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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： | 36/44頁 |
| 文件大小： | 628K |
| 代理商： | RS-485 |
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RS-422/485 Application Note
33
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH (815) 433-5100 -- FAX (815) 434-7094
Chapter 5: Software
Introduction
RS-422 and RS-485 are hardware specifications. Software protocol is not
discussed in either specification. It is up to the system designer to define a
protocol suitable for their system. This chapter we will not attempt to define a
protocol standard, but will explain some of the issues that should be considered
by the system designer, whether writing or purchasing software.
RS-422 Systems
RS-422 system software differs little from the familiar point-to-point RS-
232 communication systems. RS-422 is often used to simply extend the
distance between nodes over the capabilities of RS-232. RS-422 can also be
used as the master node in a four wire master-slave network described later in
this chapter. When selecting or writing software for RS-422 systems the
designer should be aware of the signals being used by the hardware in the
system. Many RS-422 systems do not implement the hardware handshake lines
often found in RS-232 systems due to the cost of running additional conductors
over long distances.
RS-485 Driver Control
The principle difference between RS-422 and RS-485 is that the RS-485
driver can be put into a high impedance, tristate mode, which allows other
drivers to transmit over the same pair of wires. There are two methods of
tristating an RS-485 driver. The first method is to use a control line, often the
RTS handshake line, to enable and disable the driver. This requires that the
host software raise the RTS line before beginning a transmission to enable the
driver, then lower the RTS line after the completion of the transmission. Since
only a single RS-485 driver can be enabled on a network at one time it is
important that the driver is disabled as quickly as possible after transmission to
avoid two drivers trying to control the lines simultaneously, a condition called
line contention. Under some operating systems it can be difficult to lower RTS
in a timely manner and this method of driver control should be avoided
altogether.
The second method of RS-485 driver control we refer to as Automatic Send
Data Control. This type of control involves special circuitry that senses when
data is being transmitted and automatically enables the driver as well as
disabling the driver within one character length of the end of transmission.
This is the preferred method of driver control since it reduces software
overhead and the number of potential pitfalls for the programmer.
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相關代理商/技術參數 |
參數描述 |
|---|---|
| RS485-232 | 制造商:PR 功能描述: |
| RS485-C1(AS) | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:RS232/RS485 CONV (AS) |
| 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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