JCI NAE BACnet Proxy Field Controller Points

This article explains how Johnson Controls Metasys NAE controllers proxy BACnet field bus points and the implications for integration.

Understanding NAE as BACnet Proxy

When integrating with Metasys systems, the Network Automation Engine (NAE) can act as a BACnet proxy for field controllers on its N2 or SA bus.

Architecture Overview

Key point: Field controllers use proprietary protocols (N2, SA Bus), but the NAE can expose their points as BACnet objects.

BACnet Routing vs Proxy Mode

BACnet Routing Enabled

• NAE acts as a BACnet router
• Field controllers (if BACnet-capable) have their own device instances
• Direct BACnet communication to field devices possible

BACnet Routing Disabled (Proxy Mode)

• NAE acts as a single BACnet device
• All points from field controllers appear as objects within the NAE
• Field controllers are not directly addressable via BACnet
• More common in multi-site or secured deployments

Identifying Proxy Behavior

Symptoms

• Field controller points show NAE device instance, not field controller instance
• Object IDs follow NAE numbering scheme
• Cannot directly discover field controllers via BACnet
• Higher point counts than expected on NAE device

Example

Integration Considerations

Point Addressing

When NAE proxies field points:

JCI Address	Becomes BACnet Object
N2:1/AV1	AV on NAE, object ID per NAE mapping
N2:2/BI5	BI on NAE, object ID per NAE mapping
SA:FEC1/AI3	AI on NAE, object ID per NAE mapping

Discovering Proxied Points

1. Export from Metasys - Use SCT to export point list with BACnet object mappings
2. BACnet discovery - Enumerate all objects on NAE device
3. Cross-reference - Match object names to field controller points

Object Naming Convention

JCI typically maps field points with naming that indicates source:

• N2-1-ZN-T = N2 bus, device 1, zone temperature
• FAC1-SA-T = FAC controller 1, supply air temp

Note: Naming conventions vary by site and Metasys version.

Multi-Site Integration Challenges

Problem

When integrating multiple NAEs with BACnet routing disabled:

• Each NAE appears as single device
• Must poll each NAE individually
• Point mapping becomes complex
• Network traffic increases (all through NAE)

Solutions

Option 1: Enable BACnet Routing

• Allows direct access to BACnet-capable field controllers
• Reduces NAE processing load
• Requires coordination with JCI or site owner
• May require firewall/routing changes

Option 2: Structured Point Mapping

• Create clear naming convention for third-party system
• Document NAE-to-field-controller relationships
• Use object names to maintain traceability

Option 3: Use Metasys as Data Source

• Accept proxy architecture
• Integrate at NAE level only
• Let Metasys handle field communication

Performance Considerations

Latency

• Proxied points add latency (NAE must relay)
• Typical additional delay: 1-5 seconds
• Critical for real-time applications

Polling Load

• Each polled BACnet object = NAE processing
• Large point counts can overload NAE
• Consider COV subscriptions vs polling

Recommendations

• Limit integrated points to essential values
• Use reasonable poll rates (30-60 seconds typical)
• Monitor NAE CPU/memory utilization
• Consider COV for critical points

Troubleshooting

Points Not Updating

1. Check NAE-to-field communication

   • Verify N2/SA bus is healthy in Metasys
   • Look for comm errors in NAE diagnostics

2. Check BACnet mapping

   • Verify point is configured for BACnet exposure
   • Some points may not be mapped by default

3. Check third-party polling

   • Verify correct object ID being polled
   • Check for timeout errors

Missing Points

• Not all field points are exposed via BACnet by default
• Additional points may need to be enabled in Metasys
• Contact JCI for assistance with point exposure configuration

Documentation Needed

Before integrating:

• NAE device instance assignments
• BACnet object list export from Metasys
• Network diagrams showing field bus topology
• Point naming conventions used at site