ADM485E/ADM487E/ADM1487E

Rev. 0 | Page 13 of 16

THEORY OF OPERATION

The ADM485E/ADM487E/ADM1487E are ruggedized RS-485

transceivers that operate from a single 5 V supply. They contain

protection against high levels of electrostatic discharge and are

ideally suited for operation in electrically harsh environments

or where cables can be plugged or unplugged. These devices are

intended for balanced data transmission and comply with TIA/

EIA standards RS-485 and RS-422. They contain a differential

line driver and a differential line receiver and are suitable for

half-duplex data transmission, as the driver and receiver share

the same differential pins.

The input impedance on the ADM485E is 12 kΩ, allowing up

to 32 transceivers on the differential bus. The ADM487E/

ADM1487E are 48 kΩ, allowing up to 128 transceivers on the

differential bus.

CIRCUIT DESCRIPTION

The ADM485E/ADM487E/ADM1487E are operated from

a single 5 V ± 10% power supply. Excessive power dissipation

caused by bus contention or output shorting is prevented by

a thermal shutdown circuit. If, during fault conditions, a sig-

nificant temperature increase is detected in the internal driver

circuitry, this feature forces the driver output into a high

impedance state.

The receiver contains a fail-safe feature that results in a logic

high output state if the inputs are unconnected (floating).

A high level of robustness is achieved using internal protection

circuitry, eliminating the need for external protection compo-

nents such as tranzorbs or surge suppressors.

Low electromagnetic emissions are achieved using slew-rate-

limited drivers, minimizing both conducted and radiated

interference.

The ADM485E/ADM487E/ADM1487E can transmit at data

rates up to 250 kbps.

A typical application for the ADM485E/ADM487E/ADM1487E

is illustrated in Figure 26, which shows a half-duplex link where

data can be transferred at rates up to 250 kbps. A terminating

resistor is shown at both ends of the link. This termination is

not critical, because the slew rate is controlled by the ADM485E/

ADM487E/ADM1487E and reflections are minimized.

The communications network can be extended to include

multipoint connections, as shown in Figure 29. As many as

32 ADM485E transceivers or 128 ADM487E/ADM1487E

transceivers can be connected to the bus.

ADM485E/

ADM487E/

ADM1487E

ADM485E/

ADM487E/

ADM1487E

RS485/RS-422 LINK

RO

DI

DE

DI

DE

B

A

GND

B

A

GND

RO

0.1µF

0.1µF

5V

5V

RE

RE

Figure 26. Typical Half-Duplex Link Application

Table 7 and Table 8 show the truth tables for transmitting and

receiving.

Table 7. Transmitting Truth Table

Transmitting Inputs

Transmitting Outputs

RE

DE

DI

B

A

1

1

0

1

1

0

1

0

0

0

High-Z

High-Z

1

0

High-Z

High-Z

Table 8. Receiving Truth Table

Receiving Inputs

Receiving Outputs

RE

DE

A to B

RO

0

0

≥ +0.2 V

1

0

0

≤ −0.2 V

0

0

0

Inputs Open Circuit

1

1

0

High-Z

ESD Transient Protection Scheme

The ADM485E/ADM487E/ADM1487E use protective clamping

structures on their inputs and outputs that clamp the voltage to

a safe level and dissipate the energy present in ESD (electrostatic).

The protection structure achieves ESD protection up to ±15 kV

human body model (HBM).