ADM488/ADM489

Rev. D | Page 14 of 16

APPLICATION 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 that specify the electrical

characteristics of transceivers used in differential data

transmission are approved by the EIA.

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.

To cater to true multipoint communications, the RS-485 stan-

dard was defined to meet or exceed the requirements of RS-422.

It 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 the

RS-422 and RS-485 is that the RS-485 drivers can be disabled,

thereby allowing up to 32 receivers 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 are designed for bidirectional data

communications on multipoint transmission lines. A typical

application showing a multipoint transmission network is

illustrated in Figure 30. An RS-485 transmission line can have

up to 32 transceivers on the bus. Only one driver can transmit

at a particular time, but multiple receivers can be simultane-

ously enabled.

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

minimized. This can 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.

Table 10. 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 kΩ minimum

12 kΩ minimum

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

D

R

RT

RT

D

R

Figure 30. Typical RS-485 Network