How do I support legacy N2 devices in a NCU chassis?

In order for the S4 Open Appliances to function they need to be inserted between the Metasys supervisory controller and the N2 field devices. With the 1st generation Metasys chassis based Network Control Unit (NCU) this is not physically possible because all cards sit on the chassis backplane. This includes both NCM and Modular I/O cards. Considering the age of these systems, and the difficulty of repairing them, we are recommending that you approach these systems with a co-existance and phase out strategy. This way we you can move forward with an integration project with minimal investment and even less risk. Please refer to this diagram.

N2 Router in NCU configuration

In order to keep the picture as simple as possible we showed all network connections as Ethernet. If the NCU has ARCnet then it, and whatever Metasys workstation is associated with it, would reside on a separate ARCnet network. For this discussion that variation is not important.

Normally, when we do an integration the BACnet-N2 Router or OPC-N2 Router are inserted in between the Metasys supervisory controller (NCM) and the field devices with our Downstream N2 interface being connected to the field devices and the Upstream N2 interface being connected to a stand-alone NCM. The N2 Router assumes the role of the gatekeeper for access to the N2 bus and it field devices. With the NCU this is not possible because of the integral N2 bus on the backplane.

The Co-existance and phase out strategy involves the following steps

  1. Install the BACnet-N2 Router between the NCU chassis and the stand alone N2 field devices. The new BACNet head end will not have visibility to any of the N2 modular devices in the NCU chassis. The tradeoff is that you are minimizing both risk and cost. You do not have to invest in replicating any custom code that resides in the NCM for the modular I/O cards in your new head end.
  2. We are not aware of any installations that have done this. Every installation that we are aware of has replaced the NCU and its cards.
  3. Go into the NCM logic and remove everything except what is absolutely needed to support the modular devices in the NCU chassis. You will need to look carefully at Global Variable support. For instance, if there is an outside air temperature that needs to be shared.
  4. Provide all supervisory logic for the downstream N2 devices with your new head end.
  5. When the DCM or NCM fail you simply replace that functionality with current technology, probably BACnet devices. The money that you saved with this approach should help to fund the replacements. Better yet, work out a going forward plan with your customer for planned replacements of the NCU chassis spread out over a period of time.

Modular N2 cards inserted in a Network Expansion Unit (NEU) chassis can be supported just like any of the newer stand alone N2 devices since the entire chassis connects to the N2 bus and each N2 device in the chassis has its own N2 address.

The concept is interesting, and can be generalized. The same approach could be utilized to support only a portion of a N2 Network with our S4 Open appliances. That is, install the S4 Open appliance as you would a RS-485 repeater on a segment of the N2 bus. The only caviet is that you must be careful that you do not add any new N2 devices that have duplicate N2 addresses. This approach has been used by one of our integrator partners who only wanted to publish the devices in their chiller plant to BACnet.