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Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 11
Installing the N2 Bus
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 Bus. Three
lines are required. It is important that the N2+ and N2– lines are twisted
together, which allows most induced noise (common-mode noise) from
external sources to affect both lines equally, thereby canceling the noise.
In most installations, the N2 Bus works fine with unshielded cable.
However, in noisy environments, such as near gas ignition devices and arc
welders, shielded twisted wire must be used. Otherwise, the noise disrupts
N2 communications and the ASCs. For more details, refer to the section
Metasys Installation Practices.
An important feature of the N2 Bus is opto-isolation. Isolation prevents
interruption of all N2 Bus communication if any of the controllers on the
bus become grounded. Table 1 below outlines which controllers have
opto-isolation in their N2 Bus circuitry.
Table 1: Opto-Isolated Devices
Controller or Device
Opto-Isolation?
AHU
Yes
LCP
Yes
MM-CVT101
No
UNT100/101
No
UNT110/111/120/121
Yes
VAV100/101
No
The Reference (REF) line helps to provide a common reference from
which each device connected to the N2 Bus can discern the voltage levels,
and hence, the data, on the N2+ and N2– lines. The N2 Bus may connect
devices that are far apart, such as in two different buildings, by allowing
line lengths of up to 15,000 feet with two repeaters. If the N2 Bus is wired
between buildings, a surge protection module should be used at both ends.
Realize that connecting the earth ground of one building to the earth
ground in another building can cause current to flow in the line that
connects the two grounds together. Because of this, the EIA Standard
RS-485 (the N2 Bus) specification states:
Background
Information
N2 Reference
Lines
Summary of Contents for METASYS ASC
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