Channel

A Channel lets one write reach many objects. A client writes a value to the Channel, and the device passes it on to a list of properties in other objects. It also takes part in group writes, where one message sets many channels at once.

Typical uses

• Set a whole floor of lights with one write
• Send one setpoint to several zone controllers
• Join lighting outputs into a scene
• Respond to a group write from a lighting panel

Key properties

Property	What it holds
Present_Value	The last value written to the channel. It starts out NULL.
List_Of_Object_Property_References	The properties that receive each value written to the channel.
Channel_Number	The number a group write uses to find this channel.
Control_Groups	The control groups this channel belongs to. A group write is matched against them.
Last_Priority	The priority of the most recent write.
Write_Status	Shows whether the last write to the members is idle, in progress, successful or failed.

How it behaves

A Channel is written with priorities like a commandable object, but its writes are copied to the members rather than driving hardware. With intrinsic reporting it applies the NONE algorithm, so it reports faults only.

With the CAS BACnet Stack

Enable it with the STACK_OPTION_OBJECT_TYPE_CHANNEL build option. Call BACnetStack_AddObject first, then BACnetStack_AddChannelObject with the datatype of Present_Value. The second call does not check that the object exists. Present_Value is read from the priority slots through the callback you register with BACnetStack_RegisterCallbackGetPropertyBool and the typed getter. Writes arrive at the typed setter or the BACnetStack_RegisterCallbackSetPropertyNull callback. A group write reads Channel_Number and Control_Groups through the BACnetStack_RegisterCallbackGetPropertyUnsignedInteger callback, so serve both or the channel never matches. Pass 97 as the datatype for a lighting command. Give the channel its members with BACnetStack_AddObjectPropertyReferenceToChannel, one call per referenced property; a channel with no members passes nothing on. A member in another device makes the stack bind that device’s address: it sends a Who-Is for the device at once and then on every heartbeat, 3600 seconds by default, while the reference exists. The stack keeps Write_Status and does not ask a callback for it. While Write_Status is in progress, it answers a WriteProperty or WritePropertyMultiple of Present_Value with OBJECT / BUSY itself; a member in another device can hold it there for APDU_Timeout times (Number_Of_APDU_Retries + 1). While Write_Status is failed, the stack also serves Reliability with the cause, if you enabled that property; the rest of the time your Reliability is served.

Related object types

Group, Global Group, Lighting Output, Binary Lighting Output

Properties

The properties ASHRAE 135-2024 clause 12.53 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.
Description	CharacterString	O	no	no	""
Present_Value	BACnetChannelValue	W	yes	yes	Built by the stack from the priority slots your callbacks serve; Null while every slot is empty. A lighting-command or object-identifier channel reads your plain Get callback instead.
Last_Priority	Unsigned	R	yes	no	0
Write_Status	BACnetWriteStatus	R	yes	no	Held by the stack.
Status_Flags	BACnetStatusFlags	R	yes	no	Held by the stack.
Reliability	BACnetReliability	O	no	no	0
Out_Of_Service	Boolean	R	yes	no	false
List_Of_Object_Property_References	BACnetARRAY[N] of BACnetDeviceObjectPropertyReference	W1	yes	yes	Held by the stack.
Execution_Delay	BACnetARRAY[N] of Unsigned	O1	no	no	Not presented: a read answers unknown-property.
Allow_Group_Delay_Inhibit	Boolean	O	no	no	Not presented: a read answers unknown-property.
Channel_Number	Unsigned16	W	yes	yes	0
Control_Groups	BACnetARRAY[N] of Unsigned32	W	yes	yes	0
Event_Detection_Enable	Boolean	O2,3	no	no	false
Notification_Class	Unsigned	O2,3	no	no	0
Event_Enable	BACnetEventTransitionBits	O2,3	no	no	An empty bit string.
Event_State	BACnetEventState	O2,3	no	no	0
Acked_Transitions	BACnetEventTransitionBits	O2,3	no	no	An empty bit string.
Notify_Type	BACnetNotifyType	O2,3	no	no	0
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	O2,3	no	no	No default: the read fails.
Event_Message_Texts	BACnetARRAY[3] of CharacterString	O3	no	no	""
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	O3	no	no	""
Reliability_Evaluation_Inhibit	Boolean	O4	no	no	false
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	R	yes	no	Held by the stack.
Value_Source	BACnetValueSource	O5,6,7	no	no	No default: the read fails.
Audit_Level	BACnetAuditLevel	O8	not in profile	not in profile	No default: the read fails.
Auditable_Operations	BACnetAuditOperationFlags	O8	not in profile	not in profile	No default: the read fails.
Audit_Priority_Filter	BACnetOptionalPriorityFilter	O8	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	""