參數(shù)資料
型號: RS-422
廠商: Electronic Theatre Controls, Inc.
英文描述: RS-422 AND RS-485 APPLICATION NOTE
中文描述: RS - 422和RS - 485接口應(yīng)用筆記
文件頁數(shù): 32/44頁
文件大?。?/td> 628K
代理商: RS-422
RS-422/485 Application Note
29
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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