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接口應用筆記
文件頁數(shù): 21/44頁
文件大?。?/td> 628K
代理商: RS-485
18
RS-422/485 Application Note
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Figure 2.4 Transceiver with Bias Resistors
Example 1. 10 node, RS-485 network with two 120
termination
resistors
Each RS-485 node has a load impedance of 12K
. 10 nodes in parallel
give a load of 1200
. Additionally, the two 120
termination resistors
result in another 60
load, for a total load of 57
. Clearly the termination
resistors are responsible for a majority of the loading. In order to maintain at
least 200mV between the B and A line, we need a bias current of 3.5 mA to
flow through the load. To create this bias from a 5V supply a total series
resistance of 1428
or less is required. Subtract the 57
that is already
part of the load, and we are left with 1371
. Placing half of this value as a
pullup to 5V and half as a pulldown to ground gives a maximum bias resistor
value of 685
for each of the two biasing resistors.
Bias Resistor
Bias Resistor
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