytes that you want to rearrange
Order Mode
Destination INT
DINT
REAL tag
If the Source
Is an
And You Want To Change the Bytes To
This Pattern (Each Letter Represents a
Different Byte)
INT
DINT
REAL n/a
ABCD
⇒
ABCD
⇒
ABCD
⇒ tag to store the bytes in the new order
If the Source
Is an
Then the Destination Must Be an
INT INT
DINT
REAL
DINT
DINT
REAL
Then Select
any of the options
REVERSE (or enter 0)
WORD (or enter 1)
HIGH/LOW (or enter 2)
Structured Text
SWPB(Source,OrderMode,Dest);
The operands are the same as those for the relay ladder SWPB instruction. If you select the HIGH/LOW order mode, enter it as HIGHLOW or
HIGH_LOW (without the slash).
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299
Chapter 7 Move/Logical Instructions (MOV, MVM, BTD, MVMT, BTDT, CLR, SWPB, AND, OR, XOR, NOT, BAND, BOR, BXOR, BNOT) bar code reader
B A
42969
42968
A B
Arithmetic Status Flags:
not affected
Fault Conditions:
none
Execution:
Condition
prescan rung-condition-in is false rung-condition-in is true
EnableIn is set
Relay Ladder Action
The rung-condition-out is set to false.
The rung-condition-out is set to false.
The rung-condition-out is set to true.
na instruction execution postscan
Description:
The SWPB instruction rearranges the order of the bytes of the Source. It places the result in the Destination.
When you read or write ASCII characters, you typically do not need to swap characters. The ASCII read and write instructions (ARD, ARL, AWA, AWT) automatically swap characters, as shown below.
The instruction rearranges the specified bytes.
The rung-condition-out is set to false.
Tag Name
bar_code[0]
Value Style Type
AB ASCII INT
Structured Text Action
No action taken na na
EnableIn is always set.
The instruction executes.
The instruction rearranges the specified bytes.
No action taken.
300
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Move/Logical Instructions (MOV, MVM, BTD, MVMT, BTDT, CLR, SWPB, AND, OR, XOR, NOT, BAND, BOR, BXOR, BNOT) Chapter 7
Example 1:
The three SWPB instructions each reorder the bytes of DINT_1 according to a different order mode. The display style is ASCII, and each character represents one byte. Each instruction places the bytes, in the new order, in a different Destination.
Relay Ladder
Structured Text
SWPB(DINT_1,REVERSE,DINT_1_reverse);
SWPB(DINT_1,WORD,DINT_1_swap_word);
SWPB(DINT_1,HIGHLOW,DINT_1_swap_high_low);
Example 2:
The following example reverses the bytes in each element of an array. For an
RSLogix 5000 project that contains this example, open the
RSLogix 5000\Projects\Samples folder, Swap_Bytes_in_Array.ACD file.
1.
Initialize the tags. The SIZE instruction finds the number of elements in
array and stores that value in array_length. A subsequent instruction uses this value to determine when the routine has acted on all the elements in the array.
2.
Reverse the bytes in one element of array.
•
The SWPB instruction reverses the bytes of the element number that is indicated by the value of index. For example, when index equals 0, the
SWPB instruction acts on array[0].
•
The ADD instruction increments index. The next time the instruction executes, the SWPB instruction acts on the next element in array.
3.
Determine when the SWPB instruction has acted on all the elements in the array.
•
If index is less then the number of elements in the array (array_length), then continue with the next element in the array.
•
If index equals array_length, then the SWPB has acted on all the elements in the array.
Publication 1756-RM003K-EN-P - July 2008
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301
Chapter 7 Move/Logical Instructions (MOV, MVM, BTD, MVMT, BTDT, CLR, SWPB, AND, OR, XOR, NOT, BAND, BOR, BXOR, BNOT)
Relay Ladder
Initialize the tags.
Reverse the bytes.
Determine whether the SWPB instruction has acted on all the elements in the array.
Structured Text
index := 0;
SIZE (array[0],0,array_length);
REPEAT
SWPB(array[index],REVERSE,array_bytes_reverse[index]); index := index + 1;
UNTIL(index >= array_length)END_REPEAT;
302
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