ADM3485E

Rev. D | Page 11 of 16

STANDARDS AND TESTING

Table 6 compares RS-422 and RS-485 interface standards, and

Table 7 and Table 8 show transmitting and receiving truth tables.

Table 6.

Specification

RS-422

RS-485

Transmission Type

Differential

Differential

Maximum Data Rate

10 Mbps

10 Mbps

Maximum Cable Length

4000 ft

4000 ft

Minimum Driver Output Voltage

±2 V

±1.5 V

Driver Load Impedance

100 Ω

54 Ω

Receiver Input Resistance

4 kΩ min

12 kΩ min

Receiver Input Sensitivity

±200 mV

±200 mV

Receiver Input Voltage Range

−7 V to +7 V

−7 V to +12 V

Number of Drivers/Receivers per Line

1/10

32/32

Table 7. Transmitting Truth Table

Transmitting Inputs

Transmitting Outputs

RE

DE

DI

B

A

X1

1

1

0

1

X1

1

0

1

0

0

0

High-Z2

High-Z2

1

0

High-Z2

High-Z2

1

X = don't care.

2

High-Z = high impedance.

Table 8. Receiving Truth Table

Receiving Inputs

Receiving Outputs

RE

DE

A – B

RO

0

X1

> +0.2 V

1

0

X1

< –0.2 V

0

0

X1

Inputs open

1

1

X1

X1

High-Z2

1

X = don't care.

2

High-Z = high impedance.

ESD TESTING

Two coupling methods are used for ESD testing, contact

discharge and air-gap discharge. Contact discharge calls for a

direct connection to the unit being tested. Air-gap discharge

uses a higher test voltage but does not make direct contact with

the unit under test. With air-gap discharge, the discharge gun is

moved toward the unit under test, developing an arc across the

air gap, hence the term air-gap discharge. This method is

influenced by humidity, temperature, barometric pressure,

distance, and rate of closure of the discharge gun. The contact

discharge method, while less realistic, is more repeatable and is

gaining acceptance and preference over the air-gap method.

Although very little energy is contained within an ESD pulse,

the extremely fast rise time, coupled with high voltages, can

cause failures in unprotected semiconductors. Catastrophic

destruction can occur immediately as a result of arcing or

heating. Even if catastrophic failure does not occur immediately,

the device can suffer from parametric degradation, which can

result in degraded performance. The cumulative effects of

continuous exposure can eventually lead to complete failure.

I/O lines are particularly vulnerable to ESD damage. Simply

touching or plugging in an I/O cable can result in a static

discharge that can damage or completely destroy the interface

product connected to the I/O port. It is extremely important,

therefore, to have high levels of ESD protection on the I/O lines.

The ESD discharge could induce latch-up in the device under

test, so it is important that ESD testing on the I/O pins be

carried out while device power is applied. This type of testing is

more representative of a real-world I/O discharge, where the

equipment is operating normally when the discharge occurs.

Table 9. ESD Test Results

ESD Test Method

I/O Pins

Human Body Model

±15 kV

100%

90%

36.8%

10%

TIME

t

Figure 24. Human Body Model Current Waveform