Reusing existing N2 wiring for BACnet MS/TP - Facility Explorer - LIT-12012045 - Field Device - N2 Controllers

Modernization Guide for N2 Controllers - Facility Explorer

Brand
Facility Explorer
Product name
N2 Controllers
Document type
User Guide
Document number
LIT-12012045
Revision date
2019-03-22
Language
English

When converting FX-PC controllers from N2 to BACnet MS/TP, you may encounter some challenges with reusing the existing field bus network wiring. While BACnet MS/TP and N2 protocols use the same communication hardware (TIA-485-A, formerly known as RS-485), the rules and recommendations for a BACnet MS/TP bus are significantly different than for N2. The guidelines in this section should be followed when planning to convert an existing N2 field bus network to BACnet MS/TP.

N2 network wiring guidelines have changed over time. The most recent recommendation establishes 22 AWG or heavier as the preferred size, but still allows for 18 through 26 AWG wire. Your customer's network may use a variety of these wiring options, depending on when each network segment was installed. Wire gauge recommendations are different for BACnet MS/TP. BACnet MS/TP network guidelines recommend 22 AWG wire.

N2 network uses 9600 baud rate. BACnet MS/TP can be configured for up to 76.8k baud. The higher baud rates of BACnet MS/TP may not run efficiently on 18 AWG cable. Also, electrical impedance characteristics of the wire degrade the signals.

Johnson Controls engineering conducted extensive tests using 3-wire 18 AWG cable, and they found negligible performance variation with the shield-terminated test versus the unterminated test and negligible performance improvement by reducing the number of devices on a segment at a particular baud rate.

Table 1. Recommended network wiring for BACnet MS/TP using 3-wire 18 AWG cable

Baud rate

Devices

Segment length (ft.)

Offline or online alarms

Errors

76.8k

50

2,000

0

1

38.4k

50

4,000

0

0

19.2k

50

5,000

0

1

9600

50

5,000

0

0

Consider the following workaround options, and select the most appropriate options based on your specific installation situation:

  • Use repeaters, placed at closer intervals to boost the signal
  • Reduce the BACnet MS/TP baud rate
  • Replace long N2 network cable runs with wireless routers
  • Use Wireless Field Bus connections to alter the network configuration
  • Replace long N2 network cable runs with BACnet MS/TP recommended cable
  • Use fiber-optic modems between network segments
  • Use protocol converter devices to manage the network