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485总线网络资料全集中

485总线

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这是我从网络上搜集到的所有关于485的资料，全部制成PDF。想了解485的朋友不用再去搜索了！

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RS485 资料收集

第 1 页共 57 页

EIA-485

From Wikipedia, the free encyclopedia

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In telecommunications

, EIA-485 (formerly RS-485 or RS485) is an

electrical

specification of a two-wire,[1] half-duplex, multipoint serial

communications channel. Since it uses a differential

balanced line

over twisted pair

(like EIA-422), it can span relatively large

distances (up to 4,000 feet (1,200 m)).

This standard is now administered by the TIA

and is titled

TIA-485-A Electrical Characteristics of Generators and Receivers

for Use in Balanced Digital Multipoint Systems

(ANSI/TIA/EIA-485-A-98) (R2003), indicating that the standard was

re-affirmed without technical changes in 2003.

[edit

] Overview

EIA-485 only specifies electrical characteristics of the driver and

the receiver. It does not specify or recommend any data

protocol.

EIA-485 enables the configuration

of inexpensive local networks

and multidrop communications links. It offers high data

transmission speeds (35 Mbit/s up to 10 m and 100 kbit/s at

1200 m). Since it uses a differential

balanced line over twisted pair

(like EIA-422

), it can span relatively large distances (up to 4000 feet

or just over 1200 metres

RS485 资料收集

第 2 页共 57 页

In contrast to EIA-422, which has a single driver circuit which

cannot be switched off, EIA-485 drivers need to be put in transmit

mode explicitly by asserting a signal

to the driver. This allows

EIA-485 to implement linear topologies

using only two wires. The

equipment located along a set of EIA-485 wires are

interchangeably called nodes, stations and devices. [2]

The recommended arrangement of the wires is as a connected

series of point-to-point (multidropped) nodes, a line or bus, not a

star, ring, or multiply-connected network. Ideally, the two ends of

the cable will have a termination resistor connected across the two

wires. Without termination resistors, reflections of fast driver edges

can cause multiple data edges that can cause data corruption.

Termination resistors also reduce electrical noise sensitivity due to

the lower impedance, and bias resistors (see below) are required.

The value of each termination resistor should be equal to the cable

impedance (typically, 120 ohms for twisted pairs).

Star and ring topologies are not recommended because of signal

reflections or excessively low or high termination impedance. But if

a star configuration is unavoidable, such as when controlling

multiple pan-tilt-zoom video cameras from a central video

surveillance hub, special EIA-485 star/hub repeaters are available

which bidirectionally listen for data on each span and then

RS485 资料收集

第 3 页共 57 页

retransmit the data onto all other spans.

Somewhere along the set of wires, powered resistors are

established to bias each data line/wire when the lines are not being

driven by any device. This way, the lines will be biased to known

voltages and nodes will not interpret the noise from undriven lines

as actual data; without biasing resistors, the data lines float in such

a way that electrical noise sensitivity is greatest when all device

stations are silent or unpowered.[citation needed

]

[edit

] Master slave arrangement

Often in a master-slave arrangement when one device dubbed "the

master" initiates all communication activity, the master device itself

provides the bias and not the slave devices. In this configuration,

the master device is typically centrally located along the set of

EIA-485 wires, so it would be two slave devices located at the

physical end of the wires that would provide the termination. The

master device would provide termination if it itself was located at a

physical end of the wires, but that is often a bad design[citation

needed] as the master would be better located at a halfway point

between the slave devices. Note that it is not a good idea to apply

the bias at multiple node locations, because, by doing so, the

effective bias resistance is lowered, which could possibly cause a

violation of the EIA-485 specification and cause communications to

RS485 资料收集

第 4 页共 57 页

malfunction. By keeping the biasing with the master, slave device

design is simplified and this situation is avoided.

[edit

] Full duplex operation

EIA-485, like EIA-422 can be made full-duplex

by using four wires.

Since EIA-485 is a multi-point specification, however, this is not

necessary in many cases. EIA-485 and EIA-422 can interoperate

with certain restrictions.

Converters between EIA-485 and other formats are available to

allow a personal computer

to communicate with remote devices. By

using "Repeaters" and "Multi-Repeaters" very large RS485

networks can be formed. The Application Guidelines for

TIA/EIA-485-A has one diagram called "Star Configuration. Not

recommended." Using an RS485 "Multi-Repeater" can allow for

"Star Configurations" with "Home Runs" (or multi-drop) connections

similar to Ethernet Hub/Star implementations (with greater

distances). Hub/Star systems (with "Multi-Repeaters") allow for very

maintainable systems, without violating any of the RS485

specifications. Repeaters can also be used to extend the distance

or number of nodes on a network.

[edit

] Applications

EIA-485 signals are used in a wide range of computer and

automation systems. In a computer system, SCSI

-2 and SCSI-3

RS485 资料收集

第 5 页共 57 页

use this specification to implement the physical layer for data

transmission between a controller and a disk drive. EIA-485 is used

for low-speed data communications in commercial aircraft cabins

vehicle bus

. It requires minimal wiring, and can share the wiring

among several seats, reducing weight.

EIA-485 is used as the physical layer underlying many standard

and proprietary automation protocols used to implement Industrial

Control Systems, including the most common versions of Modbus

and Profibus

. These are used in programmable logic controllers

and on factory floors. Since it is differential, it resists

electromagnetic interference from motors and welding equipment.

In theatre and performance venues, EIA-485 signals are used to

control sound systems and lighting. The EIA-485 link is typically

implemented over standard XLR

cables more usually used for

microphones, and so can be run between stage and control desk

without laying special cables. Theatrical and disco lighting is

controlled with the DMX

protocol.

EIA-485 also is used in Building automation

as the simple bus

wiring and long cable length is ideal for joining remote devices. It

may be used to control video surveillance systems or to

interconnect security control panels and devices such as access

control card readers.

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yangyuanhui19851212

2012-06-14

虽然能识别串口了，但是串口怎么也打不开，不知道为什么
l87522

2012-11-12

的确是楼主收集的资料，属于485大杂烩，百度百科、广州周立功单片机发展有限公司的《RS-485 选型及应用指南》，从标准到应用都有，辛苦了。
dbayj

2013-01-30

对我很有帮助，谢谢