Distech ECYPSE - Data Bus Shield Grounding Requirements; Modbus RTU Data Bus Shield Grounding Requirements

Modbus RTU Communication Data Bus Fundamentals
ECLYPSE User Guide 161
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 isolate the cable shield from touching any metal surface by wrapping
them with electrical tape, for example. 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 ECY Series Controller, as shown below in Figure 14-5
and Figure 14-6.
Typical Modbus RTU
Device
Typical Modbus RTU
Device
Typical Modbus RTU
Device
Typical Modbus RTU
Device
Data Bus: Shielded Twisted Pair Cable
Data Bus Shield:
Connect to the ‘S’
terminal
ECY Series
NET- NET + S
NET- NET + S NET- NET + S NET- NET + S 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 14-5: Typical Cable-Shield Grounding Requirements for a Modbus RTU
Data Bus Segment with an ECY Series Controller located at the End of the Data
Bus
Typical Modbus RTU
Device
2'-5 1/4"
Typical Modbus RTU
Device
Typical Modbus RTU
Device
Data Bus: Shielded Twisted Pair Cable
Typical Modbus RTU
Device
Data Bus Shields:
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
ECY Series
NET- NET + S
NET- NET + S NET- NET + S NET- NET + S NET- NET + S
Figure 14-6: Typical Cable-Shield Grounding Requirements for a Modbus RTU
Data Bus Segment with an ECY Series Controller located in the Middle of the Data
Bus