BACnet Multi-State Variables: Unveiling State Zero
In BACnet, the Present_Value property of a Multi-State object is not permitted to have a value of zero. This requirement is explicitly defined in the BACnet specification and applies to all Multi-State Inputs, Outputs, and Values.
For example, in Section 12.20.4 (Present_Value) of the BACnet 2008 specification, the standard states:
βThe Present_Value property shall always have a value greater than zero.β
Why State Zero Is Sometimes Observed
In practice, engineers may still observe a Present_Value of zero during system startup or commissioning. This typically occurs in gateways or protocol converters that initialize internal data buffers to zero at boot time.
Until valid data is received from the non-BACnet source protocol (for example, Modbus, proprietary serial protocols, or fieldbus devices), the gateway may temporarily expose this invalid value to BACnet clients.
Once the gateway receives valid upstream data, the Present_Value is updated to a legal BACnet value greater than zero, resolving the condition.
Engineering and Integration Considerations
When integrating BACnet Multi-State objects through gateways or software stacks, it is important to account for startup behavior. Strategies such as initializing default states to a valid non-zero value, delaying exposure of objects until data is valid, or clearly documenting startup behavior can help avoid confusion during commissioning.
Monitoring systems and BACnet clients should also be designed to tolerate brief startup anomalies, especially in systems where data is sourced from external, non-BACnet devices.
FAQ: BACnet Multi-State Variables and State Zero
Can a BACnet Multi-State object have a Present_Value of zero?
No. The BACnet specification explicitly requires that the Present_Value of a Multi-State object must always be greater than zero.
Where is this requirement defined in the BACnet specification?
The requirement is defined in Section 12.20.4 (Present_Value) of the BACnet specification, including the 2008 edition and later revisions.
Why do I sometimes see a value of zero at startup?
Some gateways and devices initialize internal data buffers to zero at boot. Until valid data is received from the non-BACnet source, this invalid value may be temporarily exposed.
Is a Present_Value of zero considered a protocol violation?
Yes, strictly speaking it is not a valid BACnet value for a Multi-State object. However, it is commonly seen as a transient startup condition in gateways.
How can integrators avoid this issue?
Integrators can initialize Multi-State values to a valid default state, delay object exposure until data is valid, or document expected startup behavior for commissioning teams.