RS-485 bias switches - Facility Explorer - Part No. 24-10143-918 - Field Device

RS-485 Option Module Installation Instructions

Facility Explorer
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Installation Guide
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Part No. 24-10143-918
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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).