Log Base 10 (LOG)
This table lists the controllers and applications that support this instruction.
Architecture | Standard applications | Safety applications |
|---|---|---|
CompactLogix 5370, ControlLogix 5570, Compact GuardLogix 5370, and GuardLogix 5570 controllers | Yes | No |
CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers | Yes | Yes |
The Log Base 10 (LOG) instruction takes the log base 10 of the Source and stores the result in the Destination.
Available Languages
Ladder Diagram
Function Block Diagram
Function Block Diagram supports these elements:
FBD Block
FBD Function
TIP:
FBD Function is applicable to CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers only.
Structured Text
This instruction is not available in structured text.
TIP:
Use LOG as an 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 | SINT INT DINT REAL | SINT INT DINT LINT USINT UINT UDINT ULINT REAL LREAL | Immediate tag | Value for which the instruction finds the log. |
Destination | SINT INT DINT REAL | SINT INT DINT LINT USINT UINT UDINT ULINT REAL LREAL | tag | Tag to store the result of the instruction. |
Function Block Diagram
FBD Block
Operand | Type | Format | Description |
|---|---|---|---|
LOG | FBD_MATH_ADVANCED | tag | LOG structure |
FBD_MATH_ADVANCED 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. |
Source | REAL | Value for which the instruction finds the log. |
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 Pin) | Data Type CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers | Description |
|---|---|---|
Source | SINT USINT INT UINT DINT UDINT LINT ULINT REAL LREAL | Value of which to find the log of this value |
Output Operand (Right Pin) | Data Type CompactLogix 5380, CompactLogix 5480, ControlLogix 5580, Compact GuardLogix 5380, and GuardLogix 5580 controllers | Description |
|---|---|---|
Dest | REAL LREAL | Result of the function. |
Description
The LOG instruction takes the log base 10 of the Source and stores the result in the Destination. The Source must be greater than zero or a minor fault will occur.
Affects Math Status Flags
Controllers | Affects Math Status Flag |
|---|---|
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 = value of log base 10 of the Source. |
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 = value of natural log of the Source. If overflow occurs Clear EnableOut to false. else Set EnableOut to true. |
Instruction first scan | N/A |
Instruction first run | 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 = value of log base 10 of the Source. |
Instruction first run | N/A |
Instruction first scan | N/A |
Postscan | N/A |
Examples
Ladder Diagram
Function Block Diagram
FBD Block
FBD Function
Structured Text
result := LOG(value);
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