RS-485 biasing - Facility Explorer - Part No. 24-10174-115 - Supervisory Device - FX Supervisory Controller

FX30E/FX60E Supervisory Controllers Installation Instructions

Facility Explorer
Product name
FX Supervisory Controller
Document type
Installation Guide
Document number
Part No. 24-10174-115
Document revision
Revision date

The FX30E/FX60E's RS-485 port has a pair of 2-pin jumpers that can be shorted with jumper blocks to provide biasing. When shipped from the factory, these pins are not shorted and therefore, the RS-485 port is unbiased.

Biasing can improve RS-485 communications by eliminating indeterminate idle states. When you install two, 2-pin shorting blocks on the controller's RS-485 bias jumper pins, the shorting blocks add two onboard 3.3K ohm resistors into the controller's RS-485 circuit, as follows:

  • from RS-485 plus (+) to 5 V
  • from RS-485 minus (-) to ground
Note: Only one device on an RS-485 trunk should be biased. Otherwise, undue loading of the circuit may result with fewer devices supported.
Note: RS-485 bias resistors are different than termination resistors, externally installed at the two physical ends of a daisy-chained RS-485 trunk, across the plus (+) and minus (-) terminals. Termination resistors are typically 100 or 120 ohm value resistors.
Note: If termination resistors are used, RS-485 biasing is typically required.