12 Introduction—ASC and N2 Bus Networking and Troubleshooting Guide
“Where circuit reference is provided by a third conductor, the
connection between circuit common and the third conductor must
contain some resistance (e.g. 100 ohms) to limit circulating
currents when other ground connections are provided for safety.”
For the Companion system, the 100 ohm resistor is located in the RS232-
to-N2 Bus converter (MM-CVT101-0) or in the Companion Panel/LTD.
With N2 Bus grounding in mind, there are a few important facts to know:
●
If earth grounded, the UNT-100/101 units and all VAVs may
experience communication problems. This is because the REF line in
these units is connected internally to digital common. The
UNT-100/101s and all VAV controllers are not designed to be earth
grounded, although they often become grounded because field wires
connect to earth-grounded field devices, and thus bring ground into
the digital common of the device. During installation and
commissioning, ensure that no UNT-100/101s or VAVs are earth
grounded. (This does not apply to the newer models that isolate the
N2 from digital common, including: UNT110/111/120/121, all AHU,
and all LCP models.)
●
According to RS-485 specifications, the REF protection on most
devices is a 100 ohm resistor. If you miswire the MM-CVT101 or
Companion Panel/LTD by connecting any voltage greater than
5 VAC, you may damage the 100 ohm resistor. The resistor is not
field serviceable, so if the REF line becomes damaged, you’ll need to
send the unit back for repair or replacement.
Most N2 devices have isolated N2 communications power supplies, and
therefore there is no direct path to earth ground through any of the N2
lines. However, there are three exceptions in which an unwanted earth
ground may be introduced into the system:
●
From the MM-CVT101, which is not isolated. It is grounded through
pin 7 of the RS-232 connector. Earth ground often comes in from the
ground pin on the PC’s power cord or via a printer connected to the
computer, which in turn connects to the MM-CVT101. The
Companion PC itself is grounded.
●
From a surge protection module (i.e., APT Transient Eliminator
) on
the N2 Bus. It produces a small amount of leakage to earth ground.
●
From the Cable Pro (CBLPRO-0 or -1 only). If the laptop PC is earth
grounded, the Cable Pro would be grounded through the RS-232
ground pin.
A single ground will not cause communication problems unless another
earth ground path exists in the system to complete a ground loop.
N2 Bus
Grounding
Avoiding
Unwanted Earth
Grounds
Summary of Contents for METASYS ASC
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