Data Bus Shield Grounding Requirements; 24V-Powered Controller Data Bus Shield Grounding Requirements - Acuity Controls nLight ECLYPSE User Manual

Figure 94: Unsupported BACnet MS/TP LAN Topologies

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 24V-Powered Controllers:
The data bus' cable shields must be twisted together and isolated with electrical tape at each device. Note that for 24V-
Powered Controllers, the power supply transformer's secondary that is connected to the 24V COM terminal is usually
grounded. This provides the ground reference for the data bus (see
troller is at the end of the BACnet MS/TP data bus, simply isolate the data bus shield with electrical tape.
ECLYPSE Series Controller:
The data bus' cable shields must be twisted together and connected to the S terminal at each ECLYPSE 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 ECLYPSE Controllers, the data bus' cable shield
provides the ground reference for the data bus (see
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.

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 ECLYPSE Controller, as shown in the figures below.
ECLYPSE
Data Bus Shield:
Connect to the 'S'
Figure 95: Typical Cable-Shield Grounding Requirements for a BACnet MS/TP Data Bus Segment with an ECLYPSE Controller located
at the End of the Data Bus
nLight ECLYPSE
ECLYPSE
Typical BACnet 24VAC-
Powered Controller
Data Bus Shields: Twist
together and Isolate
terminal
with electrical tape
Data Bus: Shielded Twisted Pair Cable
Electrical The shield of the data bus must be connected to
System the electrical system ground at one point only –
Ground usually at the Building Controller, when present
BACnet MS/TP Communication Data Bus Fundamentals
ECLYPSE
BACnet MS/TP is a Three-Wire Data
BACnet MS/TP is a Three-Wire Data
Typical BACnet 24VAC- Typical BACnet 24VAC-
Powered Controller Powered Controller
Data Bus Shields: Twist
together and Isolate
with electrical tape
Bus). If the con-
Bus). If the controller is at the end
Typical BACnet 24VAC-
Powered Controller
Data Bus Shield:
Isolate with
electrical tape
97