ADM1485

REV. A

–7–

APPLICATIONS 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. There are two main standards approved by the Electronics

Industries Association (EIA) which specify the electrical charac-

teristics 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 for true multipoint communications, the

RS-485 standard was defined. This standard meets or exceeds

all the requirements of RS-422 but 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 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 ADM1485 is designed for bidirectional data communica-

tions on multipoint transmission lines. A typical application

showing a multipoint transmission network is illustrated in

Figure 26. An RS-485 transmission line can have as many as 32

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

ticular time but multiple receivers may be enabled simultaneously.

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 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.

Thermal Shutdown

The ADM1485 contains thermal shutdown circuitry which

protects the part from excessive power dissipation during fault

conditions. Shorting the driver outputs to a low impedance

source can result in high driver currents. The thermal sensing

circuitry detects the increase in die temperature and disables the

driver outputs. The thermal sensing circuitry is designed to

disable the driver outputs when a die temperature of 150

°C is

reached. As the device cools, the drivers are re-enabled at

140

°C.

Propagation Delay

The ADM1485 features very low propagation delay ensuring

maximum baud rate operation. The driver is well balanced

ensuring distortion free transmission.

Another important specification is a measure of the skew be-

tween the complementary outputs. Excessive skew impairs the

noise immunity of the system and increases the amount of elec-

tromagnetic interference (EMI).

Receiver Open-Circuit Fail Safe

The receiver input includes a fail-safe feature which guarantees

a logic high on the receiver when the inputs are open circuit or

floating.

Table III. 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 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

No of Drivers/Receivers Per Line

1/10

32/32