Transient Threats in RS-485.pdf

7/27/2019 Transient Threats in RS-485.pdf

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Handling Transient Threats in RS-485 Systems

BU-CA085-300-WH

P40-G240-WH

MOV-10D201K

2031-23-SM-RPLF

CDSO23-SM712

ISP4015L1BJ

10/11 e/CPK1146

odays process control architectures and closed-loop systems are developed with numerous

serial drops using point-to-point, multi-drop, or multipoint systems. Te distances between serial

drops can range rom several meters to over a thousand meters. Data transer protocols such

as IA/EIA-485, IA/EIA-232, IA/EIA-422, eld buses, CAN, Probus, Interbus, and LVDS,

enable communication with various remote devices such as sensors, actuators, or monitors

connected throughout the system. Te RS-485 dierential interace is useul in long distance

communications where immunity to commonmode noise is prevalent, typically ound in theprocess control actory setting. Process control systems must be designed with robust protection

needed during installation (handling) and commissioning (miswiring), with high expectation

o surviving signicant transients (system transients/lightning). Tis application note describes

circuit protection solutions that can be implemented to meet a variety o transient standards or

RS-485 serial device ports. Bourns circuit protection evaluation boards have been introduced

to assist the system designer in developing solutions utilizing the Bourns BU High-Speed

Protector (HSP), Metal Oxide Varistor (MOV), Gas Discharge ube (GD), and ransient

Voltage Suppression (VS) diode products.

Introduction to RS-485As a standard or industrial communications, RS-485 eatures ast data rates that achieve 35 Mbps

or short distances and 100 kbps over longer distances. It is not unusual to encounter miswiring

and short circuit aults, especially when covering longer distances. ransients and surges also can

be generated by induced energy on the data lines due to environmental conditions or switching o

heavy inductive loads. Any o these issues can degrade the serial ports data transer perormance.

RS-485 commonly is deployed in building automation, security system controls, and other

industrial communication applications. One common application o RS-485 that illustrates its

sensitivity is an elevator control system. With cables in motion, and large motors and inductors in

proximity, rugged communication systems are required.

RS-485 systems are inherently more robust than many other networks because the signal is

delivered dierentially between two wires, relative to a third reerence voltage. Te reerence

voltage is oen, but not always, the local earth potential. Tis provides substantial immunity to

common mode noise such as ground noise and induced noise rom nearby motors, solenoids

and transormers. Te interace uses voltages that are high by communication system standards,

also making it more robust than networks using other protocols. For instance, Ethernet signals

are about 2 V and the popular USB standard communicates at a maximum o 5 V, though much

internal communication in a computer is even lower. RS-485, on the other hand, allows or

common mode voltages rom -7 V to +12 V. Te voltage drop between distant nodes can be

accommodated without adding expensive equipment along the path.


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able 1: RS-485 Interace Specifcations

Dierential yes

Max. number of drivers 32

Max. number of receivers 32

Modes of operation half duplex

Network topology multipoint

Max. distance 1200 m

Max. speed at 12 m 32 Mbps

Max. speed at 1200 m 100 kbps

Receiver input sensitivity +/- 200 mV

Receiver input range -7 V to +12 V

Max. driver output voltage -7 V to +12 V

Max. driver output voltage with load +/- 1.5 V

RS-485 Port Interace RequirementsDisplayed in table 1 are the interace specications or RS-485 ports. Key electrical parameterso -7 V to +12 V operating voltage with 32 Mbps data transer at distances o 1200 meters

are included. A ully compliant RS-485 port must meet IEC 61000-4-2 (ESD), IEC 61000-4-4

(EF) and IEC 61000-4-5 (Surge) standards.

Every serial port design ollows the nominal recommended voltage range or the bus pins.

RS-485 denes how much protection the device has beyond the 7 V to +12 V. I the

environment demands that the bus survives up to 24 V, then external protection must be

added or adapted to meet compliance standards. Bourns oers various circuit protection

technologies or overcurrent and overvoltage protection. Te selection o a device or

combination o devices depends on the requirements o the application and the specications

o other devices such as the serial bus driver.


