AN-727: iCoupler® Isolation in RS-485 Applications (Rev. 0) PDF

AN-727
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
iCoupler® Isolation in RS-485 Applications
by Sean Clark
INTRODUCTION
The RS-485 bus standard is one of the most widely used
physical layer bus designs. The RS-485 standard is specified to drive up to 32 driver/receiver pairs. Although no
maximum cable length is specified, lengths of 4000 meters
are possible. The versatility of the RS- 485 makes this
design popular for a wide range of applications, especially intersystem connections over long distances.
Because the RS- 485 system is typically used to connect
multiple systems, and is often run over very long cable
distances, isolation between the bus and each system
connected is critical. Digital isolation provides crucial
isolation and protection from overvoltage transients
between the RS- 485 cable network and the systems
connected to it. Digital isolation also eliminates ground
loops in the RS - 485 network. Digitally isolating the
RS- 485 bus from each system connected to the bus
reduces signal distortion and errors, and provides system and component protection from system and bus
voltage and ground mismatches.
The intention of this application note is to give the user
a brief overview of the RS- 485 system physical layer, as
well as an understanding of why isolation is so important to the system. This application note details how to
implement isolation in an RS- 485 system using Analog
Devices iCoupler products.
RS-485 OVERVIEW
RS- 485 is more properly known as EIA/TIA485, but is
commonly referred to by the older “Recommended
Standard” 485 designation. The RS- 485 specification
defines the physical layer only. Signal protocol is defined
by the user, or standards that define the protocol, and
specify RS- 485 for the physical layer.
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The RS-485 is found in products ranging from industrial systems to computers. Examples of protocols that specify this
bus design include the SCIS2 and SCIS3 (Small Computer
Systems Interface) busses, and the PROFIBUS (Process
Field Bus) high performance protocol.
Usable lengths are dependent upon system data speed
requirements. Examples of data rate and length combinations vary from 200 kbps at 1200 meters, to 12 Mbps at
100 meters for the PROFIBUS high performance RS- 485
bus. Note that the PROFIBUS also requires special high
performance RS- 485 drivers.
The RS-485 uses balanced differential signaling. RS-485
drivers send the data signal across two output lines. The
receiver determines the logic state by comparing these
two input signals to each other, rather than to a ground
reference. The receiver looks for a greater than 200 mV
difference between the A and B inputs for a valid logic
level. The RS- 485 drivers and receivers contain differential amplifiers and their circuits steer current between
the two differential signal lines.
The use of differential signaling imbues the system with
a high level of noise immunity when compared to singleended drive schemes, such as that used in the RS-232
specification.
All RS - 485 drivers also include an enable function to
allow the drivers to be placed in high impedance state.
The enable function allows multiple drivers to share one
bus and prevents bus contention problems. The driver
enable feature and the software protocol define the
arbitration procedure for line sharing between the drivers. The software protocol arbitrates between the drivers,
keeping all but one at a time in the inactive state. This
arbitration allows line sharing by up to 32 drivers.
AN-727
2-WIRE AND 4-WIRE CONFIGURATIONS
RS- 485 is specified to support up to 32 drivers and 32
receivers in a half-duplex, bidirectional, 2-wire, multidrop configuration on one bus. Each node on the line
contains a receiver and a driver. In this configuration,
all receiver and driver pairs share the same set of two
differential signal lines. (Figure 1) The 2-wire system can
be installed using just one twisted pair cable. This design
simplifies installation and keeps costs down. However,
the design requires that all drivers share the line, limiting
maximum data throughput speed.
TERMINATION AND BIASING NETWORKS
Termination is used to match the line end nodes to the
impedance value of the transmission line, reducing
or eliminating reflections. However, termination also
increases the line load. Also, since the termination is
connected only to the end nodes on the line, this can
make system modifications more difficult. Termination
is needed in high data rate and long line applications.
Although various termination designs can be used, the
most common is a resistor connected in parallel across
the differential lines (Figures 1 and 2).
A 4-wire RS-485 full-duplex design is also possible.
(Figure 2). In the 4-wire configuration, one node is the
master node that communicates to all other nodes. The
slave nodes communicate only with the master node.
