FieldServer FS-8700-73 Appendix B. Advanced Topics Appendix B.1. BACnet Object Names

FS-8700-73 BACnet MSTP Driver Manual Page 18 of 24

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
	Name of Data Array where the Priority Array Table will	Up	to	16
DA_Pri_Array	be stored. Location 0 is the Relinquish Default value	alphanumeric
	and locations 1 to 16 the different entries of the Priority	characters
	Array Table.
DA_Pri_Array_Offset	Starting location in Data Array.	1-65535 (17)
	Name of Data Array that indicates if a particular
	Priority Values is in use. Location 0 indicate if the	Up	to	16
DA_Pri_In_Use	Relinquish Default has been set and locations 1 to 16	alphanumeric
	indicate if the index is in use (1), or if it has been	characters
	Relinquished (0).
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.

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