ADM1486JN View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1486JN

RS-485 PROFIBUS Transceiver +5 V Low Power

Analog Devices 

ADM1486

PRELIMINARY TECHNICAL DATA

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 characteristics of trans-

ceivers 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.

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 addi-

tional 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 cancella-

tion of the magnetic fields generated by the current

flowing through each wire, thereby, reducing the effective

inductance of the pair.

The ADM1486 is designed for bidirectional data commu-

nications on multipoint transmission lines. A typical

application showing a multipoint transmission network is

illustrated in Figure 11. 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 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 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.

Thermal Shutdown

The ADM1486 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 ADM1486 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

between the complementary outputs. Excessive skew

impairs the noise immunity of the system and increases

the amount of electromagnetic 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.

–6–

REV. PrB

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