BACnet ReadProperty Service Results

The BACnet^® protocol includes the commonly used service ReadProperty, which is used to request data from BACnet devices like the WattNode^® meter. BACnet also includes ReadPropertyMultiple. There is a bit of confusion about what ReadProperty can return. It is allowed to return the following:

Primitive Data (see ANSI/ASHRAE 135 section 20.2.1.3.1)
Constructed Data (see ANSI/ASHRAE 135 section 20.2.1.3.2)

Primitive and Constructed data include the following:

Boolean
Integer (signed and unsigned)
Real or float
Octet (byte) string
Character string
Bit string
Enumerated
Date
Time
BACnetObjectIdentifier
BACnetARRAY (as for Device:Object_List or Multi-state Value:State_Text)

The main limitation with ReadProperty is that only a single property can be requested. However, if that property is an array property, you may request one element of the array (index = 1..n), the length of the array (index = 0) or the entire array (no index specified), so ReadProperty can return an entire array of results (assuming the entire array will fit in a single BACnet frame or segmentation is supported¹.

¹Note: The WattNode BACnet meter (MS/TP) does not currently support segmentation.

See Also