High/Low Limit (HLL)

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.

The High/Low Limit (HLL) instruction limits an analog input between two values. You can select high/low, high, or low limits.

Available Languages

Ladder Diagram

This instruction is not available in ladder diagram logic.

Function Block

Structured Text

HLL(HLL_tag);

Operands

Function Block

Operand	Type	Format	Description
HLL tag	HL_LIMIT	structure	HLL structure

HL_LIMIT Structure

Input Parameter	Data Type	Description
EnableIn	BOOL	Enable input. If false, the instruction does not execute and outputs are not updated. Default is true.
In	REAL	The analog signal input to the instruction. Valid = any float Default = 0.0
HighLimit	REAL	The high limit for the Input. If SelectLimit = 0 and HighLimit LowLimit, the instruction sets the appropriate bit in Status and sets Out = LowLimit. Valid = HighLimit > LowLimit Default = 0.0
LowLimit	REAL	The low limit for the Input. If SelectLimit = 0 and LowLimit HighLimit, the instruction sets the appropriate bit in Status and sets Out = LowLimit. Valid = LowLimit < HighLimit Default = 0.0
SelectLimit	DINT	Select limit input. This input has three settings: 0 = Use both limits 1= Use high limit 2 = Use low limit If SelectLimit is invalid, the instruction assumes SelectLimit = 0 and sets the appropriate bit in Status. Valid = 0 to 2 Default = 0

Output Parameter	Data Type	Description
EnableOut	BOOL	Indicates if instruction is enabled. Cleared to false if Out overflows.
Out	REAL	The calculated output of the algorithm.
HighAlarm	BOOL	The high alarm indicator. Set to true when In HighLimit. The HighAlarm is disabled when SelectLimit is set to 2.
LowAlarm	BOOL	The low alarm indicator. Set to true when In LowLimit. The LowAlarm is disabled when SelectLimit is set to 1.
Status	DINT	Status of the function block.
InstructFault (Status.0)	BOOL	The instruction detected one of the following execution errors. This is not a minor or major controller error. Check the remaining status bits to determine what occurred.
LimitsInv (Status.1)	BOOL	HighLimit LowLimit.
SelectLimitInv (Status.2)	BOOL	The value of SelectLimit is not a 0, 1, or 2.

Structured Text

Operand	Type	Format	Description
HLL tag	HL_LIMIT	structure	HLL structure

See Structured Text Syntax for more information on the syntax of expressions within structured text.

Description

The HLL instruction determines the value of the Out using these rules:

Selection	Condition	Action
SelectLimit = 0 (use high and low limits)	In < HighLimit and In > LowLimit	Out = In
	In HighLimit	Out = HighLimit
	In LowLimit	Out = LowLimit
	HighLimit LowLimit	Out = LowLimit
SelectLimit = 1 (use high limit only)	In < HighLimit	Out = In
SelectLimit = 1 (use high limit only)	InHighLimit	Out = HighLimit
SelectLimit = 2 (use low limit only)	In > LowLimit	Out = In
SelectLimit = 2 (use low limit only)	In LowLimit	Out = LowLimit

Affects Math Status Flags

Major/Minor Faults

None specific to this instruction. See Common Attributes for operand-related faults.

Execution

Function Block

Condition/State	Action Taken
Prescan	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is false	EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is true	EnableIn and EnableOut bits are set to true. The instruction executes.
Instruction first run	N/A
Instruction first scan	N/A
Postscan	EnableIn and EnableOut bits are cleared to false.

Structured Text

Condition/State	Action Taken
Prescan	See Prescan in the Function Block table.
Normal Execution	See Tag.EnableIn is true in the Function Block table.
Postscan	See Postscan in the Function Block table.

Example

This HLL instruction limits In between two values and sets HighAlarm or LowAlarm, if needed when In is outside the limits. The instruction sets Out = limited value of In.

Function Block

Structured Text

HLL_01.In := value;

HLL_01.HighLimit := high_limit;

HLL_01.LowLimit := low_limit;

HLL(HLL_01);

limited_value := HLL_01.Out;

high_alarm := HLL_01.HighAlarm;

low_alarm := HLL_01.LowAlarm;

Select/Limit Instructions

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