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FS-8700-73 BACnet MSTP Driver Manual Page 18 of 24
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Appendix B. Advanced Topics
Appendix B.1. BACnet Object Names
When an external BACnet Client builds a list of Object Names, the BACnet Server Map Descriptor
name determines the BACnet Object Name. If the Map Descriptor length is greater than 1, then the
Object Name will be suffixed with the index into the Map Descriptor. For example, if the Map
Descriptor name is SMD_AI_01 and the length 3, then the Object Names will be SMD_AI_01[0],
SMD_AI_01[1] and SMD_AI_01[2].
Appendix B.2. BACnet Priority Arrays
FieldServer implementation of BACnet priority Arrays
When BACnet Output objects are written to the Server side of the FieldServer, an associated write
priority is given to each write value. When the FieldServer receives the write value, it stores it to the
Map Descriptor Priority Array Table at the specified priority. The Priority Array Table is then scanned
and the value with the highest priority is stored to the Data Array location specified by the Map
Descriptor.
When a Write “Relinquished” command is received, the value is removed from the Priority Array
Table and the next highest value from the Priority Array Table is stored to the Data Array.
If all values have been “Relinquished” from the Priority Array Table, then the Map Descriptors
“Relinquish Default” value will be stored to the Data Array.
Accessing Priority Array information
The Priority Array table and its “In_Use” (or Not Relinquished) state are stored internally to every Map
Descriptor, and cannot be accessed directly. If the following data arrays are specified, however, they
will maintain an exact copy of the Priority Array Table for the Map Descriptor. Thus the Priority Array
Table can be accessed.
Section Title
Map_Descriptors
Column Title Function Legal Values
DA_Pri_Array
Name of Data Array where the Priority Array Table will
be stored. Location 0 is the Relinquish Default value
and locations 1 to 16 the different entries of the Priority
Array Table.
Up to 16
alphanumeric
characters
DA_Pri_Array_Offset Starting location in Data Array.
1-65535 (17)
DA_Pri_In_Use
Name of Data Array that indicates if a particular
Priority Values is in use. Location 0 indicate if the
Relinquish Default has been set and locations 1 to 16
indicate if the index is in use (1), or if it has been
Relinquished (0).
Up to 16
alphanumeric
characters
DA_Pri_In_Use_Offset
Starting location in Data Array.
1-65535 (17)
Appendix B.3. Limitations of the BACnet Read_Property_Multiple service.
The BACnet PTP client driver can read multiple BACnet objects using the Read_Property_Multiple
service under the following conditions:
• The number of objects to be read must not exceed 29.
• No gaps must exist in the range of object to be read.
• The Map Descriptor function must be of the Read type. (e.g. Rdbc)
• The Map Descriptor property must be Present Value.