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
Chapter 7
136
in the ground connection. For example, if the driver??™s outputs are +5V and 0V
relative to the driver??™s ground, and the driver??™s ground is 7V more positive than
the receiver??™s ground, the receiver??™s inputs relative to the receiver??™s ground will
be +12V and +7V (ignoring losses and noise in the differential lines). The common-
mode voltage at the receiver??™s inputs is:
((DriverOutputA - DriverOutputB) / 2 ) + DriverGroundVoltage - ReceiverGroundVoltage
or
((+5 - 0) / 2) + 7 = +9.5
which is within the +12V limit.
In the other direction, if the driver??™s outputs are +5V and 0V and the driver??™s
ground is 7V more negative than the receiver??™s ground, the receiver??™s inputs relative
to the receiver??™s ground will be +2V and -7V. The common-mode voltage
is:
((+5 - 0) / 2) - 7 = -4.
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