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P40-G240-WH

MOV-10D201K

2031-23-SM-RPLF

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10/11 e/CPK1146

Protection or RS-485 Ports

A general protection scheme or an RS-485 network includes a VS diode array such as the BournsModel CDSO23-SM712 device on each node. Tis single-stage bidirectional device provides

asymmetrical overvoltage protection or the RS-485 transceiver across the entire -7 V to +12 V

common mode voltage range. For RS-485 systems with limited exposure, this VS diode array may

be all that is needed or reliable operation.

Efective Circuit Protection in ActionTe single-stage VS diode solution will provide eective clamping o the transient signal up to

the peak impulse current, IPP, o the VS diode. IPP is the maximum current the VS diode can be

subjected to without ailure. As the VS diode is exposed to higher transient voltages, concern orexceeding this devices current limitation rises as well. Current limiting must be used to keep IPP

within its rated limit. A VS diode with series resistance Rx, shown in gure 1, is eective or low

level transients. As the transient voltage increases, the power rating (and physical size) o the VS

diode and series resistance also must increase to limit the VS diodes current below the devices

IPP (peak pulse current rating). Te transient energy will be a unction o the transient pulse

width, duration, and slew rate (dv/dt). Most VS diode data sheets will provide the PPP rating

or either 8/20 s or 10/1000 s surges. In addition, VS diode data sheets may provide a log-log

graph describing the diode perormance when exposed to longer or shorter transient pulses.

As the transient energy increases, the limitation o the single-stage solution becomes apparent.

For example, i the series resistance Rx is increased to limit the maximum current to the VS

diode, then the voltage drop during normal operation o the communications loop will aect

signal transition levels. Te communication loop distance is shortened as a result o the increased

resistance. Te required size and power dissipation o the Rx resistor increases as the magnitude

o the transient voltage and current increases and a practical solution is no longer achievable. As

we will discuss, the three-stage transient protection scheme oers the capability to handle large

transients with minimal impact on the length o the communication link, occupying a small area

on the printed circuit board (PCB).

Figure 1: VS Diode Plus Resistor

RS485

Transceiver Itransient

Rx

CDSOT23-SM712

TVS Diode


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Efective Circuit Protection in Action (Continued)

Te three-stage protection scheme, gure 2, provides an optimal approach when protecting the RS-485 transceiver in applications located in more exposed transient environments with large signal

levels and ast rising, long duration surges. Tis solution uses VS diodes or secondary protection,

Bourns BU HSP devices or coordination, and either MOVs or GDs or primary protection.

Te response characteristics o these circuit protection components are coordinated to provide a

robust level o protection or the RS-485 interace that ar exceeds the transient handling capability

o a single-stage component solution.

Implementing the Three-Stage Protection SolutionFigure 2 shows the topology or three-stage coordinated protection. Components are chosen

to provide coordinated protection beginning where the transient enters the protected port.

An MOV or GD primary device protects the Bourns BU HSP rom exposure to excessive

transient voltage, clamping or crowbarring to a level less than impulse. When the transient

current exceeds the Bourns BU HSP trigger current limit, the sub-microsecond response o

the Bourns BU HSP limits the current let-through to the sensitive transceiver. Te VS diodes

at the RS-485 transceiver provide a tight voltage clamp to keep the RS-485 signals within the

absolute maximum voltage ratings o the transceiver.

EquipmentPrimaryDevice Port

TBU HSP

TBU HSP

TVSDiodes

Figure 2: Tree-Stage Coordinated Protection


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Understanding the Operation o the Bourns TBU High-Speed Protector

Te BU-CA Series o Bourns BU HSP products are low capacitance, single, bidirectional, high-speed (sub-microsecond) circuit protection components constructed with MOSFE semiconductor

technology. Tey are designed to protect against aults caused by short circuits, AC power cross,

induction, and lightning surges up to rated limits, and then reset once the ault clears. Te Bourns

BU HSP oers a signicant perormance increase in terms o response time compared to solutions

previously available. Te Bourns BU HSP oers simplied place and route manuacturing

due to the small size o the Bourns BU-CA package, which measures a mere 6.5 mm by 4 mm.

