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參數(shù)資料
| 型號(hào)： | 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(yè)數(shù)： | 20/44頁(yè) |
| 文件大小： | 628K |
| 代理商： | RS-485 |
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RS-422/485 Application Note
17
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
termination resistor to eliminate the DC loading effect. Although this method
eliminates DC loading, capacitor selection is highly dependent on the system
properties. System designers interested in AC termination are encouraged to
read National Semiconductors Application Note 903
2
for further information.
Figure 2.3 illustrates both parallel and AC termination on an RS-485 two-wire
node. In four-wire systems, the termination is placed across the receiver of the
node.
Figure 2.3 Parallel and AC Termination
Biasing an RS-485 Network
When an RS-485 network is in an idle state, all nodes are in listen
(receive) mode. Under this condition there are no active drivers on the
network, all drivers are tristated. Without anything driving the network, the
state of the line is unknown. If the voltage level at the receiver’s A and B
inputs is less than
±
200 mV the logic level at the output of the receivers will
be the value of the last bit received. In order to maintain the proper idle
voltage state, bias resistors must be applied to force the data lines to the idle
condition. Bias resistors are nothing more than a pullup resistor on the data
B line (typically to 5 volts) and a pulldown (to ground) on the data A line.
Figure 2.4 illustrates the placement of bias resistors on a transceiver in a two-
wire configuration. Note that in an RS-485 four-wire configuration, the bias
resistors should be placed on the receiver lines. The value of the bias
resistors is dependent on termination and number of nodes in the system.
The goal is to generate enough DC bias current in the network to maintain a
minimum of 200 mV between the B and A data line. Consider the following
two examples of bias resistor calculation.
2
Refer to Chapter 7 for information on National Semiconductors Application
Notes.
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相關(guān)代理商/技術(shù)參數(shù) |
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| RS485-232 | 制造商:PR 功能描述: |
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| RS485-C1(EU) | 制造商:Equinox Technologies 功能描述:RS232/RS485 CONV (EU) |
| RS485-C1(UK) | 制造商:Equinox Technologies 功能描述:RS232/RS485 CONV (UK) |
| RS485-C1(US) | 制造商:Equinox Technologies 功能描述:RS232/RS485 CONV (US) |
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