ADM3485E

Rev. D | Page 12 of 16

APPLICATIONS INFORMATION

DIFFERENTIAL DATA TRANSMISSION

Differential data transmission is used to reliably transmit data

at high rates over long distances and through noisy environ-

ments. Differential transmission nullifies the effects of ground

shifts and noise signals that appear as common-mode voltages

on the line.

Two main standards that specify the electrical characteristics of

transceivers used in differential data transmission are approved

by the Electronics Industries Association (EIA). The RS-422

standard specifies data rates up to 10 Mbps and line lengths up

to 4000 feet. 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 requirements 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 under the RS-485 standard 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 additional 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

generated by the current flowing through each wire, thereby

reducing the effective inductance of the pair.

The ADM3485E is designed for bidirectional data communi-

cations on multipoint transmission lines. A typical application

showing a multipoint transmission network is illustrated in

Figure 25. 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 can be achieved by terminating the extreme

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

dance of the line. Stub lengths off the main line must 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.

Table 10. 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 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

B

A

R

D

RO

RE

DI

DE

ADM3485E

B

A

R

D

RO

RE

DI

DE

ADM3485E

B

A

R

D

RO

RE

DI

DE

ADM3485E

B

A

R

D

RO

RE

DI

DE

ADM3485E

MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 50

Figure 25. Multipoint Transmission Network