Te Bourns BU HSP is available in a surace mount DFN package meeting industry standard

requirements such as RoHS* and Pb-ree solder refow proles.

Te three-stage topology handles large transient surges through sub-microsecond coordination

o the BU HSP with the primary and secondary elements. During normal circuit operation, the

Bourns BU HSP presents a nite series resistance to the communication loop. As the transient

current increases to the BU HSP trigger current level, the BU HSP switches to a very high

resistance or protected state, eectively disconnecting the transient rom the sensitive RS-485

transceiver. Te Bourns BU HSP enters this protected state in less than 1 microsecond, providing

a fat response over operating temperature, independent o requency o the incident transient

signal. Te linear, high-speed response o the BU HSP helps to minimize transient testing

requirements and time spent preparing or regulatory compliance.

In any transient environment, power cross and high surge current are signicant challenges or

circuit designers especially when working in a nite PCB area. As discussed previously, the VSdiode and resistor single-stage solution is eective in handling small to medium transients but

impractical or power cross and large surge currents. Te Bourns BU HSP oers a signicantly

smaller ootprint that will handle power cross up to 265 Vrms and surge currents above 20 kA when

used in combination with the right primary protector. o demonstrate the response o the three-

stage protection scheme, measurements were made using the schematic in gure 3. Te 100 load

resistor eatured in the schematic is not necessary in an actual application. It is used in this instance

to mimic the load o the application circuit.

Figure 3: Bourns BU HSP ransient esting Circuit

TBU-CA085-300-WH

LoadGenerator

Schaffner 1.2/500.5/50

ECAT 0.5/70010/700

1:2 AC IsolationTransformer

Surge GeneratorCDSOT23-SM712

100

+

-100TBU-CA085-300-WH


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BU-CA085-300-WH

P40-G240-WH

MOV-10D201K

2031-23-SM-RPLF

CDSO23-SM712

ISP4015L1BJ

10/11 e/CPK1146

Selecting the Primary and Secondary Protector in a Three-Stage Topology

Te purpose o the primary protector is to prevent voltage excursions greater than Vimpulse romreaching the Bourns BU HSP. Tus the clamping or crowbar voltage o the primary protector must

be less than Vimpulse at the surge current, Isurge, where Isurge = Vgenerator/Rgenerator. Isurge is based on

the transient environment with threat levels dened per IEC 61000-4-5.

Te secondary voltage protector is chosen to clamp or crowbar at a voltage less than the absolute

maximum voltage rating o the RS-485 transceiver. Since the BU HSP has a very ast response o

less than 1 s, only a minimal amount o current is let-through to the secondary device, worst-case

leaving a small amount o energy to be handled by the transceiver internal steering diodes.

Capacitance is another key consideration when selecting the primary and secondary protectors. I

the maximal allowable capacitance o the interace is exceeded, then the bandwidth o the nodes is

limited. Te capacitance on this signal is a lumped eect rom the cabling, connector, and circuit

protection components. Estimating the capacitance rom the cabling and connector will indicate the

maximal capacitance budget remaining or the circuit protection components.

Transient Test DataTe ollowing oscillograms are helpul in understanding the perormance o the Bourns BU

HSP-based three-stage protection solution or AC power cross and a variety o transient surges.

Using the schematic rom gure 3, the ollowing waveorms demonstrate the negligible signal

energy seen at the secondary VS diode clamp, i.e. IBU is a raction o Itransient.

Figure 4: 28 Vrms Power Cross

4

2

Ch1

500 mA Ch4

10.0 VCh250.0 V

T

1

T

TrigdT

Vtransient

= 28.2 Vrms

Vtvs

= 15.47 V peak

Itbu

= 478 mA

Line 100 VM A4.00 ms

19.40 %T


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10/11 e/CPK1146

Transient and Protection Level Test Data (Continued)



2

4

Ch1

500 mA Ch4

10.0 VCh2200 V

T

1

TT

TrigdT

Vtransient

= 123.7 Vrms

Vtvs

= 15.3 V peak

Itbu

= 308 mA


19.20 %T

4

2

Ch1

500 mA Ch4

10.0 VCh2200 V

T

1

TTT

TrigdT

Vtransient

= 220.5 Vrms

Vtvs

= 15.2 V peak

Itbu

= 280 mA


19.40 %T


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Transient and Protection Level Test Data (Continued)

Te above power cross and transient surge examples demonstrate the predictable response o

the BU HSP in limiting current to the protected load. Further, the three-stage protection

topology allows the designer to easily select primary and secondary components to provide a

robust transient response with minimal verication testing.

