Networking The Controller - Johnson Controls METASYS VAV110 Technical Manual

N2 Bus
Overview
Wiring the
Controller to an
NCU
N2 Bus
Characteristics

Networking the Controller

6 3 F 6 4 F 6 5 F
If you are familiar with the N2 Bus, go to the next section of this technical
bulletin: Installing the N2 Bus.
A hardware connection between the N2 Communications Bus and the
NCM of the Network Control Unit (NCU) is required if the NCM is to
service N2 devices. Refer to Table 14 for terminal locations and to the N2
Communications Bus Technical Bulletin for termination resistor values.
When installed in a Metasys Network, the VAV Controller receives
commands from the Network Control Module (NCM) or Companion on the
N2 Bus, and transmits status reports in return. The maximum electrical
limitation on the N2 Bus is 255 devices. However, the number of
controllers on the N2 Bus is a data base memory issue at the NCU or
Companion. Refer to the N2 Communications Bus Technical Bulletin
(FAN 636) and either the Metasys Companion Technical Manual (FAN
628.1) or the Metasys Technical Manual (FAN 636) to determine practical
limitations to the number of controllers on the N2 Bus.
The N2 Bus connections are electrically isolated from other series
VAV110/111 and VAV140/141 Controller circuitry by optical and
magnetic coupling.
For runs of up to 5,000 feet/1,500 meters, use 22 AWG/0.6 mm diameter or
higher twisted pair wire. (Electrically, 26 AWG wire works, but it is too
fragile and prone to nicks and breaks.) Runs longer than
5,000 feet/1,500 meters require use of a bus repeater. An N2 network may
be extended to a maximum length of 15,000 feet/4,500 meters, using two
repeaters.
The N2 Bus is a daisy chain communications line. Essentially, it consists
of three wires, which carry three signals: N2+, N2–, and REF. The N2+
and N2– lines carry the actual data signals. The REF line provides a
common reference so that each connected device is capable of electrically
receiving and transmitting data by creating a common voltage reference
among all the devices connected together by the N2 lines. Three lines are
required for optimum reliability. It is important that the N2+ and N2– lines
must be twisted pair lines, which allows most induced noise (common-
mode noise) from external sources to affect both lines equally, thereby
canceling the noise.
Note: Do not run N2 Bus wiring in the same conduits as line voltage
wiring (above 30 VAC) or wiring that switches power to highly
inductive loads (such as contactors, coils, motors, or generators).
VAV Controller—Variable Air Volume (VAV) Controller 45