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Michael Peffers

Oct 23, 2015

Get
Connected

TIDA-00730

RS-485
Transceivers

SN65HVD3082E

SN65HVD82

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Related

Get Connected: The basics of protecting RS-485 against ESD

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Welcome back to the Get Connected blog series on
Analog Wire. In my
last post, I discussed fail-safe biasing of differential buses and how to
implement fail-safe biasing in your designs. In this post, I’ll look at
protecting your differential bus against system-level transients using a
transient voltage suppressor (TVS) diode and pulse-proof resistors.

Industrial networks such as RS-485 are expected to withstand harsh
system-level transients in their end applications without being damaged. Damage
can occur from electrostatic discharge (ESD) during handling, interruption of
inductive loads, relay contact bounce and/or lightning strikes. You can protect
your industrial design against these transient events through the addition of
external components on the differential bus.

Let’s look at the International Electrotechnical Commission (IEC) ESD
61000-4-2 system-level ESD test that manufacturers use to test for robustness.
There are two types of testing methods involved with the IEC ESD test: contact
discharge and air discharge. The contact ESD test discharges an ESD pulse from
an IEC ESD gun directly onto the device under test (DUT). The air ESD discharge
test involves moving the charged ESD gun toward the DUT until the air breaks
down enough to allow conduction of the ESD strike between the ESD gun and the
DUT. IEC ESD testing is performed with both positive and negative polarities,
and a passing score is not possible until both polarities survive at a single
level. Table 1 shows the IEC 61000-4-2 ESD test-voltage and peak-current
levels.

Contact
discharge

Air discharge

Level

Test voltage (kV)

Peak current (A)

Level

Test voltage (kV)

1

2

7.5

1

2

2

4

15

2

4

3

6

22.5

3

8

4

8

30

4

15

*

Special

Special

*

Special

*This is an open level. The level has to be specified in the dedicated
equipment speciation. If higher voltages than those shown are specified, you may
need special test equipment.

Table 1: IEC
61000-4-2 ESD test voltage levels

Figure 1 depicts the basic shape of the IEC ESD pulse and shows the timing
sequence of the test pulses.

Figure 1: Current waveform
of IEC ESD pulse and timing sequence of test

To help protect your industrial design against the high levels of energy
delivered during an ESD event, you can add a TVS diode and pulse-proof series
resistor to the bus to help divert and consume the energy. Placing a TVS diode
very close to the board connector where the bus lines enter the design ensures
that any transient energy coupled onto the bus is minimized at the point of
origin. The TVS acts as a clamping circuit to redirect any high-energy pulses to
ground, away from the RS-485 transceiver.

Adding series pulse-proof resistors on the A and B bus lines limits the
residual clamping current the transceiver sees if the TVS diode clamping voltage
is higher than the specified maximum voltage of the transceiver bus pins. These
resistors are typically very low in value (~10-20Ω). Select those resistors that
can accommodate the appropriate power levels.

Figure 2 shows this concept using the SN65HVD82
RS-485 transceiver.

Figure 2: SN65HVD82
RS-485 transient protection overview

Table 2 shows the achievable
performance of the SN65HVD82
RS-485 transceiver with the added external components.

Protection
scheme

IEC ESD (kV)

IEC EFT (kV)

IEC surge
(kV)

SN65HVD82

TVS

±30 contact

±30 air

±4

±1

SN65HVD3082E

TVS

±14 contact

±30 air

±4

±1

Table 2: Summary of test
results

For more information on IEC protection schemes, please see the RS-485
IEC ESD TI Designs reference design. Leave a comment if you’d like to hear
more about anything discussed in this post or if there’s an interface topic
you’d like to see in the future.

Additional resources

Find out more about the SN65HVD82
RS-485 transceiver.

Learn more about RS-485 in our RS-485
Design Guide.

Read more posts in the Get
Connected series.

Learn more about IEC ESD and other electromagnetic compatibility testing
in the Precision Hub blog series by Ian Williams: Get
CerTIfied, not certi-FRIED! Electromagnetic compatibility testing
explained.

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