The issue of isolation is a common one in electronic communications and one of the issues isolation is used to mitigate are those of common mode signal levels. When a signal goes outside the operating range of a receiver, transitions will no longer be recognized and component damage may occur.
A communications receiver has a range of signal input voltage over which it will function. If the signals get outside this range state changes will cease to be recognized. A typical EIA-485 transmitter generates signals between 0 and 5 volts. A typical EIA-485 receiver will respond to signals between -7 and 12 volts. This is the minimum and maximum voltage with respect to circuit common that can be applied to the receiver’s input for proper function. Depending on the receivers specification, damage may occur when voltages outside this range are applied. The functional signal input range is designated on the data sheet as the common mode range.
The ultimate and most common reference for all electronic circuits is earth. Electronic circuits typically gain reference to earth through the power distribution wiring they are supplied by. This reference to earth in many cases is broken through the use of a transformer in the DC power portion of the circuit. Additionally ground reference can be obtained through something the circuit is interfacing with. For example a device on a communications network may also be connected to an earth grounded sensor. With variations in voltage across the sensor, where the sensor is grounded, and how the sensor is biased an offset of the circuits common with respect to earth ground will be developed. If the sensor and the communications circuit are using the same circuit common the communications signals will have an offset with respect to earth.
Common mode problems occur when the signalling levels are outside the common mode range of the receiver. This happens when the circuit common of the receiver is offset from the circuit common of the transmitter by an amount that puts the signal levels outside the common mode range of the receiver. A typical cause of offset between circuit commons is both circuits being referenced to earth with different offsets.
Galvanic isolating the transmitter/receiver circuit breaks the DC connection to any other circuit. The isolated circuit can take on any offset with respect to ground up to the break-down values of the isolation components. If all the transmitter/receiver circuits on a bus are isolated and they all have approximately the same biasing of the bus then all the circuits will have similar offsets between earth and their circuit commons. More importantly any transmitter on the bus will be operating within the common mode range of all the receivers on the bus.