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THE ART AND SCIENCE OF RS-485

by Bob Perrin

Start • Arm Yourself • RS-485 101 • Getting Grounded • Shielding • Topology • Termination • Idle-state Biasing • Transients • Review Time • Sources

IDLE-STATE BIASING

An article on RS-485 wouldn’t be complete without mentioning idle-state biasing, also called failsafe biasing. Once again, National Semiconductor and John Goldie have the seminal treatise on the subject, and I would encourage you to refer to this existing work for analytical details [8].

RS-485 networks with multiple transmitters on the same communication channel rely on the line drivers to tristate when not talking. This arrangement allows the two conductors in the transmission line to float, which can cause the line receivers listening to the network to register false data. TIA/EIA-485-A purposely leaves the region of less than 200 mV of differential voltage as an undefined state.

To get around this situation, two resistors are often used to pull one line high while the other line is pulled low. This process is referred to as idle-state biasing because the line is said to be idle when it is not being actively driven by a transmitter.

The impact of the idle-state bias resistors on line termination must be considered, as should their physical location in the network. Depending on the application, it may be better to use a series of high-valued resistors distributed across many nodes than two smaller-valued resistors placed at the end of the line.

Another situation to consider is what happens when power goes down to a node with idle-state bias resistors installed. And likewise, what effect on the network’s idle state is there when a node with failsafe biasing is removed from the network? These issues and more are adequately addressed by Goldie [8].

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