Although more complex, this design greatly improves
data throughput rates.
Every stub line adds capacitance and can generate
reflections and disruptions to the signal on the RS- 485
bus. To minimize these effects, stub connections to all
receivers and transmitters on the line should be as short
as possible.
TERMINATION AT LINE ENDS ONLY
TERMINATION AT LINE ENDS ONLY
TX
TX
ENABLE
ENABLE
RX
RX
TX
TX
ENABLE
ENABLE
RX
RX
Figure 1. RS-485 2-Wire Multidrop, Half-Duplex Network
TERMINATION AT LINE ENDS ONLY
TERMINATION AT LINE ENDS ONLY
TX
RX
ENABLE
MASTER
RX
TX
TX
SLAVE
ENABLE
SLAVE
TX
ENABLE
ENABLE
RX
SLAVE
RX
Figure 2. RS-485 4-Wire Multidrop, Full-Duplex Network
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AN-727
A resistor biasing network, (also known as a fail-safe, or
idle line network) is necessary if the network is placed in
an idle condition where all drivers are in a high impedance state (Figure 3).
causes rapid changes in the ground potential. These
changes can generate a current flow through any nearby
lines to equalize the ground potential. Other induction
surge sources include electrostatic discharge (ESD) and
lightning strikes. These induced surges can result in
hundreds, or even thousands of volts of potential on
the line, and manifest themselves as transient current
and voltage surges.
The biasing resistors ensure that all receiver A inputs are
200 mV above the B inputs when no driver signal is present. This prevents receivers from going into oscillations.
Oscillations can appear on receiver outputs as erroneous
data. The biasing network does add loading to a system,
and values are in direct relation to the specific system
design parameters.
Thus, a remote node may receive a 5 V switching signal
superimposed on a high voltage level with respect to
the local ground. These uncontrolled voltages and currents can corrupt the signal, and can be catastrophic to
the device and system, causing damage or destruction
of the components connected to the bus, and resulting
in system failure. RS- 485 systems that run over long
distances and connect multiple systems are especially
susceptible to these events.
A
TX
B
ENABLE
A
RX
VCC
(+5V)
To protect against this potentially destructive energy, all
devices on the bus, and systems connected to the bus,
must be referenced to only one ground. Isolating the
RS- 485 system devices from each of the systems connected to the bus prevents ground loops and electrical
surges from destroying circuits.
B
Figure 3. RS-485 Fail-Safe Biasing Network
SYSTEM ISOLATION OVERVIEW
Unwanted currents and voltages on a cable bus connecting multiple systems have the potential to cause severe
problems. High voltages and currents can destroy components connected to the bus. These unwanted voltages
and currents come primarily from two sources: ground
loops and electrical line surges.
Isolation prevents ground loops because each system
connected to RS- 485 cable bus, and each RS- 485 circuit, has a separate and isolated ground. By referencing
each RS- 485 circuit only to one ground, ground loops
are eliminated.
Ground loops occur when a bus or system utilizes
multiple ground paths. It cannot be assumed that two
system grounds connected to the bus and separated by
hundreds or thousands of meters will be at the same
potential. Because these grounds are unlikely to be at the
same potential, current will flow between these points.
This unintended current flow can damage or destroy
components.
Isolation also allows the RS- 485 circuit reference voltage levels to rise and fall with any surges that appear
on the cable line. Allowing the circuit voltage reference
to move with surges, rather than being clamped to a
fixed ground, prevents devices from being damaged or
destroyed.
To accomplish system isolation, both the RS- 485 signal
lines and power supplies must be isolated. Power isolation is obtained through the use of an isolated dc to dc
power supply. Signal isolation is typically accomplished
with optocouplers, or with Analog Device’s innovative
iCouplers.
Electrical surges can be caused by many sources, these
surges are the result of currents coupled onto cable
lines through induction. Long cable lines in industrial
environments are especially susceptible to this phenomena. The operation of electric motors, in particular,
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AN-727
ISOLATION IMPLEMENTATION
The implementation of isolation is not overly complex.
However, the designer must consider several important
factors when implementing the isolation circuitry.
ISOLATION DEVICE SELECTION
System performance requirements have the most impact
on the selection of an isolation device. Other considerations include space constraints and cost.
