BACnet PTP Serial Driver Manual
Page 23 of 34
Appendix A.9. 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, an 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:
o
Function, e.g. Rdbc, Arcs, Wrbc, Wrbx
o
Scan_interval
o
Timeout
o
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:
o
Linked_Map_Descriptor
– this is a reference by name to the active Map Descriptor described
above, which controls the transaction
o
Function
– Passive_Client
o
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.
