Unitronics’s BACnet Gateway User Guide Page 8 of 28
Unitronics
2.2.1 Biasing the Modbus RS-485 Network
An RS-485 network with more than one device needs to have biasing to ensure proper communication.
The biasing needs to be done on one device.
The GW-BAC1 has 530 Ohm resistors that can be used to set the biasing. The GW-BAC1’s default positions
from the factory for the Biasing jumpers are OFF.
The OFF position is when the 2 RED biasing jumpers straddle the 4 pins closest to the outside of the board of the GW-
BAC1. See Error! Reference source not found..
Only turn biasing ON:
o IF the BMS cannot see more than one device connected to the GW-BAC1
o AND you have checked all the settings (Modbus COM settings, wiring, and DIP switches).
To turn biasing ON, move the 2 RED biasing jumpers to straddle the 4 pins closest to the inside of the
board of the GW-BAC1.
Figure 3: Modbus RS-485 Biasing Switch on the GW-BAC1
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