Figure 7: 1.2 s trise/50 s tallSurge

Figure 8: 0.5 s trise/700 s tallSurge

42

Ch1

500 mA Ch4

10.0 VCh2200 V

T

T

1

TT

TrigdT

Vtransient

= 864 Vrms

Vtvs

= 18.2 V peak

Itbu

= 1.78 mA

Line 100 VM A1.00 s

19.40 %T

214

Ch1

500 mA Ch4

10.0 VCh2200 V

T

TTTT

TrigdT

Vtransient

= 856 Vrms

Vtvs

= 18.6 V peak

Itbu

= 2.6 mA

Line 100 VM A1.00 s

19.40 %T


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10/11 e/CPK1146

RS-485Transceiver

P40-G240-WH

D5 D6

Output_P

Output_N

TISP4015L1BJ TISP4015L1BJ

1

1 1

3

2 2

4 6

24 VDCTransient

Handling DC Power Faults using the TBU HSP

A DC bus, used or relay control and actuators, is typically ound in the implementation oindustrial communication systems. Tis leaves the potential or DC power being miswired rom

installation mishaps resulting in a DC power connection to the RS-485 dierential signal lines.

Tis hard ault (vs. transient ault) requires special consideration when designing a BU HSP

electronic current limiter-based protection circuit. As the BU HSP is a semiconductor, the

power dissipation with a sustained ault rom direct current is signicant and a design which

ensures the BU HSP is in the protected (high-impedance) state is critical to a repeatable and

robust solution. Te circuit conguration in gure 4 utilizes a P40-G240-WH HSP to handle

the 24 V power ault case:

Figure 9: Handling DC power aults with the BU HSP


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10/11 e/CPK1146

Handling DC Power Faults using the TBU HSP (Continued)Te BU HSP operates in the high-impedance (protected) state when the voltage across the

BU HSP is greater than Vreset (typically 7 volts). During the DC power ault, the thyristor

(ISP4015L1BJ) is used to crowbar the power ault to zero volts at the RS-485 transceiver. Te 24

V across the BU HSP exceeds the P40-G240-WH Vreset orcing the BU HSP into the protected

state. Aer the power ault is cleared, the BU HSP resets and normal RS-485 circuit operation

resumes. Te P40-G240-WH solution can handle aults less than or equal to 40 V (Vimpulse).

By combining the transient solution rom gure 3, and the DC power ault solution o gure 4,

high-speed transients and AC and DC power cross are able to be handled in the ollowing

BU HSP-based circuit, gure 5:

In the robust transient solution above, the MOV provides ESD/EF protection while the

GD handles high surge current, protecting the BU-CA085 rom signals greater than 850 V

(BU-CA085 Vimpulse). At the input to the P40, back-to-back VS diodes are used to protect

the P40-G240-WH, clamping signals to less than 40 V. (P40-G240-WH Vimpulse). Te DC

ault is handled by the combination o the thyristor (ISP4015L1BJ) and P40-G240-WH HSP.

As the transient environment varies in each application, the BU HSP congurations

discussed are or the specic conditions outlined within. Please contact Bourns to discuss

your specic application requirements.

