ADM488A/ADM489A

Rev. 0 | Page 13 of 16

APPLICATIONS INFORMATION

DIFFERENTIAL DATA TRANSMISSION

Differential data transmission reliably transmits 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 charac-

teristics of transceivers used in differential data transmission

are approved by the EIA.

The RS-422 standard specifies data rates up to 10 Mbps and line

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

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 causes cancellation of the magnetic fields generated

by the current flowing through each wire, thereby reducing the

effective inductance of the pair.

The ADM488A/ADM489A are designed for bidirectional data

communications on multipoint transmission lines. A typical

application with a multipoint transmission network is illustrated

in Figure 22. An RS-485 transmission line can have up to 32

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

ticular time, but multiple receivers can be simultaneously enabled.

As with any transmission line, it is important to minimize reflec-

tions. This can be achieved by terminating the extreme ends of the

line using resistors equal to the characteristic impedance of the

line. Keep stub lengths of the main line as short as possible. A

properly terminated transmission line appears purely resistive

to the driver.

Table 8. Comparison of RS-422 and RS-485 Interface Standards

Specification

RS-422

RS-485

Transmission Type

Differential

Differential

Maximum Data Rate

10 Mbps

10 Mbps

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 22. Typical RS-485 Network