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28 RS-422/485 Application Note

 Copyright B&B Electronics -- Revised October 1997

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

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

Isolation Devices

Optical isolation can be implemented in a number of ways. If a conversion

from RS-232 to RS-422 or RS-485 is being made, optically isolated converters

are available. Optically isolated ISA bus serial cards can replace existing ports

in PC systems. For systems with existing RS-422 or RS-485 ports, an optically

isolated repeater can be installed. Examples of each of these type devices can be

found in the B&B Electronics Data Communications catalog.

Transient Protection using Shunting

Shunting Theory

Creating one common ground at the host device provides a safe place to

divert surge energy as well as a voltage reference to attach surge suppression

devices to. Shunting harmful currents to ground before they reach the data port

is the job of components such as TVS (often referred to by the trade name

Tranzorb), MOV or gas discharge tubes. These devices all work by “clamping”

at a set voltage, once the clamp voltage has been exceeded, the devices provide a

low impedance connection between terminals.

Since this type of device diverts a large amount of energy, it cannot tolerate

very long duration or continuous transients. Shunting devices are most often

installed from each data line to the local earth ground, and should be selected to

begin conducting current at a voltage as close as possible above the systems

normal communications levels. For RS-422 and RS-485 systems, the voltage

rating selected is typically 6 - 8 volts. These devices typically add some

capacitive load to the data lines. This should be considered when designing a

system and can be compensated for by derating the total line length to

compensate for the added load. Several hundred feet is usually an adequate

figure.

To apply these type products correctly they should be installed as close to

the port to be protected as possible, and the user must provide an extremely low

impedance connection to the local earth ground of the unit being protected. This

ground connection is crucial to proper operation of the shunting device. The

ground connection should be made with heavy gauge wire and kept as short as

possible. If the cable must be longer than one meter, copper strap or braided

cable intended for grounding purposes must be used for the protection device to

be effective. In addition to the high frequency nature of transients, there can be

an enormous amount of current present. Several thousand amps typically result

from applications of the combination wave test in the ANSI and IEC

specification.

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