November 2011

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Control Solutions, Inc. - Minnesota

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Turning a Metasys Supervisory Controller into a BACnet Client
 The supervisory controller as a BACnet client enhancement moves the BACnet-N2 Router beyond its original N2 gateway functionality, encapsulating the entire Metasys system.

Steve Jones
Steve Jones,

Managing Partner,
The S4 Group, Inc.

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A significant percentage of building automation systems (BAS) installations are based on Johnson Controls MetasysŪ legacy N2 technology. These systems were designed prior to the adoption of BACnet as the primary open standard for the building automation industry.

Originally, Metasys systems were on the leading edge. But as building environments age and become increasingly outdated, owners, managers, and their BAS support partners found that the S4Open: BACnet-N2 Router enabled a seamless upgrade to legacy systems, resulting in a cost-effective extension of their useful life, while adding new capabilities to them. With this enhanced BACnet-N2 Router, points on Metasys N2 field devices are published as standard BACnet data objects, which can be accessed by any BACnet client device or application.
The initial design of the BACnet-N2 Router allowed the legacy Metasys supervisory controller to co-exist with an upgraded BACnet-based, head-end system via our unique Upstream N2 interface, a virtual implementation of the N2 bus. No other Johnson Controls, or third-party integration product, had this capability. The Upstream N2 interface operated in Passthrough mode, providing transparent access for the supervisory controller--through the BACnet-N2 Router and its Downstream N2 interface--to N2 field devices on the physical N2 bus. This meant that supervisory controller transactions were independent of any BACnet operator workstation, controller, or application.

Providing this Passthrough operation for Metasys supervisory controller transactions served our integration partners well for the first few years of the BACnet-N2 Router deployment. However, as these integration companies took on larger and more complex projects, they defined the need for supervisory-controller-initiated transactions to participate in the BACnet priority array mechanism.

The priority array is defined by the BACnet standard to coordinate and prioritize access to data objects by BACnet entities. The model assumes that the entity utilizing the higher-priority array entry will control the value of the data object’s present value. As the entity relinquishes its entry in the priority array, the next higher one is given control of the present value data attribute.

Our partner suggesting this enhancement was integrating to a legacy Metasys system in a high-rise facility requiring the installation of 19 BACnet-N2 Routers. As they studied the existing installation, they found a high degree of complexity in the Metasys NCM supervisory controllers’ logic, as well as that of a smoke control application. Our partner requested that transactions coming from the NCM act as a peer to BACnet applications and clients, essentially providing full participation in the BACnet priority array mechanism, rather than being passed through transparently to the N2 field devices.

The supervisory controller as a BACnet client enhancement moves the BACnet-N2 Router beyond its original N2 gateway functionality, encapsulating the entire Metasys system. For example, a supervisory controller is capable of sending Read, Write, Override, and Release commands to N2 devices. Rather than routing those transactions transparently to the N2 device, the BACnet-N2 Router now inserts the BACnet priority array mechanism as a filter for the entire transaction, not just the simple data value. The command will only be passed to the N2 device if the supervisory controller’s status in the BACnet priority array for the target point indicates it is the highest-priority entity trying to manipulate the point.

When a Metasys supervisory controller-initiated command becomes the highest-priority entry in the array, it is sent to the N2 device for execution. (Note these transactions do not directly change the emulation of the BACnet present value for the point in the BACnet-N2 Router.) This N2 field device has the final say if the command is accepted and applied to the target point. Only then is the BACnet present value point attribute updated to reflect the results of the transaction. This guarantees our emulation of the BACnet present value for the point reflects the real-world condition of the N2 device.

Control Solutions, Inc Supervisory controller as a BACnet client functionality is configured by a global parameter in the BACnet-N2 router through our S4 Open Management Console. The selected configuration is applied to all transactions created from any generation Metasys N2-based supervisory controller. The option works equally well for the Facilitation, Companion, NCM, N30, NAE, or Facility Explorer supervisory controllers. Configuration options include:

•    Supervisory controller transparency: The BACnet-N2 Router and all associated functionality are transparent to the Metasys system. Supervisory controller transactions are sent to N2 field devices in Passthrough mode.

•    Supervisory Controller as a BACnet Client: N2 transactions initiated by the supervisory controller operate at any user-specified priority level defined by the BACnet priority array mechanism.

•    Supervisory Controller Pseudo Priority 17: N2 transactions initiated by the supervisory controller operate at a pseudo-BACnet priority level that is below those specified by BACnet. The net effect is that the supervisory controller transactions are executed only if there are no active transactions from actual BACnet operator workstations, controllers, or applications.

•    Supervisory Controller Read-only Mode: The Metasys system can have read-only access to all N2 points, facilitating ongoing monitoring of building operations, but prohibiting the supervisory controller from initiating any Write, Override, or Release transactions.

Without question, there are many solutions to integration projects of all scopes and sizes. The most effective way to integrate complex projects is using the S4 Open: BACnet-N2 Router to turn Metasys supervisory controllers into BACnet clients. This results in salvaging legacy Metasys systems, extending their useful life, and adding new value at minimal cost by being able to act as a peer to state-of-the-art BACnet products. In addition, this approach ensures business continuity during the upgrade process, avoids (or defers) construction and controller replacement costs, and facilitates an incremental and planned migration to new technologies and capabilities.

Learn more about the S4 Group here, by calling (801) 621-1970, or send an email to

About the Author

Before forming The S4 Group, Inc., Mr. Jones held multiple positions with the Johnson Controls, Inc. Controls Group. As Manager, Architecture and Planning for the IT Communications team, he played a key role in the design, implementation, operations, and management of the corporation-wide WAN. As a senior member of their System Integration Services group, he focused on developing enterprise solutions and integrating the MetasysŪ Building Automation System into customers’ IT environments, network infrastructure, and management platforms. He has extensive experience in communications systems design, development, implementation, and management, as well as in systems engineering and integration. The early years of Steve’s career were in the telecommunications industry with Bell Laboratories, Siemens Corporation, and TRT Telecommunications, developing industry-specific applications. More recent work was with system integration firms in the Washington, D.C., area, providing both federal and commercial customers with IT facilities management, network integration, management, and outsourcing services.


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