Each RS-485 port has an adjacent 3-position biasing switch.
Figure 1. RS-485 module bias switches
Often, adding RS-485 biasing can improve communications by eliminating indeterminate idle states.
Typical scenarios for the best RS-485 bias switch position are as follows:
- BIA - (Default, middle) Often best if the RS-485 trunk needs biasing, but when the controller is not installed at the end of the trunk.
- END - Often best if the controller is installed at the end of an RS-485 trunk of devices that is not already biased.
- MID - Often best if the controller is put in the middle of an already-biased RS-485 trunk.
If desired, you can change the position of an RS-485 port’s bias switch while the controller is running. Each RS485 port also has two LEDs.
Note: FX-PC controllers implement biasing. When using RS485 for BACnet MS/TP with FX-PC
controllers, we recommend using the MID position (with FX-PC
controllers at the ends of the trunks with their EOLs set). The BIA
position is not required for an all FX-PC controller trunks. For more information, refer to
the FX-PC Series Controllers MS/TP Communications Bus Technical Bulletin
(LIT-12011670).