122 ECLYPSE APEX
Only a Daisy-Chained Data Bus Topology is Acceptable
Use a daisy-chained BACnet MS/TP data bus topology only. No other data bus topology is allowed.
Distech Controls devices exclusively
END EOL ON
ECLYPSE Controller
EOL Externally Set
Figure93: Typical BACnet MS/TP LAN Topology Showing How Devices are Daisy-Chained Together to Form
One Data Bus Segment
Only linear, daisy-chained devices provide predictable data bus impedances required for reliable data bus operation.
Only a daisy-chained data bus topology should be specified during the planning stages of a project and implemented
in the installation phase of the project.
A spur is only permitted when it is connected to the data bus through a repeater (see Using Repeaters to Extend the
Data Bus).
Figure94: 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 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 con-
nected 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). If the controller is at the end of the BACnet MS/TP data bus,
simply isolate the data bus shield with electrical tape.
BACnet MS/TP Communication Data Bus Fundamentals
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