ADM3491

–11–

REV. 0

Table III. Comparison of RS-422 and RS-485 Interface Standards

Specification

RS-422

RS-485

Transmission Type

Differential

Differential

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 k

Ω min

12 k

Ω min

Receiver Input Sensitivity

±200 mV

±200 mV

Receiver Input Voltage Range

–7 V to +7 V

–7 V to +12 V

APPLICATIONS INFORMATION

Differential Data Transmission

Differential data transmission is used to reliably transmit data at

high rates over long distances and through noisy environments.

Differential transmission nullifies the effects of ground shifts

and noise signals which appear as common-mode voltages on

the line.

Two main standards are approved by the Electronics Industries

Association (EIA) which specify the electrical characteristics of

transceivers used in differential data transmission. The RS-422

standard specifies data rates up to 10 MBaud and line lengths

up to 4000 ft. A single driver can drive a transmission line with

up to 10 receivers.

The RS-485 standard was defined to cater to true multipoint

communications. This standard meets or exceeds all the re-

quirements of RS-422, but also allows multiple drivers and

receivers to be connected to a single bus. An extended common

mode range of –7 V to +12 V is defined.

The most significant difference between RS-422 and RS-485 is

the fact that the drivers may be disabled thereby allowing more

than one to be connected to a single line. Only one driver should

be enabled at a time, but the RS-485 standard contains addi-

tional specifications to guarantee device safety in the event of

line contention.

Cable and Data Rate

The transmission line of choice for RS-485 communications is a

twisted pair. Twisted pair cable tends to cancel common-mode

noise and also causes cancellation of the magnetic fields gener-

ated by the current flowing through each wire, thereby reducing

the effective inductance of the pair.

The ADM3491 is designed for bidirectional data communica-

tions on multipoint transmission lines. A typical application

showing a multipoint transmission network is illustrated in

Figure 23. Only one driver can transmit at a particular time, but

multiple receivers may be enabled simultaneously.

As with any transmission line, it is important that reflections are

minimized. This may be achieved by terminating the extreme

ends of the line using resistors equal to the characteristic im-

pedance of the line. Stub lengths of the main line should also be

kept as short as possible. A properly terminated transmission

line appears purely resistive to the driver.

Receiver Open-Circuit Fail Safe

The receiver input includes a fail-safe feature that guarantees a

logic high on the receiver when the inputs are open circuit or

floating.