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AN-960 (Analog Devices) : Total 12 Pages

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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.
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AN-960
TABLE OF CONTENTS
Introduction ...................................................................................... 1 
Why Use Differential Data Transmission? .................................... 1 
RS-485 or RS-422?............................................................................ 3 
RS-422 ................................................................................................ 3 
RS-485 and the Unit Load Concept ............................................... 3 
Half-Duplex RS-485 ......................................................................... 4 
Full-Duplex RS-485.......................................................................... 4 
Termination....................................................................................... 5 
No Termination ........................................................................ 5 
Parallel Termination................................................................. 5 
APPLICATION NOTE
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AC Termination.........................................................................5 
Stub Length ................................................................................5 
Data Rate and Cable Length ........................................................5 
Fail-Safe Biasing.................................................................................6 
Differential Input Threshold Voltage of a Receiver..............6 
Open Fail-Safe ...........................................................................6 
True Fail-Safe Receivers ...........................................................7 
Isolation ..............................................................................................8 
Transient Overvoltage Stress Protection ........................................9 
References...........................................................................................9 
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APPLICATION NOTE
RS-485 OR RS-422?
RS-422 is specified as a simplex multidrop standard, which
means that only one driver and up to ten receivers can be
connected to the same bus. If more than one driver needs to
be connected on the same bus, then RS-485 is recommended.
RS-485 is specified as a multipoint standard, which means up
to 32 transceivers can be connected on the same bus.
Figure 2 shows a typical RS-422 interface circuit. Although
an RS-485 circuit may appear similar, the main difference is
in the bus architecture. Figure 3 shows a typical RS-485 appli-
cation circuit.
RS-422
The RS-422 standard specifies data rates up to 10 Mbps and
line lengths of up to 4000 feet. A single driver can drive a
transmission line with up to 10 receivers. The common-mode
voltage (VCM) is defined as the mean voltage of A and B
terminals with respect to signal ground (VCM = (VIA + VIB)/2).
The RS-422 receivers can withstand a common-mode
voltage (VCM) of ±7 V. When all ten receivers are placed on the
bus, a maximum load condition occurs. The input impedance
of the RS-422 receiver must be larger than or equal to 4 kΩ.
RS-485 AND THE UNIT LOAD CONCEPT
The input impedance of the RS-485 receiver is specified as
larger than or equal to 12 kΩ. This impedance is defined as
having one unit load (UL). The RS-485 specification specifies
the capability to sustain up to 32 ULs.
Some RS-485 receivers are specified as having ¼ UL or
⅛ UL. A receiver specified to have ¼ UL means that the
receiver only loads the bus by ¼ of the standard UL and,
therefore, 4 times as many of these receivers can be connected
to the bus (4 × 32 = 128 nodes).
RO RO RO
AN-960
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Similarly, if an RS-485 receiver is specified to have ⅛ UL,
the receiver only loads the bus by ⅛ of the standard UL and,
therefore, 8 times as many of these receivers can be connected
to the bus (8 × 32 = 256 nodes). See Table 1 for UL and receiver
input impedance details.
Many of the RS-485 transceivers characteristics are the same as
for RS-422. The common-mode voltage range for RS-485 is
expanded to −7 V to +12 V. The RS-485 transceivers must
withstand this common-mode voltage range while tristated
(disconnected from the bus).
An RS-485 system must have a driver that can be disconnected
from the transmission line when a particular node is not
transmitting. The DE (RTS) pin on the RS-485 transceiver
enables the driver when a logic high is set to DE (DE = 1).
Setting the DE pin to low (DE = 0) puts the driver in a tristate
condition. This effectively disconnects the driver from the
bus and allows other nodes to transmit over the same twisted
pair cable.
RS-485 transceivers also have an RE pin that enables/disables
the receiver. The DE and RE pins combined allow some RS-485
transceivers to be put into a low power shutdown mode. This is
important for battery-powered applications.
Table 1. UL Receiver Input Impedance
Unit Load No. of Nodes Min. Receiver Input Impedance
1 32
12 kΩ
½ 64
24 kΩ
¼ 128
256
48 kΩ
96 kΩ
RO RO
R1 R2 R3 R4 R5
DI D
RT R10
R6 R7 R8 R9
RO
RO RO RO RO
Figure 2. Typical RS-422 Interface Circuit
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AN-960
HALF-DUPLEX RS-485
Half-duplex RS-485 links have multiple drivers and receivers on
the same signal path. This is the reason why RS-485 transceivers
must have driver/receiver enable pins enabling only one driver
to send data at a time. See Figure 3 for a half-duplex bus config-
uration. This configuration is also known as a 2-wire RS-485
network connected in a multipoint configuration and allows for
data transmission in both directions, but only in one direction
at a time.
APPLICATION NOTE
FULL-DUPLEX RS-485
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Figure 4 shows an example of an RS-485 bus connected in
a full-duplex bus configuration. This configuration is also
known as a 4-wire RS-485 network connected in a multipoint
master/slave configuration. Full-duplex RS-485 allows for
simultaneous communication in both directions between
master and slave nodes.
RO R
RE
DE
DI D
A
RT
B
GND
AB
AB
R RO
A RE
RT DE
B
D DI
GND
R
....
R
D GND
D GND
RO RE DE DI
RO RE DE DI
Figure 3. Half-Duplex RS-485 Bus Configuration
MASTER
RO R
RE
DE
DI D
A
RT
B
Z
Y
GND
A BZ Y
A BZ Y
SLAVE
Y
D
Z
DE
DI
A
RT R
B
RO
RE
GND
GND
SLAVE
R
.......
R
D
GND
SLAVE
D
RO RE DE DI
RO RE DE DI
Figure 4. Full Duplex RS-485 Bus Configuration
Rev. 0 | Page 4 of 12





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