AO (Analog Output) Block Documentation

Overview

The AO block converts a real-valued analog signal (e.g., a PID output) into a hardware-compatible raw value to be sent to an analog output module (DAC). The output can be scaled and reversed, and alarms are triggered based on configured thresholds with hysteresis and acknowledgment logic.

This block can be used for: - Valve or actuator control - VFD output signal - Pressure or level control - Process output signal representation

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AO Logic Block Illustration

The following illustration shows how the AO block is linked with the physical analog output channel. It receives inputs from logic (e.g., PID output) and sends processed, scaled, raw values to the hardware.

Showing the AO block with AO channel in the above picture.

In the above picture, we are showing that how to use the AO block

Pins Information

Signal	Type	Direction	Description
AO_IN	REAL	Input	Analog value from logic (e.g., PID block output)
IN_LO	REAL	Input	Minimum Input value (e.g., 0.0)
IN_HI	REAL	Input	Maximum Input value (e.g., 100.0)
OP_LO	WORD	Input	Minimum raw Output value for AO Channel
OP_HI	WORD	Input	Maximum raw Output value for AO Channel
DIR	BOOL	Input	FALSE = Direct scaling, TRUE = Reverse
ACK	BOOL	Input	Alarm acknowledge input
RTN_ACK_REQD	BOOL	Input	Return-to-normal condition requires acknowledgment if TRUE
OPLL	REAL	Input	Very low limit of process value
OPLO	REAL	Input	Low limit of process value
OPHI	REAL	Input	High limit of process value
OPHH	REAL	Input	Very high limit of process value
HYST	REAL	Input	Hysteresis for alarm reset
OP	WORD	Output	Final raw value to be written to DAC module
OP_LL_ALM	BOOL	Output	Alarm: TRUE if AO_IN ≤ OPLL
OP_LO_ALM	BOOL	Output	Alarm: TRUE if AO_IN ≤ OPLO
OP_HI_ALM	BOOL	Output	Alarm: TRUE if AO_IN ≥ OPHI
OP_HH_ALM	BOOL	Output	Alarm: TRUE if AO_IN ≥ OPHH
Acknowledged	BOOL	Output	Becomes TRUE after alarm is acknowledged

Functional Description

Input Scaling Logic

The AO block linearly maps AO_IN from the engineering range [IN_LO … IN_HI] to a corresponding raw value between [OP_LO … OP_HI].

Alarm Handling

The AO block monitors AO_IN for the following alarm conditions:

Condition	Alarm Bit
AO_IN <= OPLL	OP_LL_ALM = TRUE
AO_IN <= OPLO	OP_LO_ALM = TRUE
AO_IN >= OPHI	OP_HI_ALM = TRUE
AO_IN >= OPHH	OP_HH_ALM = TRUE

Example:

If OPHH = 100.0 and HYST = 2.0, then:

• Alarm triggers when AO_IN >= 100.0
• Alarm resets when AO_IN < 98.0

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Alarm Acknowledgment Logic

• When any alarm becomes active, Acknowledged = FALSE
• Operator must set ACK = TRUE to acknowledge it
• If RTN_ACK_REQD = TRUE, the alarm remains latched even after AO_IN normalizes, and must be acknowledged manually

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Typical Application Example

Parameter	Example Value	Description
AO_IN	62.5	PID output value
IN_LO	0.0	Minimum setpoint (engineering unit)
IN_HI	100.0	Maximum setpoint (engineering unit)
OP_LO	800	Raw output for 0% (e.g., 4 mA)
OP_HI	4000	Raw output for 100% (e.g., 20 mA)
DIR	FALSE	Output action (Direct/Reverse)

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Advanced View

• Configure IN_LO, IN_HI, OP_LO, OP_HI
• Set alarm thresholds (OPLL, OPLO, etc.)
• Configure RTN_ACK_REQD behavior

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Summary

Feature	Description
Input to Output	Scales REAL input to WORD output using defined engineering and raw ranges
Direction	Selectable direct/reverse output
Alarm Generation	4 levels of process alarm detection
Hysteresis	Reduces flickering by buffering alarm limits
Acknowledgment	Manual ACK with auto/manual return-to-normal control
Faceplate Support	Structured views for Operator, Technician, Engineer

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Best Practices

• Always scale IN_LO/HI and OP_LO/HI based on actual signal range.
• Keep HYST small but effective (e.g., 1–5% of signal range).
• Display OP and alarm bits on HMI faceplate.
• Use consistent logic for ACK and RTN_ACK_REQD across all analog signals.

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Notes

• For current output use (4–20mA), scale OP_LO = 800, OP_HI = 4000
• Ensure AO_IN is bounded within [IN_LO, IN_HI] to avoid incorrect scaling