|
 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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THE
ART AND SCIENCE OF RS-485