Figure 10: Handling AC & DC Power Fault & High-Speed ransients with the BU HSP

SMBJ25A

RS-485Transceiver

TBU-CA085-300-WHInput_P

Input_N

MOV-10D201

MOV-10D201

2027-15

2027-15

TBU-CA085-300-WH

P40-G240-WH

D5 D6

D1

SMBJ25A

SMBJ25A

SMBJ25A

D3

D2 D4

Output_P

Output_N

TISP4015L1BJ TISP4015L1BJ

1

1

1 1

1

3

3

3

2 2

4 6

Transient Levels (minimum):ESD IEC61000-4-2 Level 4, 8 kV contact;

15 kV air dischargeEFT IEC61000-4-4 Level 3, 1 kV, 20 ACurrent surge IEC61000-4-5 Level 4, 4 kV, 95 AAC power cross 120 VrmsDC power cross +/- 24 VDC

Transient


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Bourns Evaluation Boardswo evaluation board options are available to designers when evaluating the bidirectional

Bourns BU-CA HSP Series. RS-485 Evaluation Board 1 provides a very high current primary

protector solution with a GD. Tis solution will support high bandwidth signals with no

signal degradation because o the GDs extremely low 1 p capacitance loading. RS-485

Evaluation Board 2 will support medium bandwidth signal interaces above 10 MHz using an

MOV primary protector. Tis level o protection provides a low cost solution or most RS-485

interaces and transient currents up to 2.5 kA. o oer maximal fexibility these evaluation

boards support many protection combinations. It is important to review the transient

requirements prior to use to ensure the current surge and power cross levels are capable o the

required perormance. Please contact a local Bourns representative or Bourns customer service

to order evaluation boards.

Figure 11: RS-485 Evaluation Board 1: GD Primary Protector

Figure 12: RS-485 Evaluation Board 2: MOV Primary Protector


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Summary

Te recommended three-stage protection solution eaturing Bourns BU HSP devices oersenhanced perormance over competing solutions. Design engineers can take advantage o this three-

stage solution to increase the surge and transient protection levels on RS-485 ports and have robust

protection against signicant transient events such as AC power cross, ESD, EF, and surge current.

Bourns approach protects the RS-485 transceiver by combining the Bourns BU HSP to limit the

let-through voltage and energy, the VS diode to keep the signal within the maximum limits o the

RS-485 transceiver, and a primary GD or MOV to protect the BU HSP by limiting the maximum

voltage to which it is exposed. Bourns oers developers evaluation boards to allow easy prototyping.

Board conguration options assist in selecting the most suitable part number or this Bourns BU

HSP-based three-stage solution. Te compact size o the BU HSP can reduce the overall board area

compared to what was required previously to meet the protection requirements.

Te use o Bourns BU HSP devices brings a new level o overvoltage and overcurrent protection to

RS-485 drivers.

Why Bourns?Te BU High-Speed Protector technology is unique to Bourns, a company that has been a proven

leader in the circuit protection industry or several decades. Bourns invests in innovative new products

through internal development and strategic acquisition o companies and products lines. Bourns serves

diverse markets rom computers and peripherals to telecommunications. Te companys commitment

to excellence in design and customer service and its established track record in circuit protection or

over hal a century sets Bourns apart rom its competition in terms o quality and integrity.

Reerences:More inormation on the Bourns BU HSP and other Bourns circuit protection solutions can be

ound online:

http://www.bourns.com/data/global/pdfs/bourns_tbu_white_paper.pdf

http://www.bourns.com/data/global/pdfs/TBU-CA_MDS.pdf

http://www.bourns.com/ProductFamily.aspx?name=circuitprotection

http://www.bourns.com/data/global/pdfs/Bourns_FU1106_RS-485_Evalboard_

DesignNote_1.pdf

http://www.bourns.com/data/global/pdfs/Bourns_FU1106_RS-485_Evalboard_

DesignNote_2.pdf

For urther technical support and or complete port solutions,please visit

www.bourns.com

Americas: el+1-951 781-5500 Fax+1-951 781-5700 Europe: el+41-(0)41 768 55 55 Fax+41-(0)41 768 55 10 Asia-Pacifc: el+886-2 256 241 17 Fax+886-2 256 241 16

Bournsand TBU are registered trademarks of Bourns, Inc. in the U.S. and other countries.

COPYRIGHT 2011 BOURNS, INC. 10/11 e/CPK1146

*RoHS Directive 2002/95/EC Jan. 27, 2003 including Annex.

Documents

Transient Threats in RS-485.pdf