Specifications

Chapter 4 - Transient Protection of RS-422 and RS-485 Systems RS-422 And RS-485 Applications Ebook
Page 56 of 137 Manual Documentation Number: <DocNumber>
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method separates the signal reference from any fixed ground. Optical
isolators, transformers and fiber optics are all methods commonly used
in many types of data networks to isolate one section of a system from
another.
In RS-422 and RS-485 applications, optical isolators have been the
most common type of isolation for some time. An optical isolator is an
integrated circuit that converts the electrical signal to light and back,
eliminating electrical continuity. With an isolated port, the entire
isolated circuitry floats to the level of the transient without disrupting
data communications. As long as the floating level of the circuitry does
not exceed the breakdown rating of the isolators (typically 1000 to
2500 volts) the port will not be damaged.
Figure 40: An Optocoupler
Some newer technologies are now using "digital isolators". Digital
isolators combine CMOS technology with chip-scale micro-transformers
into a single integrated circuit packages and provide superior
performance over opto-couplers.
Figure 41: Digital Isolator Chip
Isolation technologies do not attempt to absorb or shunt excess energy
so they are not sensitive to the length of the transient. Even
continuous potential differences will not harm isolated devices.
It is important to note that isolators work on common mode transients;
they cannot protect against large voltage differences between