Supervisory Controller as a BACnet Application

When we initially released the S4 Open: OPC-N2 Router we knew from our market research that one of the important features of the product was the Upstream N2 Interface. This is the mechanism that we use to allow a Metasys supervisory controller, of any generation as long as it supports N2, to co-exist with your new head end system. The marketplace quickly embraced this technology and validated that our assumptions were correct. When we developed the S4 Open: BACnet-N2 Router this feature was included in it as well. The Upstream N2 Interface is completely transparent to the Metasys supervisory controller and comes standard with the full size (-256) models. It is an extra cost option on each of the smaller models.


As part of our BACnet integration we needed to emulate the BACnet Priority Array and Relinquish Default functionality of BACnet because the Metasys N2 protocol does not have an equivalent feature set. As each N2 device is discovered by our Configure Wizard the associated Priority Array and Relinquish Default attributes are created for all point types allowed by the BACnet standard. The Upstream N2 Interface was designed to operate in a passthrough mode to maintain complete transparency to the Metasys supervisory controller. Logically, this meant that the supervisory controller was operating at a priority higher than the highest entry provided by the BACnet Priority Array.

As our integration partners realized the value in the BACnet-N2 Router they started taking on more complex and much larger integration projects. As a result it became a best practice for all large integration projects to install the Upstream N2 Interface. This can be done either as a short term transition approach or as a long term co-existence strategy, as determined by each project. One of the biggest benefits of utilizing the Upstream N2 Interface is that the interruption to a building is minimized while the new head end services are being developed and tested.

Then, something surprising happened. Our integration partner who is working on the largest Metasys integration project to date came to us and requested that we enhance the BACnet-N2 Router Upstream Interface so that transactions coming from the Metasys supervisory controller participate as a peer to BACnet applications and clients. This means full participation in the Priority Array mechanism rather than transactions transparently being passed through to the N2 field devices. This was a very large and risky development task but we understood the importance of the request. This enhancement is completed and is now a feature of the production S4 Open: BACnet-N2 Router product.

The options that this enhancement provides are:

  • Supervisory Controller Transparent – Backwards compatibility with current release of the BACnet-N2 Router. Logically, it is operating at a priority higher than any BACnet transaction.
  • Supervisory Controller as a BACnet Client - transactions operate at any of the priority levels defined by the BACnet Priority Array mechanism. This is a system wide user definable parameter.
  • Supervisory Controller Read Only Mode – The Metasys supervisory controller is allowed read access to all N2 points. However write and override transactions are intercepted by the BACnet-N2 Router and not passed on to the N2 device. Logically, transactions from the supervisory controller are treated as if they have a lower priority than the lowest priorty defined by the BACnet priority array mechanism.

This enhancement is the first example of us going beyond the gateway functionality implemented in the original BACnet-N2 Router, encapsulating the Metasys system, and having it work as a full peer to the BACnet environment. Look forward to future case studies describing in more detail how this feature was utilized.