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參數(shù)資料
| 型號： | RS-485 |
| 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)： | 29/44頁 |
| 文件大小： | 628K |
| 代理商： | RS-485 |
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26
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Ground Differences
Realizing that transient energy can be high frequency in nature leads to
some disturbing observations. At frequencies of this magnitude, it is difficult to
make a low impedance electrical connection between two points due to the
inductance of the path between them. Whether that path is several feet of
cable or thousands of feet of earth between grounding systems, during a
transient event there can be hundreds or thousands of volts potential between
different “grounds”. We can no longer assume that two points connected by a
wire will be at the same voltage potential. To the system designer this means
that although RS-422/485 uses 5V differential signaling, a remote node may
see the 5V signal superimposed on a transient of hundreds or thousands of volts
with respect to that nodes local ground. It is more intuitive to refer to what is
commonly called “signal ground” as a “signal reference”.
How do we connect system nodes knowing that these large potential
differences between grounds may exist The first step towards successful
protection is to assure that each device in the system is referenced to only one
ground, eliminating the path through the device for surge currents searching
for a return. There are two approaches to creating this idyllic ground state.
The first approach is to isolate the data ground from the host device ground,
this is typically done with transformers or optical isolators as shown is Figure
4.4. The second approach is to tie each of the grounds on a device together
(typically power ground and data ground) with a low impedance connection as
shown in Figure 4.5. These two techniques lead us to the two basic methods of
transient protection.
Figure 4.4 Isolated RS-485 Device
Device
Isolated Power
Vcc
Optical
Isolation
Port
Data Lines Out
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