FieldServer FS-8704-06 - Manual (Page 34)

FS-8704-06 BACnet IP Driver Manual Page 34 of 57
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Appendix A.10. Specifying ReadPropertyMultiple and WritePropertyMultiple Transactions using Linked
Map Descriptors
The ReadPropertyMultiple (RPM) and WritePropertyMultiple (WPM) BACnet services allow many objects and
attributes to be read and written in a single transaction. Since FieldServer Map Descriptors can only refer to a
single object type and address range, a RPM or WPM transaction can be constructed by linking multiple Map
Descriptors. This is done using the following components:

an active read or write Map Descriptor that defines the behavior of the RPM or WPM transaction via:

Function, e.g. Rdbc, Arcs, Wrbc, Wrbx

Scan_interval

Timeout

Length

any number of Linked Map Descriptors specifying additional objects and data array locations to be included in
the transaction. Each Linked Map Descriptor is specified with the following settings:

Linked_Map_Descriptor - this is a reference by name to the active Map Descriptor described above,
which controls the transaction

Function - Passive_Client

Length
The number of Linked Map Descriptors is limited by the maximum message length allowed for the BACnet driver in
question. When too many Map Descriptors have been linked a SEGMENTATION_NOT_SUPPORTED error message
will be generated on the first poll attempt. This message will recommend splitting the RPM or WPM transaction
into multiple transactions, e.g.
DRV->BACnet : Linked Map Desc. "CMD_WPM" is too long.
Message Segmentation not supported.
Please split the transaction into multiple Linked Map Descriptors.