ADM488/ADM489

–11–

REV. 0

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

Specification

RS-422

RS-485

Transmission Type

Differential

Differential

Maximum Data Rate

10 MB/s

10 MB/s

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

Number of Drivers/Receivers Per Line

1/10

32/32

D

R

D

R

DD

RR

RT

RT

Figure 25. Typical RS-485 Network

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, which appear as common-mode volt-

ages 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 transmis-

sion line with up to 10 receivers.

In order to cater to true multipoint communications, the RS-

485 standard was defined. This standard meets or exceeds all

the requirements of RS-422 and also allows for 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 the fact

that the drivers may be disabled thereby allowing more than

one (32, in fact) 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 gener-

ated by the current flowing through each wire, thereby reducing

the effective inductance of the pair.

The ADM488/ADM489 is designed for bidirectional data com-

munications on multipoint transmission lines. A typical applica-

tion showing a multipoint transmission network is illustrated in

Figure 25. 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 may simultaneously

be enabled.

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 imped-

ance 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.