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PDF AN-960 Data sheet ( Hoja de datos )
| Número de pieza | AN-960 | |
| Descripción | RS-485/RS-422 Circuit Implementation Guide | |
| Fabricantes | Analog Devices | |
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AN-960
APPLICATION NOTEwww.DataSheet4U.com
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
RS-485/RS-422 Circuit Implementation Guide
by Hein Marais
INTRODUCTION
Industrial and instrumentation applications (I&I) require
transmission of data between multiple systems often over
very long distances. The RS-485 bus standard is one of the
most widely used physical layer bus designs in I&I applica-
tions. The key features of RS-485 that make it ideal for use
in I&I communications applications are
• Long distance links—up to 4000 feet.
• Bidirectional communications possible over a single pair of
twisted cables.
• Differential transmission increases noise immunity and
decreases noise emissions.
• Multiple drivers and receivers can be connected on the
same bus.
• Wide common-mode range allows for differences in
ground potential between the driver and receiver.
• TIA/EIA-485-A allow for data rates of up to 10 Mbps.
Devices meeting the TIA/EIA-485-A specifications do not
have to operate over the entire range and are not limited
to 10 Mbps.
The purpose of this application note is to discuss the imple-
mentation of RS-485/RS-422 in an industrial environment.
Applications for RS-485/RS-422 include process control
networks; industrial automation; remote terminals; building
automation, such as heating, ventilation, air conditioning
(HVAC), security systems; motor control; and motion control.
TIA/EIA-485-A, the telecommunication industry’s most widely
used transmission line standard, describes the physical layer of
the RS-485 interface and is normally used with a higher-level
protocol, such as Profibus, Interbus, Modbus, or BACnet. This
allows for robust data transmission over relatively long distances.
The RS-422 physical layer is described in TIA/EIA-422-B. The
TIA/EIA-485-A standards are similar to those described in
TIA/EIA-422-B, and the values used to specify the drivers and
receivers in TIA/EIA-485-A standards are specified so that it
can meet both standards.
WHY USE DIFFERENTIAL DATA TRANSMISSION?
The main reason why RS-485 can communicate over long
distances is the use of differential or balanced lines. A com-
munication channel requires a dedicated pair of signal lines
to exchange information. The voltage on one line equals the
inverse of the voltage on the other line.
TIA/EIA-485-A designates the two lines in this differential pair
as A and B. Line A is more positive than Line B (VOA > VOB) on
the driver output if a logic high is received on the input of the
transmitter (DI = 1). If a logic low is received on the input of the
transmitter (DI = 0), the transmitter causes Line B to be more
positive than Line A (VOB > VOA). See Figure 1.
A
DI VOD
DE
VOA VOB B
VIA VIB
RO
RE
Figure 1. Differential Transmitter and Receiver
If Line A is more positive than line B (VIA − VIB > 200 mV)
on the input of the receiver, the receiver output is a logic high
(RO = 1). If Line B is more positive than Line A (VIB − VIA >
200 mV) on the input of the receiver, the receiver output is a
logic low (RO = 0).
Figure 1 shows that a differential signaling interface circuit
consists of a driver with differential outputs and a receiver with
differential inputs. This circuit has increased noise performance
because the noise coupling into the system is equal on both
signals. One signal emits the opposite of the other signal and
electromagnetic fields cancel each other. This reduces the
electromagnetic interference (EMI) of the system.
Rev. 0 | Page 1 of 12
1 page  
APPLICATION NOTE
TERMINATION
In a transmission line, there are two wires, one to carry the
currents from the driver to the receiver and another to provide
the return path back to the driver. RS-485 links are a little more
complicated because of the fact that they have two signal wires
that share a termination as well as a ground return path.
However, the basic principles of transmission lines are the same.
For reliable RS-485 and RS-422 communications, it is essential
that the reflections in the transmission line be kept as small as
possible. This can only be done by proper cable termination.
Reflections happen very quickly during and just after signal
transitions. On a long line, the reflections are more likely to
continue long enough to cause the receiver to misread logic
levels. On short lines, the reflections occur much sooner and
have no effect on the received logic levels.
In RS-422 applications there is only one driver on the bus and
if termination is to be used it must be placed at the end of the
cable near the last receiver. RS-485 applications require termin-
ation at the master node and the slave node furthest from the
master. Table 2 shows a comparison of different termination
techniques.
No Termination
The time required for a signal to propagate down the line to a
receiver determines if a line is considered a transmission line.
Physically long wires have longer propagation times, whereas
physically short wires have shorter propagation times. When
the propagation time is short relative to the data bit duration,
the effect on the signal quality is minimized. A cable is not seen
as a transmission line if the signal rise time is more than four
times the propagation delay of the cable.
Parallel Termination
When two or more drivers share a pair of wires, each end of
the link has a termination resistor equal to the characteristic
impedance of the cable. There should be no more than two
terminating resistors in the network regardless of how many
nodes are connected.
In a half-duplex configuration, both ends of the cable must be
terminated (see Figure 3). In a full duplex configuration only
the master receiver and most remote slave receiver need to be
terminated.
AC Termination
AC termination is used to reduce the power consumption of
idle links as well as to reduce ringing voltages. The negative
effect though is a reduction in cable length and bit rate. A
resistor and capacitor can be placed in series across the bus
(between A and B) as shown in Figure 5. The Capacitor CT is
selected by using the following formula:
2(One-Way Cable Delay (ps))
CT (pF) > Characteristic Impedance (Ω)
AN-960
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DIFFERENTIAL
DRIVER
A
DI
DIFFERENTIAL
RECEIVER
RO
RT
B RE
Figure 5. Parallel Termination
DIFFERENTIAL
DRIVER
A
DI
B
DIFFERENTIAL
RECEIVER
RT
CT
RO
RE
Figure 6. AC Termination
Table 2. Termination Advantages and Disadvantages
Termination Advantages
Disadvantages
None
Simple, low power
Suitable only for short
links with slow drivers
Parallel
Simple
High power
AC
Low power
Suitable only for low bit
rates and short links
Stub Length
Stub length should be much less than ¼ of a wavelength of the
frequency equal to the inverse of the bit period.
DATA RATE AND CABLE LENGTH
When high data rates are used, the application is limited to a
shorter cable. It is possible to use longer cables when low data
rates are used. The dc resistance of the cable limits the length of
the cable for low data rate applications by increasing the noise
margin as the voltage drop in the cable increases. The ac effects
of the cable limit the quality of the signal and limit the cable
length to short distances when high data rates are used.
Examples of data rate and cable length combinations vary from
90 kbps at 4000 feet to 10 Mbps at 15 feet for RS-422.
Figure 7 can be used as a conservative guide for cable length vs.
data rate.
10000
1000
100
10
10k
100k
1M
DATA RATE (bps)
Figure 7. Cable Length vs. Data Rate
10M
Rev. 0 | Page 5 of 12
5 Page 
APPLICATION NOTE
NOTES
AN-960
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Rev. 0 | Page 11 of 12
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| PDF Descargar | [ Datasheet AN-960.PDF ] | |
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