Designing RS-485 Links and NetworksIn some RS-485 circuits, all of the nodes and the cable that connects them
share a common ground. In other circuits, the line is isolated from the nodes it
connects to. In either case, all of the drivers and receivers should share a ground
connection, which can have any of several sources. Most obviously, the RS-485
cable can include a wire that connects to signal ground at each node. Or the
nodes’ power supplies can share a common ground either through electrical
wiring or via an earth ground. In a very short line, multiple nodes can share a
power supply.
The specifications for RS-485 interface chips limit the permitted difference in
ground potentials. Isolating the line is sometimes easier than ensuring that
earth grounds at distant nodes are within the required limits.
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TIA-485-A doesn’t specify the voltage between each output and signal ground
except to say that the common-mode voltage must be 7V or less. The com-
mon-mode voltage is the mean, or average, of the voltages on the two differen-
tial lines referenced to signal ground. The common-mode voltage measured at
the receiver is a function of the driver offset voltage (1/2 the sum of the voltages
at the A and B outputs), the difference in ground potentials at the driver and
receiver, and any noise that appears equally on both lines. If the interface is per-
fectly balanced, the inputs are offset equally from 1/2 the supply voltage. Any
imbalance raises or lowers the offset.
To comply with TIA-485-A, components must work properly with com-
mon-mode voltages from -7V to +12V. In addition, each of the receiver’s inputs
must be in the range of -7V to +12V referenced to the receiver’s ground.
With differential signals as large as ±5V, the ground potentials at the driver and
receiver can vary as much as ±7V and still comply with the standard’s com-
mon-mode limit. The data sheets for interface chips specify a common-mode
limit, which is often larger than the minimum requirement.
This brings us to guideline #6 for RS-485:
Limit common-mode voltages.
If the ground potentials of the driver and receiver are equal, the common-mode
voltage at the receiver is the mean of the two inputs, or +2.5V with a 5V supply.
The common-mode voltages also remain within the limits when the ground
potentials of two nodes vary by up to ±7V. A difference in grounds is a result of
any DC differences in the ground potentials and any AC oscillations or spikes