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RS-422/485 Application Note 29

 Copyright B&B Electronics -- Revised October 1997

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

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

Connecting Signal Grounds

Since a local ground connection is required at each node implementing

shunt type protection, the consequences of connecting remote grounds together

must be considered. During transient events a high voltage potential may exist

between the remote grounds. Only the impedance in the wire connecting the

grounds limits the current that results from this voltage potential. The RS-422

and RS-485 specification both recommend using 100 ohm resistors in series with

the signal ground path in order to limit ground currents. Figure 4.6 illustrates the

ground connection recommended in the specification.

Figure 4.6 Signal Ground Connection between two nodes

with 100 ohm resistor

Shunting Devices

There are two types of shunting devices to choose from. The least

expensive type is single stage, which usually consists of a single TVS device on

each line. Three stage devices are also available. The first stage of a three stage

device is a gas discharge tube, which can handle extremely high currents, but has

a high threshold voltage and is too slow to protect solid state circuits. The

second stage is a small series impedance which limits current and creates a

voltage drop between the first and third stage. The final stage is a TVS device

which is fast enough to protect solid state devices and brings the clamping

voltage down to a safe level for data circuits.

Combining Isolation and Shunting

Installing a combination of both types of protection can offer the highest

reliability in a system. Figures 4.7 and 4.8 illustrate two means of implementing

this level of protection.

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B&B Electronics RS-422 Especificaciones Pagina 32