System Description
The following discussion centers on an open control system architecture (see diagram) in which components from several vendors
interoperate over a BACnet-adapted Ethernet LAN. Subsidiary control buses both isolate and unite multi-vendor products, using either
LonMark or BACnet standards.
User Workstations
The BACnet specification defines how data is formatted and how certain building management functions (alarm limits, scheduling, etc.) are
handled by the user workstation. Multiple workstations with common operating characteristics provide convenient central interfaces. The
benefits of a consistent operator interface to the system include reduced operator training, better utilization of the systems by building staff,
lower operations costs, coordination of energy users within a facility and a more informative control system, to name just a few.
Routers
The term "router" is relatively new to building control systems but quite familiar to information networks. It may be a passive (extending the
distance limitations of a communications line) or play a more active communications role by routing information to requesters or by limiting
traffic on bus segments. Routers usually are transparent to the operator.
Three routers are shown in the diagram. The Ethernet/MSTP router (EM) Separates the high-speed LAN from the RS485 MSTP bus. Both
sides of the bus are defined in the ASHRAE BACnet specification. This allows for BACnet operator interface devices to interface with low
cost BACnet MSTP devices.
The Ethernet/LonMark router (EL) separates LonMark compatible devices from the Ethernet/ BACnet LAN. LonMark is a communications
standard defined by and used by many control industries - building control, industrial control and transpiration control to name a few. The
LonMark standard utilizes the LonTalk communication protocol across a Local Operating Network (LON). The router allows BACnet operator
workstations to interface to LonMark devices from man industries, providing a great deal of flexibility in integration.
The custom-designed Ethernet/Proprietary router (EP) allows proprietary devices to be connected the BACnet LAN. It may interface with
prestandard (e.g., earlier generation) digital control systems or with third-party non-DDC prestandard devices. Although development costs
may be significant, it is quite likely that routers for popular proprietary applications will be available from multiple sources. Over time, EP
routers will no longer be needed as vendors convert to open standards. They do, however, support openness during this period of transition by
providing a gateway from existing propriety system and devices to the open world afforded by LonMark and BACnet technologies.
Controllers
The BACnet/Ethernet controller (BC), also referred to as a BACnet native controller, is very similar to digital controllers available throughout
the '80s and '90s, but with the addition of BACnet/Ethernet compatibility. The software and firmware must comply with the message and
communications provisions of the BACnet specification, and be compatible with the defined BACnet user workstation features. These
provisions represent the minimum feature set required to be BACnet compatible.
The BACnet/MSTP controller (MC) is of a lower communications order (non "peer to peer") but is otherwise similar to the BC. It may be fully
programmable or application-specific.
The LonMark controller (LC) is similar to the two above controllers except that standard LonTalk communication is performed with a Neuron
chip. This communications LON (local operating network) utilizes a full peer-to-peer (no master) protocol similar to that of the Ethernet LAN.
One of the key advantages to LonMark Technology is that the communications software is embedded in the Neuron chip that, complete with
micro-processor and control memory, costs less that $5.00.
Proprietary digital controllers and devices (XDC) from both the traditional control system sources as well as new and emerging sources such
as power monitoring, motor control, variable-speed drive, lighting, etc. feature non-standard communications protocols. These may connect to
the EP router, or, in some cases, connect directly to a user workstation over the proprietary bus. This is an excellent avenue for obtaining the
benefit of the facility-wide integration. It can also provide a first step toward "open" for building owners with control systems who want to start
enjoying the benefits of an open system, without replacing an existing one.
In many cases, controllers will also contain a number of vendor-specific features to enhance system operation and performance.
Tools
For a multi-vendor system such as this, unique tools must be provided for each vendor's controllers In most cases, a standard notebook
computer can be used with vendor-specific software and cable connectors for programming, DDC loop tuning and parametric adjustments.
Common network management tools are available for the Ethernet and LON environments and are expected soon for BACnet environments.
