Introduction to RS485Introduction to RS485

Author: Zhu TaoAuthor: Zhu Tao

Date: 2008-10-24Date: 2008-10-24

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DefinitionDefinition[1][1]

RS485, namely EIA-485, is an electrical RS485, namely EIA-485, is an electrical specification of a specification of a two-wiretwo-wire, , half-duplexhalf-duplex, , multipointmultipoint serial communicationserial communication channel. channel.

supporting transmission in two directions sim

ultaneouslyThe transmission of data in both directions, but only one direction at a time.

Refers to a communications line (net

work) that provides a path from one location to many.

the process of sending data one bit at one time, sequentially, over a communication channel or comput

er bus.

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Use a differential balanced line over Use a differential balanced line over twisted pairtwisted pair

Can span a large distance (up to 1200 m)Can span a large distance (up to 1200 m)

Linear topologyLinear topology is the preferred is the preferred recommended wires arrangementrecommended wires arrangement

Not star or ring topology, because of

signal reflection or excessively low or high termination

impedance.

RS485 topology

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Often use the master-slave arrangementOften use the master-slave arrangement

Not specify or recommend any data Not specify or recommend any data protocol (i.e. the overlaying protocol)protocol (i.e. the overlaying protocol)

Can be extended to Can be extended to full-duplexfull-duplex using 4- using 4-wirewire

Components cost very lowComponents cost very low

NO integral method to detect data collision NO integral method to detect data collision (often be implemented using software)(often be implemented using software)

A full-duplex (or rarely double-duplex) system allows communication in both directions, and unlike half-duplex, allows this to happen simultaneousl

y.

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摘自摘自 [3][3]

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AdvantagesAdvantages

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AdvantagesAdvantages[2][2]

1.1. Small overhead (without defining the Small overhead (without defining the overlaying protocol, fault detect, etc.)overlaying protocol, fault detect, etc.)

2.2. Cheaper hardwareCheaper hardware3.3. Maximum data transmission speed is Maximum data transmission speed is

10Mbps10Mbps4.4. Has good noise immunityHas good noise immunity5.5. Maximum 256 transceivers are allowed Maximum 256 transceivers are allowed

to connect to the busto connect to the bus

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DisadvantagesDisadvantages[2][2]

1.1. Only one node can transmit data to the bus at Only one node can transmit data to the bus at any one moment and all other nodes must be any one moment and all other nodes must be in receiving status, otherwise the transmission in receiving status, otherwise the transmission will fail (when 2 or more nodes transmit data will fail (when 2 or more nodes transmit data simultaneously).simultaneously).

2.2. A special communication cable is needed. A special communication cable is needed. And the maximum communication distance And the maximum communication distance depends on the type of the cable.depends on the type of the cable.

3.3. Need software to ensure no over 2 nodes Need software to ensure no over 2 nodes transmit data simultaneously. transmit data simultaneously.

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RS485RS485 is used as the electrical layer for is used as the electrical layer for many well known interface standards, many well known interface standards, including Profibus and Modbus. Therefore including Profibus and Modbus. Therefore RS485RS485 will be in use for many years in the will be in use for many years in the future. future.

And in the TCN area, the train control And in the TCN area, the train control system can be abstracted to a master-system can be abstracted to a master-slaves model. So RS485 can be used.slaves model. So RS485 can be used.

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1.1. http://en.wikipedia.org/wiki/EIA-485http://en.wikipedia.org/wiki/EIA-485

2.2. http://www.metering.com/node/12618http://www.metering.com/node/12618

3.3. http://www.lammertbies.nl/comm/info/RS-485.hhttp://www.lammertbies.nl/comm/info/RS-485.htmltml

4.4. 基于基于 RS485 RS485 的的 25T 25T 客车电源串行通信系统客车电源串行通信系统 .pd.pdff

5.5. 422 and 485 Standards Overview.pdf422 and 485 Standards Overview.pdf

6.6. The Art and Science of RS-485.pdfThe Art and Science of RS-485.pdf

7.7. EIA TIA-485-A an Overview.pdfEIA TIA-485-A an Overview.pdf

More can be seen in the next week.More can be seen in the next week.

Thanks a lot!Thanks a lot!