Data Bus Shield Grounding Requirements; Modbus Rtu Data Bus Shield Grounding Requirements - Distech Controls ECLYPSE APEX User Manual

ECLYPSE
Figure 134: Unsupported Modbus RTU Data Bus Topologies

Data Bus Shield Grounding Requirements

The EIA-485 data bus standard requires that the data bus must be shielded against interference. A
Modbus RTU data bus must also be properly grounded.
The data bus' cable shields must be twisted together and connected to the S or shield terminal at each
ECY Series Controller. Keep the cable shield connections short and take steps at each device to iso-
late the cable shield from touching any metal surface by wrapping them with electrical tape, for exam-
ple. Note that for ECY Series Controllers, the data bus' cable shield provides the ground reference for
the data bus. 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.

Modbus RTU 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 below.
ECLYPSE
RS-485
- S
Data Bus Shield:
Connect to the 'S'
terminal
Electrical
System
Ground
Figure 135: Typical Cable-Shield Grounding Requirements for a Modbus RTU Data Bus Segment with an
ECLYPSE Controller located at the End of the Data Bus
ECLYPSE APEX
Modbus RTU Communication Data Bus Fundamentals
Spur / Backbone
Topologies are
Unsupported
Typical Modbus RTU Typical Modbus RTU
Device
Data Bus: Shielded Twisted Pair Cable
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
ECLYPSE
Star Topologies are
Unsupported
Typical Modbus RTU
Device Device
Typical Modbus RTU
Device
161