BACnet - Commandable versus Writable

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In BACnet the Analog-Value object contains a Present Value property.

In the BACnet spec, the Present Value property for the Analog-Value has the following footnote:

“If Present_Value is commandable, then it is required to be writable. This property is required to be writable when Out_Of_Service is TRUE.”

There are two descriptions of the Present Value contained in this footnote.

1) The Present Value can be commandable.  This means that when writing to the Present Value property, a user assigns a priority to that value.  If the Present Value is commandable, then two other properties must be present, Priority Array and Relinquish Default.

Note: For more information about a property being commandable, refer to section 19.2 Command Prioritization in the BACnet spec.

2) The present Value can be writable.   This means that a user can write to the Present Value property.  In the case of the footnote above, if the Out of Service property is set to true, then the Present Value must be writable.

Commandable and Writable are two different aspects of the Present Value of the Analog Value.  What this means is that the Present Value property of the Analog-Value can be either Commandable and Writable (which requires the use of the Priority Array and Relinquish Default properties), Writable ( a user can write to it any time), or only Writable when the Out Of Service property is true.

Quotes were taken from the ANSI/ASHRAE Standard 135-2008 for BACnet

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