Tag Number

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There are two types of Tags, and the Tag Number usage varies with each.

  • Application Tags
    • These have fixed type - boolean, int, date etc.
  • Context Specific Tags
    • The types of these tags depend on where they are found, in the variable part of the APDU. Their type cannot be determined by inspection alone.
    • They have an associated context number; this context number specifies the meaning of the data.
    • Context Tags also allow Constructed Tags (lists) to be created.
    • See section 20.2.1, pg 377 of the specification

Another resource is | BAC Cave, from BACnet Bill]



BACnet Tags
Octets Meaning
7 6 5 4 3 2 1 0
1 Tag Number Class Length/Value/Type
n Variable amount of data


Class = 0 for Application Tags (will indicate the Type of tag)

Class = 1 for Context Specific Tags (Will indicate the sequence of the tag)








Keyword List
APDU BBMD BVLC BVLL DADR DNET Encoding I-Am LSAP MAC NPCI NPDU NSAP NSDU SA SADR SNET

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