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14
Prelim Draft - Brand Neutral
BACnet IP/MSTP VAV Controller - INSTALLATION
INSTRUCTION
NOTE:
NOTE:
NOTE:
When attaching two or more wires to the same ter- minal,
other than 14 AWG (2.0 mm2), be sure to twist them
together. Deviation from this rule can result in improper
electrical contact, see
Figure. 10
below.
The RS485 Standard
According to the RS485 standard (TIA/EIA 485: “Electrical
Characteristics of Generators and Receivers for Use in Balanced
Digital Multipoint Systems”), only one driver communicating via
an RS485 interface may transmit data at a time. Further,
according to U.L. requirements, each RS485 interface may be
loaded with a max. of 32 unit loads. For example, if a controller
utilizes as little as 1/8 unit load each, up to 256 devices can be
connected.
BACnet connections to the RS485 interfaces must comply with
the RS485 standard. Thus, it is recommended that each end of
every bus should be equipped with a termination resistor (not
included in shipment) having a resistance equal to the cable
impedance (120
Ω
; the wattage should be in the range of 0.25 –
0.5 W).
RS485 systems frequently lack a separate signal ground wire.
However, the laws of physics still require that a solid ground
connection be provided for in order to ensure error-free
communication between drivers and receivers – unless all of
the devices are electrically isolated, and no earth grounding
exists.
Wiring Method
Each terminal can accommodate the following gauges of wire:
•
Single wire: from 22 AWG to 18 AWG solid or stranded
•
Multiple wires: up to two 18 AWG stranded, with 1/4 watt wire-
wound resistor
—
Prepare wiring for the terminal blocks, as follows:
—
Strip 1/2 in. (13 mm) insulation from the conductor.
—
Cut a single wire to 3/16 in. (5 mm). Insert the wire in the
required terminal location and tighten the screw.
—
If two or more wires are being inserted into one termi- nal
location, twist the wires together a minimum of three turns
before inserting them, see
Figure. 10
below.
—
Cut the twisted end of the wires to 3/16 in. (5 mm) before
inserting them into the terminal and tightening the screw.
—
Pull on each wire in all terminals to check for good
mechanical connection.
TIA/EIA 485 Cable Specifications
The following cable specification is valid for BACnet MS/TP
EIA 485 buses.
Table 5. TIA/EIA 485 cable specifications
Maximum length
4000 ft (9.6–78.8 kbps) or
2600 ft (115.2 kbps).
Cable type
Twisted pair shielded (foil or
braided shields are
acceptable)
Characteristic
impedance
100-130
Ω
Distributed
capacitance between
conductors
Less than 100 pF per meter
(30 pF per foot)
Distributed cap.
between conductors and
shield
Less than 200 pF per meter
(60 pF per foot)
The following cables fulfill this requirement:
—
AWG 18
—
Shielded, twisted pair cable J-Y-(St)-Y 4 x 2 x 0.8 mm2.
—
CAT 5,6,7 cable - use only one single pair for one bus
—
Belden 9842 or 9842NH.
Figure 10. Attaching two or more wires at terminal block
IMPORTANT:
A separate signal ground wire must be used. Fail- ing to
obey this requirement can lead to unpre- dictable
behavior if other electrically non-isolated devices are
connected and the potential differ- ence is too high.
NOTE:
Do not over-tighten the terminal screws to avoid
deformation and damage of the terminal block. The
maximum torque for the terminal screws 4.4 lb.-in. (0.5
Nm).