Because digital isolators do not support the RS-485 standard, it is not possible to insert a digital isolator between
the RS- 485 receivers and drivers, and the RS- 485 cable.
Theoretically, transformers could be used to supply
isolation at that location. However, the slow speeds of
the bus would require large transformers, making this
solution impractical.
DATA RATE REQUIREMENTS
System data rate requirements are likely to be the
single most important parameter for device selection.
If a system uses high data rates, such as the high speed
PROFIBUS protocol, the minimum data rate speed
requirement of 9.6 Mbps will narrow the device selection
to the high performance products available. Conversely,
if the RS - 485 network runs at much lower data rate
speeds, the possible device selection options are more
numerous.
RS- 485 system signal path isolation is accomplished by
designing isolators into the digital signal path between
the RS- 485 driver and receiver, and the local system.
The isolator contains input and output circuits that are
electrically isolated from one another.
Device costs typically rise in proportion to data rate
performance. Therefore, a designer should take care
not to specify a device with more performance than is
required. However, low performance device selection
can make future system performance upgrades more costly and involved, because all devices not compatible with
upgraded system data speeds will require replacement.
To complete the isolation of the RS- 485 circuits from
the local system, a dc-to-dc isolated power converter
is required. The isolated power supply is used to supply
power to the local RS- 485 driver, receiver, and RS- 485
side of the isolator. The isolated power supply is typically
supplied from the local system.
The combination of digital isolators and an isolated
dc-to-dc power supply creates an effective protection
against surge damage and eliminates ground loops
(Figure 4).
SUPPLY
ISOLATED SUPPLY
VCC
LOCAL BUS/PROCESSOR
LOCAL VCC
VDD1
VDD2
GND1
GND2
VIA
VOA
ISOLATED SUPPLY
GROUND
TX
iCOUPLER
ADuM1301
VIB
VOB
VOC
VIC
NC
NC
VE1
GND1
ENABLE
RX
VE2
GND2
LOCAL GROUND
Figure 4. Isolated RS-485 Circuit: Receiver, Driver, iCoupler Signal Isolator, and Isolated DC Supply
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AN-727
Additional cost benefits of integrating as many channels
into one device include reduction in board space and
assembly costs. A lower device count results in smaller
boards. Also, lower device count typically results in a
less complex board layout. The combination of smaller
boards and less complex layout reduces board costs. In
addition, circuit board assembly costs typically decrease
proportionally as the number of devices required for the
board assembly process decreases. Therefore, designing
with fewer devices results in lower manufacturing costs.
SPACE REQUIREMENTS
Space constraints are a second area of concern that can
also limit a designer’s choices. Maximum dimension
requirements are a concern for virtually all applications. However, some implementations can be severely
space-limited. Fortunately, there are solutions for these
situations.
Solutions for systems where space is an issue include
the ADuM1301 and ADuM1401 iCouplers for isolation of
a half-duplex RS- 485 receiver and transmitter pair. The
ADuM1301 is a three-channel isolation device in a 16-lead
SOIC package, replacing three optocouplers and associated circuitry. The ADuM1401 is a four-channel isolation
device in a 16-lead SOIC package, taking the place of four
optocouplers and associated circuitry. The ADuM1401
is used in applications requiring separate control of the
receiver and transmitter enable functions.
ADI iCOUPLER PRODUCTS
ADI’s iCoupler device technology has created products
that possess distinct advantages for the system designer.
The unique technology results in a new option for implementing isolation that provides superior performance,
lower power consumption, higher reliability, and lower
component count, with cost characteristics comparable
with optocouplers.
For even more severely space-constrained systems, the
ADM2483 and ADM2486 are RS- 485 transceivers with
iCoupler isolation built in. These devices reduce part
count to one device, plus decoupling.
ADI iCOUPLER TECHNOLOGY OVERVIEW
ADI’s iCoupler technology provides isolation based on chip
scale transformers, rather than LEDs and photodiodes used
in optocouplers. By fabricating the transformers directly on
chip using wafer-level processing, iCoupler channels can
be integrated with other semiconductor functions at low
cost (Figure 5).
