Analog Output

An Analog Output object models a controlled analog signal sent to equipment, such as a valve position or a fan speed. A client writes the value at a priority, and the device drives the output. Several writers can share the output, with the highest priority winning.

Typical uses

• A chilled water valve position in percent
• A variable frequency drive speed command
• A damper actuator position
• A dimmer level sent to an analog lighting driver

Key properties

Property	What it holds
Present_Value	The value the output is driving, a real number. It is the result of the priority array.
Priority_Array	Sixteen slots, one for each write priority. The lowest-numbered occupied slot wins.
Relinquish_Default	The value used when every slot in the priority array is empty.
Units	The engineering unit of the output.
COV_Increment	The smallest change that triggers a change-of-value notification.

How it behaves

Present_Value is commandable. A client writes at a priority and later writes NULL to give that priority up. The standard pairs the type with the OUT_OF_RANGE algorithm for intrinsic reporting.

With the CAS BACnet Stack

Enable it with the STACK_OPTION_OBJECT_TYPE_ANALOG_OUTPUT build option and add each object with BACnetStack_AddObject. The stack always treats it as commandable. It reads the priority array through the callback you register with BACnetStack_RegisterCallbackGetPropertyBool (is the slot empty) and the BACnetStack_RegisterCallbackGetPropertyReal callback (the slot value), and writes through the BACnetStack_RegisterCallbackSetPropertyReal callback. It never calls the plain Present_Value getter. Arm alarms with BACnetStack_SetIntrinsicOutOfRangeAlgorithm.

Related object types

Analog Input, Analog Value, Binary Output

Properties

The properties ASHRAE 135-2024 clause 12.3 defines for this object type.

How to read the table:

• Conformance is what the standard asks of every object of this type: R is required, W is required and writable by a client, O is optional. A digit after the letter marks a condition in the standard that decides when the property is present or writable, such as support for intrinsic reporting.
• Present by default: yes means the stack presents the property as soon as you add the object; no means it is optional and you turn it on with BACnetStack_SetPropertyEnabled. This is the stack’s starting setup, so it can differ from the conformance code.
• Writable is whether a client can write the property by default. Change it with BACnetStack_SetPropertyWritable.
• not in profile means the stack does not offer this property for this object type.
• Value when your callback does not answer: “Held by the stack” means the stack stores and serves the value itself and does not ask your callbacks. Otherwise it is the value the stack answers when your callback declines or is not registered.

Property	Datatype	Conformance	Present by default	Writable	Value when your callback does not answer
Object_Identifier	BACnetObjectIdentifier	R	yes	no	Held by the stack.
Object_Name	CharacterString	R	yes	no	"Object <type>,<instance>", such as "Object 0,1" for Analog Input 1.
Object_Type	BACnetObjectType	R	yes	no	Held by the stack.
Present_Value	Real	W	yes	yes	Built by the stack from the priority array your callbacks serve, or Relinquish_Default when every slot is empty.
Description	CharacterString	O	no	no	""
Device_Type	CharacterString	O	no	no	""
Status_Flags	BACnetStatusFlags	R	yes	no	Built by the stack from your callbacks.
Event_State	BACnetEventState	R	yes	no	0
Reliability	BACnetReliability	O	no	no	0
Out_Of_Service	Boolean	R	yes	no	false
Units	BACnetEngineeringUnits	R	yes	no	95 (no units)
Min_Pres_Value	Real	O	no	no	0.0
Max_Pres_Value	Real	O	no	no	0.0
Resolution	Real	O	no	no	0.0
Priority_Array	BACnetARRAY[16] of BACnetOptionalReal	R	yes	no	Built by the stack from your callbacks.
Relinquish_Default	Real	R	yes	no	No default: value-not-initialized until a callback or a client write supplies one.
COV_Increment	Real	O1	no	no	0.0
Time_Delay	Unsigned	O2,4	no	no	0
Notification_Class	Unsigned	O2,4	no	no	0
High_Limit	Real	O2,4	no	no	0.0
Low_Limit	Real	O2,4	no	no	0.0
Deadband	Real	O2,4	no	no	0.0
Limit_Enable	BACnetLimitEnable	O2,4	no	no	An empty bit string.
Event_Enable	BACnetEventTransitionBits	O2,4	no	no	An empty bit string.
Acked_Transitions	BACnetEventTransitionBits	O2,4	no	no	An empty bit string.
Notify_Type	BACnetNotifyType	O2,4	no	no	0
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	O2,4	no	no	No default: the read fails.
Event_Message_Texts	BACnetARRAY[3] of CharacterString	O4	no	no	""
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	O4	no	no	""
Event_Detection_Enable	Boolean	O2,4	no	no	false
Event_Algorithm_Inhibit_Ref	BACnetObjectPropertyReference	O4	no	no	No default: the read fails.
Event_Algorithm_Inhibit	Boolean	O4,5	no	no	false
Time_Delay_Normal	Unsigned	O4	no	no	0
Reliability_Evaluation_Inhibit	Boolean	O6	no	no	false
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	R	yes	no	Held by the stack.
Interface_Value	BACnetOptionalReal	O	no	no	Null
Current_Command_Priority	BACnetOptionalUnsigned	R	yes	no	Built by the stack from your callbacks.
Value_Source	BACnetValueSource	O7,9,11	no	no	No default: the read fails.
Value_Source_Array	BACnetARRAY[16] of BACnetValueSource	O8,10	no	no	No default: the read fails.
Last_Command_Time	BACnetTimeStamp	O8,10	no	no	No default: the read fails.
Command_Time_Array	BACnetARRAY[16] of BACnetTimeStamp	O10	no	no	No default: the read fails.
Audit_Level	BACnetAuditLevel	O12	not in profile	not in profile	No default: the read fails.
Auditable_Operations	BACnetAuditOperationFlags	O12	not in profile	not in profile	No default: the read fails.
Audit_Priority_Filter	BACnetOptionalPriorityFilter	O12	not in profile	not in profile	No default: the read fails.
Tags	BACnetARRAY[N] of BACnetNameValue	O	no	no	No default: the read fails.
Profile_Location	CharacterString	O	no	no	""
Profile_Name	CharacterString	O	no	no	""