Subtract (SUB)

This information applies to the CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, GuardLogix 5570, Compact GuardLogix 5380, CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, and GuardLogix 5580 controllers. Controller differences are noted where applicable.
When enabled, the SUB instruction and the operator '-' subtracts Source B from Source A.
Available Languages
Ladder Diagram
SUB_LD_avail_v31
Function Block Diagram
Function Block Diagram supports these elements:
FBD Block
SUB_FBD Block
FBD Function
TIP:
FBD Function is applicable to CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers only.
FBD Function
Structured Text
This instruction is not available in structured text.
TIP:
Use the operator '-' in an expression to compute the same result. Refer to Structured Text Syntax for more information on the syntax of expressions and assignments within structured text.
Operands
IMPORTANT:
Unexpected operation may occur if:
  • Output tag operands are overwritten.
  • Members of a structure operand are overwritten.
  • Except when specified, structure operands are shared by multiple instructions.
There are data conversion rules for mixing numeric data types within an instruction. See Data Conversions.
Ladder Diagram
Operand
Data Type
CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, and GuardLogix 5570 controllers
Data Type
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers
Format
Description
Source A
SINT
INT
DINT
REAL
SINT
INT
DINT
LINT
USINT
UINT
UDINT
ULINT
REAL
LREAL
LTIME*
TIME*
TIME32*
LDT*
DT*
immediate
tag
Value from which to subtract Source B.
Source B
SINT
INT
DINT
REAL
SINT
INT
DINT
LINT
USINT
UINT
UDINT
ULINT
REAL
LREAL
LTIME*
TIME*
TIME32*
LDT*
DT*
immediate
tag
Value to subtract from Source A.
Dest
SINT
INT
DINT
REAL
SINT
INT
DINT
LINT
USINT
UINT
UDINT
ULINT
REAL
LREAL
LTIME*
TIME*
TIME32*
LDT*
DT*
tag
Tag to store result of the instruction.
TIP:
*Keep these considerations in mind when using relative time (LTIME, TIME32, TIME) and absolute time (LDT, DT) data types in SUB instructions:
  • If both Source A and Source B are relative time, the Dest must be relative time.
  • If Source A is relative time and Source B is absolute time or vice versa, the Dest must be absolute time.
  • In ADD instructions, Source A and Source B cannot both be absolute time.
See Time and date data types for a complete description of Relative Time (LTIME, TIME, and TIME32) and Absolute Time (LDT and DT) data types.
Function Block Diagram
FBD Block
Operand
Data Type
Format
Description
SUB
FBD_MATH
tag
SUB structure
FBD_MATH Structure
Input Members
Data Type
Description
EnableIn
BOOL
Enable input. If false, the instruction does not execute and outputs are not updated.
Default is true.
SourceA
REAL
Value from which to subtract SourceB.
SourceB
REAL
Value to subtract from SourceA.
Output Members
Data Type
Description
EnableOut
BOOL
Indicates if the instruction executed without fault when it was enabled.
Dest
REAL
Result of the instruction.
FBD Function
TIP:
FBD Function is applicable to CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers only.
Input Operands (Left Pins)
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers
Data Type
Description
SourceA (top)
SINT
USINT
INT
UINT
DINT
UDINT
LINT
ULINT
REAL
LREAL
Value from which to subtract SourceB.
SourceB (bottom)
SINT
USINT
INT
UINT
DINT
UDINT
LINT
ULINT
REAL
LREAL
Value to subtract from SourceA.
Output Operand
(Right Pin)
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers
Data Type
Description
Dest
DINT
UDINT
LINT
ULINT
REAL
LREAL
Result of the function.
Affects Math Status Flags
Controllers
Affects Math Status Flags
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers
Conditional
CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, and GuardLogix 5570 controllers
Yes
Major/Minor Faults
None specific to this instruction. See Index Through Arrays for array-indexing faults.
Execution
Ladder Diagram
Condition/State
Action Taken
Prescan
N/A
Rung-condition-in is false
Set Rung-condition-out to Rung-condition-in
Rung-condition-in is true
Set Rung-condition-out to Rung-condition-in
Dest = Source A - Source B
Postscan
N/A
Function Block Diagram
FBD Block
Condition/State
Action Taken
Prescan
N/A
EnableIn is false
Set EnableOut to EnableIn
EnableIn is true
Dest = SourceA - SourceB
If overflow occurs
Clear EnableOut to false
else
Set EnableOut to true
Instruction first run
N/A
Instruction first scan
N/A
Postscan
N/A
FBD Function
TIP:
FBD Function is applicable to CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers only.
Condition/State
Action Taken
Prescan
N/A
Normal Scan
Dest = SourceA - SourceB
Instruction first run
N/A
Instruction first scan
N/A
Postscan
N/A
Examples
Ladder Diagram
SUB_LD_ex_v31
Function Block Diagram
FBD Block
SUB_FBD_ex_v31
FBD Function
SUB_FBD Function
Structured Text
DINT_dest := DINT_srcA - DINT_srcB;
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