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參數資料
| 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： | 22/44頁 |
| 文件大?。?/td> | 628K |
| 代理商： | RS-485 |
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RS-422/485 Application Note
19
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Example 2. 32 node, RS-485 network without termination
Each RS-485 node has a load impedance of 12K
. 32 nodes in parallel
gives a total load of 375
. In order to maintain at least 200 mV across
375
we need a current of 0.53 mA. To generate this current from a 5V
supply requires a total resistance of 9375
maximum. Since 375
of this
total is in the receiver load, our bias resistors must add to 9K
or less.
Notice that very little bias current is required in systems without termination.
Bias resistors can be placed anywhere in the network or can be split
among multiple nodes. The parallel combination of all bias resistors in a
system must be equal to or less than the calculated biasing requirements.
B&B Electronics uses 4.7K
bias resistors in all RS-485 products. This
value is adequate for most systems without termination. The system designer
should always calculate the biasing requirements of the network. Symptoms
of under biasing range from decreased noise immunity to complete data
failure. Over biasing has less effect on a system, the primary result is
increased load on the drivers. Systems using port powered RS-232 to RS-485
converters can be sensitive to over biasing.
Extending the Specification
Some systems require longer distances or higher numbers of nodes than
supported by RS-422 or RS-485. Repeaters are commonly used to overcome
these barriers. An RS-485 repeater such as B&B Electronics’ 485OP can be
placed in a system to divide the load into multiple segments. Each “refreshed”
signal is capable of driving another 4000 feet of cable and an additional 31 RS-
485 loads.
Another method of increasing the number of RS-485 nodes is to use
low load type RS-485 receivers. These receivers use a higher input impedance
to reduce the load on the RS-485 drivers to increase the total number of nodes.
There are currently half and quarter load integrated circuit receivers available,
extending the total allowable number of nodes to 64 and 128.
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相關代理商/技術參數 |
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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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