rs485简要介绍

admin管理员组

文章数量:1567260

2024年7月3日发(作者：)

Introduction to RS485

RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices.

RS232 is without doubt the best known interface, because this serial interface is implemented on almost

all computers available today. But some of the other interfaces are certainly interesting because they can

be used in situations where RS232 is not appropriate. We will concentrate on the RS485 interface here.

RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data communication

equipment at a maximum speed of 20 kbps with a maximum cable length of 50 feet. This was sufficient

in the old days where almost all computer equipment were connected using modems, but soon after

people started to look for interfaces capable of one or more of the following:

•

•

•

•

Connect DTE's directly without the need of modems

Connect several DTE's in a network structure

Ability to communicate over longer distances

Ability to communicate at faster communication rates

RS485 is the most versatile communication standard in the standard series defined by the EIA, as it

performs well on all four points. That is why RS485 is currently a widely used communication interface

in data acquisition and control applications where multiple nodes communicate with each other.

Differential signals with RS485:

Longer distances and higher bit rates

One of the main problems with RS232 is the lack of immunity for noise on the signal lines. The

transmitter and receiver compare the voltages of the data- and handshake lines with one common zero

line. Shifts in the ground level can have disastrous effects. Therefore the trigger level of the RS232

interface is set relatively high at ±3 Volt. Noise is easily picked up and limits both the maximum distance

and communication speed. With RS485 on the contrary there is no such thing as a common zero as a

signal reference. Several volts difference in the ground level of the RS485 transmitter and receiver does

not cause any problems. The RS485 signals are floating and each signal is transmitted over a Sig+ line

and a Sig- line. The RS485 receiver compares the voltage difference between both lines, instead of the

absolute voltage level on a signal line. This works well and prevents the existence of ground loops, a

common source of communication problems. The best results are achieved if the Sig+ and Sig- lines are

twisted. The image below explains why.

Noise in straight and twisted pair cables

In the picture above, noise is generated by magnetic fields from the environment. The picture shows the

magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field.

In the straight cable, all noise current is flowing in the same direction, practically generating a looping

current just like in an ordinary transformer. When the cable is twisted, we see that in some parts of the

signal lines the direction of the noise current is the oposite from the current in other parts of the cable.

Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.

Shielding—which is a common method to prevent noise in RS232 lines—tries to keep hostile magnetic

fields away from the signal lines. Twisted pairs in RS485 communication however adds immunity which

is a much better way to fight noise. The magnetic fields are allowed to pass, but do no harm. If high noise

immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded

twisted pair and FTP, foiled twisted pair networking cables. Differential signals and twisting allows

RS485 to communicate over much longer communication distances than achievable with RS232. With

RS485 communication distances of 1200 m are possible.

Differential signal lines also allow higher bit rates than possible with non-differential connections.

Therefore RS485 can overcome the practical communication speed limit of RS232. Currently RS485

drivers are produced that can achieve a bit rate of 35 mbps.

Characteristics of RS485 compared to RS232, RS422 and

RS423

Characteristics of RS232, RS422, RS423 and RS485

Differential差动

Max number of drivers驱动

Max number of receivers接收端

RS232

no

1

1

RS423

no

1

10

RS422

yes

1

10

RS485

yes

32

32

half duplex半双工

Modes of operation 操作模式

Network topology网络布局

full duplex全双工

point-to-point

half

duplex

multidrop

half

duplex

multidrop

half

duplex

multipoint

Max distance (acc. standard)

Max speed at 12 m

Max speed at 1200 m

Max slew rate转换速率

Receiver input resistance输入阻抗

Driver load impedance驱动阻抗

Receiver input sensitivity接收灵敏

度

Receiver input range输入范围

Max driver output voltage

Min driver output voltage (with

load)

15 m

20 kbs

(1 kbs)

30 V/μs

3..7 kΩ

3..7 kΩ

1200 m

100 kbs

1 kbs

adjustable

≧ 4 kΩ

≧ 450 Ω

1200 m

10 Mbs

100 kbs

n/a

≧ 4 kΩ

100 Ω

1200 m

35 Mbs

100 kbs

n/a

≧ 12 kΩ

54 Ω

±3 V

±15 V

±25 V

±200 mV

±12 V

±6 V

±200 mV

±10 V

±6 V

±200 mV

–7..12 V

–7..12 V

±5 V ±3.6 V ±2.0 V ±1.5 V

What does all the information in this table tell us? First of all we see that the speed of the differential

interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423. We also

see that there is a maximum slew rate defined for both RS232 and RS423. This has been done to avoid

reflections of signals. The maximum slew rate also limits the maximum communication speed on the line.

For both other interfaces—RS422 and RS485—the slew rate is indefinite. To avoid reflections on longer

cables it is necessary to use appropriate termination resitors.

We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the

signal level is lower for the faster interfaces. Because of this RS485 and the others can be used in

situations with a severe ground level shift of several volts, where at the same time high bit rates are

possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.

Interesting is, that RS232 is the only interface capable of full duplex communication. This is, because on

the other interfaces the communication channel is shared by multiple receivers and—in the case of

RS485—by multiple senders. RS232 has a separate communication line for transmitting and receiving

which—with a well written protocol—allows higher effective data rates at the same bit rate than the other

interfaces. The request and acknowledge data needed in most protocols does not consume bandwidth on

the primary data channel of RS232.

Network topology with RS485

Network topology is probably the reason why RS485 is now the favorite of the four mentioned interfaces

in data acquisition and control applications. RS485 is the only of the interfaces capable of

internetworking multiple transmitters and receivers in the same network. When using the default RS485

receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. Currently

available high-resistance RS485 inputs allow this number to be expanded to 256. RS485 repeaters are

also available which make it possible to increase the number of nodes to several thousands, spanning

multiple kilometers. And that with an interface which does not require intelligent network hardware: the

implementation on the software side is not much more difficult than with RS232. It is the reason why

RS485 is so popular with computers, PLCs, micro controllers and intelligent sensors in scientific and

technical applications.

RS485 network topology

In the picture above, the general network topology of RS485 is shown. N nodes are connected in a

multipoint RS485 network. For higher speeds and longer lines, the termination resistances are

necessary on both ends of the line to eliminate reflections. Use 100 Ω resistors on both ends. The RS485

network must be designed as one line with multiple drops, not as a star. Although total cable length

maybe shorter in a star configuration, adequate termination is not possible anymore and signal quality

may degrade significantly.

RS485 functionality

And now the most important question, how does RS485 function in practice? Default, all the senders on

the RS485 bus are in tri-state with high impedance. In most higher level protocols, one of the nodes is

defined as a master which sends queries or commands over the RS485 bus. All other nodes receive

these data. Depending of the information in the sent data, zero or more nodes on the line respond to the

master. In this situation, bandwidth can be used for almost 100%. There are other implementations of

RS485 networks where every node can start a data session on its own. This is comparable with the way

ethernet networks function. Because there is a chance of data collosion with this implementation, theory

tells us that in this case only 37% of the bandwidth will be effectively used. With such an implementation

of a RS485 network it is necessary that there is error detection implemented in the higher level protocol

to detect the data corruption and resend the information at a later time.

There is no need for the senders to explicity turn the RS485 driver on or off. RS485 drivers

automatically return to their high impedance tri-state within a few microseconds after the data has been

sent. Therefore it is not needed to have delays between the data packets on the RS485 bus.

RS485 is used as the electrical layer for many well known interface standards, including Profibus and

Modbus. Therefore RS485 will be in use for many years in the future.

本文标签： 模式网络速率驱动输入