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BAS Physical Architecture
Why Is It Important To The Design?
October Issue - BAS Column
last two columns dealt with point selection and sequence writing.
These determine a majority of a BAS project’s scope of work.
However, there is one other issue necessary to the core of the design:
the system architecture. In this case, I am referring to the
“Physical Architecture” which deals mainly with the layout/quantity of
the controllers of various types along with the wiring topology &
transport technology used to connect them. (In my June ’18 column
I covered BAS “Application Architecture” which is a different issue
typically only important to integration challenges.) Why is it
important to define the Physical Architecture (or simply “Architecture”
in this column) in the design?
IP vs. MS/TP – The two major communications wiring & transport technologies used in BAS is IP and various forms of EIA-485 (mainly BACnet’s MS/TP). EIA-485 is a very old and slow technology that, because of its use in MS/TP, is still the dominant choice for lower-end controllers (e.g., VAV boxes). IP is the dominant choice for higher-level controllers (i.e., BACnet “B-BC’s”) and necessary for connecting the system to the operator interface and internet remote access. IP is much faster and more robust than MS/TP, allows the controller to communicate directly with the operator interface (i.e., its communications is not routed through a B-BC controller), and can also allow for BAS networking via the building’s IP infrastructure (a cost-effective approach if allowed by the building owner).
Many believe that critical facilities should maximize the use of IP over MS/TP (even at the VAV box level), though how much this actually improves BAS reliability is still a subject of debate. For most current BAS choices the IP vs. MS/TP choice mainly applies to middle-level controllers (e.g., BACnet “B-AAC’s”) and this is where I’d recommend IP communications be used in all but low-criticality projects. However, there’s more to assuring reliable communications and system operations than the use of IP.
Controller Quantities and Segregation – To minimize the effect from a loss of a controller the following are some guidelines to follow in further developing the architecture:
Fault Immunity & Redundancy – Mission-critical facilities should consider further steps like:
There are more architecture issues to consider that are covered in a
typical building project. The need to consider these issues
depends upon the criticality of the building’s operations. Either
way, the architecture details can be (mostly) documented in the project
design using a detailed architecture drawing.
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