Echelon IzoT BACnet Developes - Manual (Page 11)

This is best understood by examining a L ON W ORKS nvi (in Figure 2), in particular
nviSpaceTemp, based on the L ON M ARK VAV Functional Profile. In the VAV Functional
Profile specification, this input can receive data either from a physical sensor, or from
another L ON W ORKS device. The rules for choosing which value to take are laid out by
L ON M ARK and they say that if there is a valid value from another L ON W ORKS device, this
will override the physical input. If the validity of the value expires for any reason,
usually due to the failure to refresh the value in a timely basis, the Functional Block
reverts to using the physical input.
The BACnet Interface extends this model: If there is valid BACnet data available, then
this overrides both the physical input as well as the input from the other L ON M ARK
device.
BACnet has a mechanism for “Commandable Objects”, such as Analog Outputs, that
makes this process seamless; the “Priority Array” and “Relinquish Default”.
The VBS intercepts all LonTalk writes and diverts these to the Relinquish Default. Thus,
if no other BACnet Priority has been written, this value flows through to the NV in the
Function Block.
If a BACnet write occurs at a given priority, then this value is forwarded to the nvi.
However a write to a lower priority is blocked. A write of a NULL value clears the
position in the priority array allowing lower priorities to flow through.
If all the BACnet writes are “Relinquished” (A NULL value is written at all the
appropriate BACnet Priorities), then the Relinquish Default, and hence the nvo from the
external L ON W ORKS device, is used again. This means, after a predetermined time with
no further writes by another BACnet Device, the Priority Array will clear and the system
will revert to using the previous source.
BACnet statuses such as “Out Of Service” and “Override” and “Fault” are treated as follows:
Out of Service This is a BACnet-only concept. It allows a BACnet Point to be
‘disconnected’ from the live data, in this case the L ON W ORKS
Network Variable, for testing and diagnostic purposes. It is fully
functional within the VBS but does not impact the functioning of
L ON W ORKS .
Override Override is a flag that indicates that the BACnet Value being
reported is no longer a true reflection of the physical value. It is
not appropriate for this model, and is an optional BACnet
Property and so is not included here.
Fault A fault indicates some sort of problem with that value or
measurement. This condition can be detected by the VBS and is
passed on to the BACnet Client to indicate the condition in a
logical manner.
Note that BACnet allows the Present Value to be read back from a BACnet Output. This
means that a matching BACnet Input is not required for reading back the true value of
any BACnet output. This is significant because it means that the BACnet Outputs in the
VBS allow the BACnet Client to “observe” the Network Variable values set by the
physical sensors or other LonWorks devices at all times, without having to write
anything, and without having to create a ‘shadow’ BACnet input specifically for this
purpose.
Using BACnet and LONWORKS with the FT 6000 EVK 11