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 common-
mode voltage is the mean, or average, of the voltages on the two differential
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 perfectly
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 common-
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 common-
mode limit.
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