Distech Controls ECB-VAV, User Manual

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24 Network Guide
Data Bus Shield Grounding Requirements
The EIA-485 data bus standard requires that the data bus must be shielded against interfer-
ence. A BACnet MS/TP data bus must also be properly grounded.
For ECB 24V-Powered Controllers: The data bus' cable shields must be twisted together and
isolated with electrical tape at each device. Note that for ECB 24V-Powered Controllers, the
power supply transformer's secondary that is connected to the 24V COM terminal is grounded.
This provides the ground reference for the data bus (see
BACnet MS/TP is a Three-Wire Data
Bus on page 41). If the controller is at the end of the BACnet MS/TP data bus, simply isolate
the data bus shield with electrical tape.
For ECB-PTU Line-Powered Controllers: The data bus' cable shields must be twisted together
and connected to the COM terminal at each ECB PTU Line-Powered Controller. Keep the
cable shield connections short and take steps at each device to isolate the cable shield from
touching any metal surface by wrapping them with electrical tape, for example. Note that for
ECB-PTU Line-Powered Controllers, the data bus' cable shield provides the ground reference
for the data bus (see
BACnet MS/TP is a Three-Wire Data Bus on page 41). If the controller is
at the end of the BACnet MS/TP data bus, simply connect the data bus shield to the COM ter-
minal (see
Figure 15 ).
ECB 24V Powered Controller Data Bus Shield Grounding
Requirements
The shield on each data bus segment must be connected to the electrical system ground at
one point only, for example, at the Data Bus Master (Building Controller) if there is one, as
shown below in
Figure and Figure 8 . On data bus segments without a Data Bus Master, the
ideal point to ground the shield is in the middle of the data bus segment as shown in Figure 9 .
Figure 7: Typical Cable-Shield Grounding Requirements for a BACnet MS/TP Data Bus Segment with a Building
Controller located at the End of the Data Bus
Grounding the shield of a data bus segment in more than one place will more than
likely reduce shielding effectiveness.
Typical BACnet 24VAC-
Powered Controller
Data Bus Shield:
Isolate with
electrical tape
Typical BACnet 24VAC-
Powered Controller
Data Bus Shields: Twist
together and Isolate
with electrical tape
Typical BACnet 24VAC-
Powered Controller
Data Bus Shields: Twist
together and Isolate
with electrical tape
Typical BACnet 24VAC-
Powered Controller
Bus Master (Building Controller)
Typical EC-BOS Device
Data Bus: Shielded Twisted Pair Cable
+ - S
NE
T
+
NE
T
-
NE
T
+
NE
T
-
NE
T
+
NE
T
-
NE
T
+
NE
T
-
Data Bus Shield:
Connect to the ‘S’
terminal
Electrical
System
Ground
The shield of the
data bus must be
connected to the
electrical system
ground at one point
only – usually at the
Building Controller,
when present