BACnet MS/TP Communication Data Bus Fundamentals
Network Guide 21
When to use EOL Terminations with BACnet MS/TP Thermo-
stats
BACnet MS/TP thermostats support external EOL termination resistors only. When a BACnet
MS/TP thermostat is the first or last daisy-chained device, add a 120 Ohm resistor across the
- and + BACnet MS/TP data bus connections.
The BACnet MS/TP data bus must be biased. This bias can only be provided by built-in EOL
termination resistors (ones set with jumpers or DIP switches - refer to the controller's Hard-
ware Installation Guide for how to identify and set a controller's built-in EOL terminations). If a
BACnet MS/TP data bus has a BACnet MS/TP thermostat at one end of the BACnet MS/TP
data bus and an EC-BOS at the other end, you must set the built-in EOL termination in the EC-
BOS so that proper biasing is provided to the BACnet MS/TP data bus.
Figure 3: Typical EOL Terminations with BACnet MS/TP Thermostats with Biasing Provided by the EC-
BOS Built-in EOL Termination set to ON
When a BACnet MS/TP data bus has a BACnet MS/TP thermostat at one end of the BACnet
MS/TP data bus and an EC-BOS at the other end without its built-in EOL termination enabled,
fish-tail the BACnet MS/TP data bus back to a controller in which the built-in EOL termination
can be set. This will provide the necessary biasing to the BACnet MS/TP data bus.
Figure 4: Fish-tailing the BACnet MS/TP Data Bus to use a Controller with Built-in EOL Termination that
will Provide Biasing to the BACnet MS/TP Data Bus
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