BACnet MS/TP Communication Data Bus Fundamentals
ECLYPSE User Guide 119
Data Bus Shield Grounding Requirements
The EIA-485 data bus standard requires that the data bus must be shielded against
interference. A BACnet MS/TP data bus must also be properly grounded.
For ECB Series 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 130). 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 Series 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 130). If the controller is at the end of the BACnet MS/TP data bus, simply
connect the data bus shield to the COM terminal.
ECY Series Controller: The data bus’ cable shields must be twisted together and
connected to the S terminal at each ECY Series 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 ECY
Series Controller, 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 130). If the controller is at the end of
the BACnet MS/TP data bus, simply connect the data bus shield to the S terminal.
Grounding the shield of a data bus segment in more than one place will more than
likely reduce shielding effectiveness.
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 ECY Series Controller, as shown below in Figure 11-6
and Figure 11-7.
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
Data Bus: Shielded Twisted Pair Cable
NET+
NET-
NET+
NET-
NET+
NET-
NET+
NET-
Data Bus Shield:
Connect to the ‘S’
terminal
ECY Series
NET-
NET +
S
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
Figure 11-6: Typical Cable-Shield Grounding Requirements for a BACnet MS/TP
Data Bus Segment with an ECY Series Controller located at the End of the Data
Bus
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