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AN-960

RS-485/RS-422 Circuit Implementation Guide

Analog Devices 

APPLICATION NOTE

TRANSIENT OVERVOLTAGE STRESS PROTECTION

In I&I applications, lightning strikes, power source fluctuations,

inductive switching, and electrostatic discharge can cause

damage to RS-485 transceivers by generating large transient

voltages. The following ESD protection, EFT protection, and

surge protection specifications are relevant to RS-485

applications:

• IEC 61000-4-2 ESD protection

• IEC 61000-4-4 EFT protection

• IEC 61000-4-5 surge protection

Analog Devices offers a broad range of RS-485 devices with

enhanced ESD protection. An E appended to the part number,

such as ADM3072E, indicates enhanced ESD protection. For a

full range of the Analog Devices RS-485 portfolio, refer to

http://www.analog.com/RS485.

The level of protection can be further enhanced when using

external clamping devices, such as TVS diodes. TVS diodes

are normally used to protect silicon devices, like RS-485

transceivers, from transients. The protection is accomplished

by clamping the voltage spike to a limit, by the low impedance

avalanche breakdown of a PN junction. TVS diodes are ideally

open-circuit devices. A TVS diode can be modeled as a large

resistance in parallel with some capacitance while working

below its breakdown voltage. When a transient is generated and

the surge voltage is larger than the breakdown voltage of the

TVS, the resistance of the TVS decreases to keep the clamping

voltage constant. The TVS clamps the pulse to a level that does

not damage the device that it is protecting. The transients are

clamped instantaneously (< 1 ns) and the damaging current is

diverted away from the protected device (see Figure 15).

AN-960

The function of a TVS in RS-485 applications is to clamp the

voltage on the bus to the common-mode voltage range of the

RS-485 transceiver (−7 V to +12 V). Some TVS devices have

been specifically designed for RS-485 applications. For higher

power transients, protection can be increased by adding

Resistors RS (between 10 Ω and 20 Ω) between the protected

device and the input pin as shown in Figure 15 and Figure 16.

TRANSIENT

CURRENT

10Ω TO

20Ω

TVS

PROTECTED

DEVICE

Figure 15. Transient Voltage Supressor

RS-485 TRANSCEIVER

RO

R

MICRO-

RE

PROCESSOR

AND

DE

UART

DI

D

A RS

RS

B

RT

TVS

Figure 16. TVS Application Circuit

REFERENCES

ANSI/TIA/EIA-485-A-1998: Electrical Characteristics of

Generators and Receivers for use in Balanced Digital Multipoint

Systems.

ANSI/TIA/EIA-422-B-1994: Electrical Characteristics of

Balanced Voltage Digital Interface Circuits.

Axelson, Jay. 1998. Serial Port Complete : Programming and

Circuits for RS-232 and RS-485 Links and Networks, Lakeview

Research.

Clark, Sean. 2004. AN-727, iCoupler® Isolation in RS-485

Applications Application Note. Analog Devices, Inc. (June).

Rev. 0 | Page 9 of 12

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