Distech Controls RS485 Installation Manual page 6

Figure 5: RS485 module bias switches
Settings of each RS485 bias switch are as follows:
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BIA - (Default, middle) RS485 biasing and termination: 2.7K bias resistors with no termination resistor
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END - RS485 biasing and a termination: 562 Ohm bias resistors and 150 Ohm termination resistor
£ MID - RS485 biasing or termination: 47.5K bias resistors with no termination resistor
Often, adding RS-485 biasing can improve communications by eliminating indeterminate idle states.
Typical scenarios for the best RS485 bias switch position are as follows:
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BIA - (Default, middle) Often best if the RS485 trunk needs biasing, but when the EC-BOS is not installed at the end of the trunk.
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END - Often best if the EC-BOS is installed at the end of an RS485 trunk of devices that is not already biased.
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MID - Often best if the EC-BOS is put in the middle of an already-biased RS485 trunk.
If desired, you can change the position of an RS485 port's bias switch while the EC-BOS is running.
Each RS485 port also has two LEDs. See
LEDs
Five LEDs are on the front of the Dual RS485 option module.
Figure 6: LEDs on Dual RD485 option module
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"LEDs [pg. 6]".