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RS-422/485 Application Note 27

 Copyright B&B Electronics -- Revised October 1997

B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350

PH (815) 433-5100 -- FAX (815) 434-7094

Vcc

Port

Device

Data Lines

Local Chassis Ground Connection

Ground line

Figure 4.5 RS-485 Device with Signal Ground

Connected to Chassis Ground

Transient protection using Isolation

Isolation Theory

The most universal approach to protecting against transients is to

galvanically isolate the data port from the host device circuitry. This 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 I/O circuitry from its host device. In RS-422 and RS-

485 applications, optical isolators are most common. 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 - 2500 volts) the port will not be damaged. This type of

protection does not attempt to absorb or shunt excess energy so it is 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

conductors of a data cable such as those caused by short circuits between data

and power circuits.

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