BACnet MS/TP Communication Data Bus Fundamentals
ECLYPSE User Guide 123
A repeater can be used to create a spur as shown below.
EOL Terminator
MS/TP Data Bus Segment:
- 1220 m (4 000 ft) Maximum
- 50 Connected Devices Maximum Total
MS/TP Data Bus Segment:
1220 m (4 000 ft) Maximum
MS/TP
Repeater
EOL Internally Set
EOL Internally Set
EOL
ON
EOL
ON
ECY-S1000
EOL Internally Set
EOL
ON
Figure 11-13: Adding a Spur by Using a Repeater
A repeater is counted as a device on each data bus to which it is connected.
When third party devices are connected to a data bus segment, the number of devices that
can be connected to that data bus segment may be reduced. See Device Loading on page
113.
Figure 11-14: Repeater Connections when it is the First or Last Device on its
Respective Data Bus Segment
The BACnet MS/TP Data Bus must be biased. This bias can only be provided by built-in
EOL termination resistors (ones set with a jumper or DIP switch). When a repeater is the first
or last device on its respective data bus segment, use the following methods to provide
MS/TP Data Bus biasing and EOL termination as applicable to your situation:
1. On the BACnet MS/TP data bus segment shown in Figure 11-14, bias and EOL
termination is provided by a controller’s built-in EOL termination being set to ON. In this
case the connection to the repeater cannot be more than 7.6 meters (25 feet) from this
controller.
2. On the BACnet MS/TP data bus segment shown in Figure 11-14, a 120Ω EOL
Termination resistor is added to the repeater’s terminals. Biasing for this BACnet MS/TP
data bus segment is provided by the built-in EOL termination being set to ON at the last
controller at the other end of this data bus.
Data Bus Shields: Twist
together and Isolate
with electrical tape
Typical BACnet Device
Data Bus: Shielded
Twisted Pair Cable
NET+
NET-
NET+
NET-
The data bus shield must be connected to the electrical system ground at
one point only – usually at the Building Controller, when present
120
Ω
Data Bus Shields: Twist
together and Isolate with
electrical tape
EOL ON
BACnet MS/TP data bus
Bias and EOL termination is
provided by this controller’s
internal EOL Termination
being set to ON
< 7.6 m
< 25 ft
1 2
Typical
BACnet
Device
No Built-in EOL
Termination
Typical BACnet Device
NET+
NET-
EOL ON
BACnet MS/TP data bus
Bias and EOL termination is
provided by this controller’s
internal EOL Termination
being set to ON
Repeater
Data + (1)
Data – (2)
Data + (20)
Data – (19)
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