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)： 19/44頁
文件大?。?/td> 628K
代理商： RS-422
16
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Termination
Termination is used to match impedance of a node to the impedance of the
transmission line being used. When impedance are mismatched, the
transmitted signal is not completely absorbed by the load and a portion is
reflected back into the transmission line. If the source, transmission line and
load impedance are equal these reflections are eliminated. There are
disadvantages of termination as well. Termination increases load on the
drivers, increases installation complexity, changes biasing requirements and
makes system modification more difficult.
The decision whether or not to use termination should be based on the
cable length and data rate used by the system. A good rule of thumb is if the
propagation delay of the data line is much less than one bit width, termination
is not needed. This rule makes the assumption that reflections will damp out in
several trips up and down the data line. Since the receiving UART will sample
the data in the middle of the bit, it is important that the signal level be solid at
that point. For example, in a system with 2000 feet of data line the propagation
delay can be calculated by multiplying the cable length by the propagation
velocity of the cable. This value, typically 66 to 75% of the speed of light (c), is
specified by the cable manufacture.
For our example, a round trip covers 4000 feet of cable. Using a
propagation velocity of 0.66
×
c, one round trip is completed in approximately
6.2
μ
s. If we assume the reflections will damp out in three “round trips” up
and down the cable length, the signal will stabilize 18.6
μ
s after the leading
edge of a bit. At 9600 baud one bit is 104
μ
s wide. Since the reflections are
damped out much before the center of the bit, termination is not required.
There are several methods of terminating data lines. The method
recommended by B&B is parallel termination. A resistor is added in parallel
with the receiver’s “A” and “B” lines in order to match the data line
characteristic impedance specified by the cable manufacture (120
is a
common value). This value describes the intrinsic impedance of the
transmission line and is not a function of the line length. A terminating
resistor of less than 90
should not be used. Termination resistors should be
placed only at the extreme ends of the data line, and no more than two
terminations should be placed in any system that does not use repeaters. This
type of termination clearly adds heavy DC loading to a system and may
overload port powered RS-232 to RS-485 converters. Another type of
termination, AC coupled termination, adds a small capacitor in series with the
相關(guān)PDF資料
PDF描述
RS-485 RS-422 AND RS-485 APPLICATION NOTE
RS-MK-X2010 High Integrity FM Transceiver
RS3FS SDR Cable Assembly; Connector Type A:micro D Ribbon Plug; Connector Type B:micro D Ribbon Plug; Cable Length:2m; No. of Contacts:26; Pitch Spacing:0.8mm RoHS Compliant: Yes
RS3FS Damper Diodes (For TV)
RS4FS SDR Cable Assembly; Connector Type A:micro D Ribbon Plug; Connector Type B:micro D Ribbon Plug; Cable Length:5m; No. of Contacts:26; Pitch Spacing:0.8mm RoHS Compliant: Yes
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
RS422-GE1 制造商:Carlo Gavazzi 功能描述:CABLE FOR GE90
RS422-MI1 制造商:Carlo Gavazzi 功能描述:CABLE FOR MITS MASTER
RS422-MI2 制造商:Carlo Gavazzi 功能描述:CABLE FOR MITS MASTER
RS422-MI3 制造商:Carlo Gavazzi 功能描述:CABLE FOR MITS MASTER
RS422-OM2 制造商:Carlo Gavazzi 功能描述:CABLE FOR OMRON MASTER
發(fā)布緊急采購，3分鐘左右您將得到回復。

采購需求

(若只采購一條型號，填寫一行即可)

發(fā)布成功！您可以繼續(xù)發(fā)布采購。也可以進入我的后臺，查看報價

發(fā)布成功！您可以繼續(xù)發(fā)布采購。也可以進入我的后臺，查看報價

*型號 *數(shù)量 廠商 批號 封裝
添加更多采購

我的聯(lián)系方式

*
*
*