Echelon IzoT BACnet Developes - Manual (Page 10)

For example, in Figure 1 there is a nviSpaceTemp that is normally connected to some
other nvo output from another device. If a BACnet Client is required to supply this
parameter, then the BACnet Client will execute a write to the Analog Output
“AO:TempSetpoint” in the Virtual BACnet Server. Internally the BACnet Stack will map
this write to the nviSpaceTemp.
Similarly, if a BACnet Client wants to know the value of a L
ON
W
ORKS
output, it would
read the BACnet point mapped to that Network Variable output (nvo).
BACnet Read Operations
When the BACnet Client polls for data, it can request any BACnet Property contained
within the BACnet Object. Most of these properties are rather static (e.g. Object ID,
Object Name) and are seldom polled, sometimes only once, and these types of reads are
handled completely within the VBS.
Reads for live data normally are a BACnet read for the “Present Value” Property of the
BACnet Object, e.g. An Analog Input, Instance 1. This results in the VBS interface
accessing the appropriate “live” data in the L ON W ORKS Network Variable associated with
the BACnet Object.
Write Operation Resolution
BACnet Write operations are more complex within BACnet, and this is further
complicated by the fact that there is a conflict as to who is in control of the point being
written to.
It would be possible to have implemented a “last device to write wins” method. However,
using this approach makes it very difficult to diagnose problems in a system. Luckily,
BACnet already presents a solution, as outlined below.
FT 6000 based LonWorks Device
External LonWorks Device
Physical Sensor
LonWorks Write
Temp from External
Device
Virtual BACnet Server
Standard LonWorks Function Block
BACnet Write
From BACnet
Device
BACnet Read
nviSpaceTemp
AO: Space Temp
Figure 2. Write Operation Resolution
10 Using BACnet