Function Blocks
ALM Alarm Function Block
Revision 11 HC900 Hybrid Control Designer Function Block Reference Guide 49
2/07
ALM Alarm Function Block
Description
The ALM label stands for the Analog Alarm function.
This block is part of the Alarms/Monitor category.
Function
The analog alarm block accepts an analog signal as a process variable and compares it to a limit value
(setpoint) to determine an alarm condition. The setpoint may be entered by the user or be another analog
signal in the controller.
Alarm actions may be high, low or high deviation, low deviation or band deviation. For deviation alarming,
a second analog signal provides the reference and setpoints represent deviation from the reference.
The alarm output may be inverted to create normally active digital output. A user selection for latching
until acknowledged or automatically reset is provided.
A user-specified hysteresis value in the engineering units of the process variable is provided.
An on-delay time value up to 240 seconds is available to prevent momentary alarm actions. A digital reset
input is available to disable alarm actions.
Alarm type function
(PV>SP) High Process Variable/Local Setpoint
• OUT = ON If the PV is greater than the local Setpoint
• OUT = OFF If the PV is less than the Local Setpoint minus Hysteresis
(PV>CV) High Process Variable/Compare Value
• OUT = ON If the PV is greater than the Compare Value (CV) i.e. Alarm Setpoint
• OUT = OFF If the PV is less than the Compare Value minus Hysteresis
(PV<SP) Low Process Variable/Local Setpoint
• OUT = ON If the PV is less than the Local Setpoint
• OUT = OFF If the PV is greater than the Local Setpoint + Hysteresis
(PV<CV) Low Process Variable/Compare Value
• OUT = ON If the PV is less than the Compare Value (CV)
• OUT = OFF If the PV is greater than the Compare Value + Hysteresis
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