Loop

A Loop object represents a control loop, usually PID, in a controller. It exposes the loop’s output, the controlled variable, the setpoint and the tuning gains, so a client can watch and tune the loop. A controller exposes one per loop that operators need to see.

Typical uses

• Tune the gains of a temperature control loop
• Show setpoint, process value and output on one screen
• Alarm when the process drifts from the setpoint
• Switch a loop between automatic and manual

Key properties

Property	What it holds
Present_Value	The loop’s current output.
Controlled_Variable_Value	The measured value the loop is controlling.
Setpoint	The target for the controlled variable.
Proportional_Constant	The proportional gain. Integral and derivative gains have their own properties.
Manipulated_Variable_Reference	The property the loop writes its output to.
Action	Whether the loop acts directly or in reverse.

How it behaves

The loop algorithm itself is a local matter in the device. The standard pairs the Loop with the FLOATING_LIMIT algorithm, which alarms when the controlled variable drifts too far from the setpoint. It sends change-of-value notices when Present_Value moves by COV_Increment.

With the CAS BACnet Stack

Enable it with the STACK_OPTION_OBJECT_TYPE_LOOP build option and add each object with BACnetStack_AddObject. Serve Present_Value with the callback you register with BACnetStack_RegisterCallbackGetPropertyReal and accept writes with the BACnetStack_RegisterCallbackSetPropertyReal callback; Object_Name uses the BACnetStack_RegisterCallbackGetPropertyCharacterString callback. The stack does not run the PID algorithm; your application does. Arm the drift alarm with BACnetStack_SetAlarmsAndEventsForObjectEnabled and then BACnetStack_SetIntrinsicFloatingLimitAlgorithm, which takes the setpoint and separate low and high difference limits.

Related object types

Analog Input, Analog Output, Analog Value

Properties

The properties ASHRAE 135-2024 clause 12.17 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	R7	yes	no (yes while Out_Of_Service is true)	No default: value-not-initialized.
Description	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	O7	no	no (yes while Out_Of_Service is true)	0
Out_Of_Service	Boolean	R	yes	no	false
Update_Interval	Unsigned	O	no	no	0
Output_Units	BACnetEngineeringUnits	R	yes	no	0
Manipulated_Variable_Reference	BACnetObjectPropertyReference	R	yes	no	No default: the read fails.
Controlled_Variable_Reference	BACnetObjectPropertyReference	R	yes	no	No default: the read fails.
Controlled_Variable_Value	Real	R	yes	no	0.0
Controlled_Variable_Units	BACnetEngineeringUnits	R	yes	no	0
Setpoint_Reference	BACnetSetpointReference	R	yes	no	No default: the read fails.
Setpoint	Real	R	yes	no	0.0
Action	BACnetAction	R	yes	no	0
Proportional_Constant	Real	O1	no	no	0.0
Proportional_Constant_Units	BACnetEngineeringUnits	O1	no	no	0
Integral_Constant	Real	O2	no	no	0.0
Integral_Constant_Units	BACnetEngineeringUnits	O2	no	no	0
Derivative_Constant	Real	O3	no	no	0.0
Derivative_Constant_Units	BACnetEngineeringUnits	O3	no	no	0
Bias	Real	O	no	no	0.0
Maximum_Output	Real	O	no	no	0.0
Minimum_Output	Real	O	no	no	0.0
Priority_For_Writing	Unsigned(1..16)	R	yes	no	0
COV_Increment	Real	O4	no	no	0.0
Time_Delay	Unsigned	O5,8	no	no	0
Notification_Class	Unsigned	O5,8	no	no	0
Error_Limit	Real	O5,8	no	no	0.0
Deadband	Real	O5,8	no	no	0.0
Event_Enable	BACnetEventTransitionBits	O5,8	no	no	An empty bit string.
Acked_Transitions	BACnetEventTransitionBits	O5,8	no	no	An empty bit string.
Notify_Type	BACnetNotifyType	O5,8	no	no	0
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	O5,8	no	no	No default: the read fails.
Event_Message_Texts	BACnetARRAY[3] of CharacterString	O8	no	no	""
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	O8	no	no	""
Event_Detection_Enable	Boolean	O5,8	no	no	false
Event_Algorithm_Inhibit_Ref	BACnetObjectPropertyReference	O8	no	no	No default: the read fails.
Event_Algorithm_Inhibit	Boolean	O8,9	no	no	false
Time_Delay_Normal	Unsigned	O8	no	no	0
Reliability_Evaluation_Inhibit	Boolean	O10	no	no	false
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	R	yes	no	Held by the stack.
Low_Diff_Limit	BACnetOptionalReal	O	no	no	No default: the read fails.
Audit_Level	BACnetAuditLevel	O11	not in profile	not in profile	No default: the read fails.
Auditable_Operations	BACnetAuditOperationFlags	O11	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	""