COST REQUIREMENTS
Cost constraints and concerns are a reality in virtually all
system design work, and therefore must be considered.
Cost considerations can have an effect on the design
choices for a system. As noted above, isolator device
cost rises in proportion with data rate performance.
Specifying a device with only the system performance
required can reduce costs.
The technology used in iCoupler design eliminates the
inefficient electro-optical conversions that take place in
optocouplers. This is because iCouplers eliminate the
LEDs used in optocouplers. Also, because channels are
fabricated entirely with wafer level processing, multiple
iCoupler channels can be easily integrated within a
single package. iCoupler technology provides increased
performance, reduced power consumption, smaller size,
increased reliability, and cost benefits.
Other cost issues include a consideration of the number
of devices used. The iCoupler device cost increases with
channel count. However, the cost per channel decreases
as the device channel count increases.
TOP COIL
BOTTOM COIL
POLYIMIDE LAYERS
Figure 5. Cross Section of iCoupler Configuration
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AN-727
Another distinct advantage of iCouplers over optcouplers
is the elimination of external components. In addition to
bypass capacitors, optocouplers require external discrete
devices to bias the output transistors and drive the LEDs—
whereas iCoupler devices require no external components
other than bypass capacitors. The iCoupler solution results
in less circuit complexity and lower cost.
As noted, ADI offers a wide selection of iCoupler products.
The combination of performance and channel configuration allows the system designer options for optimizing
system and device match. Figures 6, 7, and 8 illustrate
some of the configuration options for iCoupler integration. Figures 9 and 10 show the RS- 485 transceivers with
iCoupler isolation. Table 1 has also been included to allow
a comparison of product options including the number of
channels and data speed performance.
iCoupler products also incorporate unique refresh and
watchdog circuits. In the absence of logic transitions at the
input for more than 2 µs, a periodic set of refresh pulses,
indicative of the correct input state, is generated to ensure
dc correctness at the output. If the iCoupler output side circuit receives no pulses for more than about 5 µs, the input
side circuit is assumed to be unpowered or nonfunctional,
in which case the isolator output is forced to a default state
by the watchdog timer.
BYPASS CAPACITORS
iCoupler products need no external components other
than bypass capacitors. A bypass capacitor is strongly
recommended for the input and output supply pins.
The bypass capacitor value should be between 0.01 µF
and 0.1 µF. The total lead length between both ends
of the capacitor and the power supply pins should not
exceed 20 mm.
ADI iCOUPLER SELECTION
ADI’s iCoupler broad portfolio of products allow the
system designer to select a product ideally suited for the
design. The iCoupler device portfolio includes options
from one channel up to four channels. These options
include devices designed for bidirectional communication, which aids in flowthrough board design. iCoupler
devices are also available for a range of data rate performances, allowing the designer to select the perfect
product for the application.
OUTPUT ENABLE CONTROL
Many iCoupler products have Output Enable control pins,
to allow outputs to be placed into a high impedance state.
The outputs are in an active logic state when the Output
Enable pins are high or floating. The outputs are disabled
when the Output Enable pin is low. It is recommended
that the Output Enable pins be pulled to a known logic
level, either high or low, in noisy applications.
The iCoupler portfolio of features and options allows a
system to be designed with fewer devices, thus creating
a better match for the system data performance requirements (Table 1).
Also available are ADI’s RS-485 transceivers with iCoupler
isolation built in. These devices reduce part count and board
space requirements even further by combining the transceiver and isolation in one device. The devices are available
in two performance levels, the ADM2483, with 250 kbps
data rate, and the PROFIBUS-compatible ADM2486, with
20 Mbps data rate. (Table 1 and Figures 9 and 10).
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Figure 6. Half-Duplex Single-Channel System Isolation Using Three ADuM1100s. GND2 Pins Are Connected Internally.
VDD1 Pins Are Connected Internally. Either or Both Can Be Used For External Connection.
ISOLATED SUPPLY
VCC
LOCAL BUS/PROCESSOR
LOCAL VCC
VDD1
VDD2
GND1
GND2
VOA
VIA
ISOLATED SUPPLY
GROUND
TX
iCOUPLER
ADuM1301
VIB
VOB
VOC
VIC
NC
NC
VE1
GND1
ENABLE
RX
VE2
GND2
LOCAL GROUND
Figure 7. Half-Duplex Single-Channel System Isolation Using One ADuM1301. GND1 Pins Are Connected Internally.
