ADM483

Rev. 0 | Page 11 of 16

APPLICATIONS

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 that appear as common-

mode voltages on the line. There are two main standards

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.

To achieve true multipoint communications, the RS-485

standard was defined. This standard meets or exceeds all the

requirements of RS-422, but also allows up to 32 drivers and 32

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 that the drivers may

be disabled, allowing up to 32 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.

Table 5. 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 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

Drivers/Receivers per Line

1/10

32/32

CABLE AND DATA RATE

The preferred transmission line for RS-485 communications

is a twisted pair. Twisted pair cable tends to cancel common-

mode noise and the magnetic fields generated by the current

flowing through each wire, thereby reducing the effective

inductance of the pair.

The ADM483 is designed for bidirectional data communica-

tions on multipoint transmission lines. A typical application

showing a multipoint transmission network is shown in

Figure 24

. An RS-485 transmission line can have as many as

32 transceivers on the bus. Only one driver can transmit at a

particular time, but multiple receivers can be enabled simul-

taneously. As with any transmission line, it is important to

minimize reflections. This can be done by terminating the

extreme ends of the line by using resistors equal to the char-

acteristic impedance 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.

RO

RE

DE

DI

D

R

A

B

MAXIMUM NUMBER OF TRANSCEIVERS ON BUS: 32

ADM483

RO

RE

DE

DI

D

R

A

B

ADM483

R

ADM483

D

RO

RE DE

DI

AB

R

ADM483

D

RO

RE DE

DI

AB

Figure 24. Typical Half-Duplex RS-485 Network Topology