I need to add bias resistors to an RS422/RS485 connection to a UDS. How do I connect them?

Bias resistors are used to ensure that the signal lines on an idle RS422 or RS485 connection are pulled to a known state when the connection is idle, e.g. there is no data flowing. Here is how bias resistors should be connected to a UDS for both 4-wire RS422/RS485 and 2-wire RS485 connections:

The resistor value is not critical, as long as they are not too low a value. If you have resistors that you use for termination (usually somewhere around 120 Ohms) you can use those. Any value between 120 Ohm and 100 kOhm should work. Higher resistor values reduce the load on the UDS1100's power supply and cause slightly slower stabilization of the signals.

[Originally Published On: 03/02/2010 11:43 AM]