GND2 Pins Are Connected Internally. Either or Both Can Be Used For External Connection.
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AN-727
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Figure 8. Full-Duplex Dual-Channel System Using One ADuM1200 and One ADuM1402
EXTERNAL ISOLATED
POWER SUPPLY
EXTERNAL ISOLATED
POWER SUPPLY
VDD1
VDD1
GND2
VDD2
GND2
DE
VDD2
TxD
RxD
RE
GND1
A
TxD
B
RxD
DE
GALVANIC ISOLATION
GALVANIC ISOLATION
RTS
RE
ADM2483
GND1
Figure 9. ADM2483 Isolated RS-485 Transceiver
A
B
ADM2486
Figure 10. ADM2486 Isolated RS-485 Transceiver
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Table I. iCoupler and Isolated RS-485 Transceiver Products
Max
Data
Rate,
5V
Number of
Channel
UL
Insulation
Rating
Channels
Configuration
(kV)
(Mbps) 5 V (ns) (°C)
Package
ADuM1100 ADuM1100AR
1
1/0
2.5
25
18
105
8-Lead, Narrow Body SOIC
ADuM1100BR
1
1/0
2.5
100
18
105
8-Lead, Narrow Body SOIC
ADuM1100UR
1
1/0
2.5
100
18
125
8-Lead, Narrow Body SOIC
ADuM120x ADuM1200AR
2
2/0
2.5
1
150
105
8-Lead, Narrow Body SOIC
ADuM1200BR
2
2/0
2.5
10
50
105
8-Lead, Narrow Body SOIC
ADuM1200CR
2
2/0
2.5
25
45
105
8-Lead, Narrow Body SOIC
ADuM1201AR
2
1/1
2.5
1
150
105
8-Lead, Narrow Body SOIC
ADuM1201BR
2
1/1
2.5
10
50
105
8-Lead, Narrow Body SOIC
ADuM1201CR
2
1/1
2.5
25
45
105
8-Lead, Narrow Body SOIC
ADuM130x ADuM1300ARW 3
3/0
2.5
1
100
105
16-Lead, Wide Body SOIC
ADuM1300BRW 3
3/0
2.5
10
50
105
16-Lead, Wide Body SOIC
ADuM1300CRW 3
3/0
2.5
90
32
105
16-Lead, Wide Body SOIC
ADuM1301ARW 3
2/1
2.5
1
100
105
16-Lead, Wide Body SOIC
ADuM1301BRW 3
2/1
2.5
10
50
105
16-Lead, Wide Body SOIC
ADuM1301CRW 3
2/1
2.5
90
32
105
16-Lead, Wide Body SOIC
ADuM140x ADuM1400ARW 4
4/0
2.5
1
100
105
16-Lead, Wide Body SOIC
ADuM1400BRW 4
4/0
2.5
10
50
105
16-Lead, Wide Body SOIC
ADuM1400CRW 4
4/0
2.5
90
32
105
16-Lead, Wide Body SOIC
ADuM1401ARW 4
3/1
2.5
1
100
105
16-Lead, Wide Body SOIC
ADuM1401BRW 4
3/1
2.5
10
50
105
16-Lead, Wide Body SOIC
ADuM1401CRW 4
3/1
2.5
90
32
105
16-Lead, Wide Body SOIC
ADuM1402ARW 4
2/2
2.5
1
100
105
16-Lead, Wide Body SOIC
ADuM1402BRW 4
2/2
2.5
10
50
105
16-Lead, Wide Body SOIC
ADuM1402CRW 4
2/2
2.5
90
32
105
16-Lead, Wide Body SOIC
ADM2483
3
RX / TX Enable
2.5
0.25
1000
85
16-Lead, Wide Body SOIC
ADM2486
3
RX / TX Enable
2.5
20
55
85
16-Lead, Wide Body SOIC
Product
ADM248x
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Model
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Max
Prop.
Delay
Max
Operating
Temp.
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AN04871–0–6/04(0)
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.
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