Review RS-485 Idle Bias Across the Complete Network

An RS-485 receiver can behave correctly while its driver is active and produce unwanted transitions when every driver releases the bus. Idle behavior depends on the receivers, termination and any external bias network. Inspect the complete network rather than copying bias resistors onto every node.

Separate receiver behavior from bus voltage

Read the selected receiver’s guaranteed behavior for open, shorted and idle conditions. These are distinct conditions, and support for one should not be assumed to establish all three. Texas Instruments’ RS-485 failsafe discussion distinguishes built-in receiver behavior and external biasing. Internal failsafe in one receiver does not define the behavior of every other receiver on a mixed network.

Record the minimum differential input needed for the intended idle decision, including the chosen noise margin. Verify polarity against the actual device truth table and connector naming. Letter labels used by different products can be confusing; the intended electrical state should be explicit.

Calculate the loaded idle network

Include both end terminations, input loading and all fitted pullup and pulldown networks. Check minimum and maximum supply and resistor tolerances. Adding a second product with its own default bias can change the load and common-mode condition even when each board worked alone.

Document which node owns external bias and whether that node can be disconnected or powered off. If reliable idle behavior depends on a powered master, that dependency belongs in the system specification. A network cannot rely on a bias supply that disappears in a supported operating state.

Check the active driver as well

A stronger bias network is a load on an active driver. Verify differential output margin and common-mode limits with the actual terminations and maximum intended nodes. Do not reduce resistor values indefinitely to suppress noise without evaluating those consequences.

Distinguish termination from bias. Termination addresses transmission-line behavior, while bias establishes an idle differential condition. One resistor network can influence both, but changing it for one purpose requires checking the other. Node count alone does not describe the network’s electrical load.

Test transitions into and out of idle

Observe bus differential voltage and receiver output when the last driver disables, when one node powers down and when a supported cable segment is removed. Exercise the longest intended idle period and representative interference conditions using the product’s validation plan.

Keep the network drawing, bias ownership and tested node combinations with the interface documentation. A production board can pass a point-to-point bench test while its default bias option causes trouble in a larger installation. The approved configuration should make those options and their limits visible.