ADM488/ADM489

–8–

REV. 0

GENERAL INFORMATION

The ADM488/ADM489 is a ruggedized RS-485 transceiver that

operates from a single +5 V supply.

It contains protection against radiated and conducted interference.

It is ideally suited for operation in electrically harsh environ-

ments or where cables may be plugged/unplugged. It is also

immune to high RF field strengths without special shielding

precautions. It is intended for balanced data transmission and

complies with both EIA Standards RS-485 and RS-422. It con-

tains a differential line driver and a differential line receiver, and

is suitable for full duplex data transmission.

The input impedance on the ADM488/ADM489 is 12 k

Ω,

allowing up to 32 transceivers on the differential bus.

The ADM488/ADM489 operates from a single +5 V

± 10%

power supply. Excessive power dissipation caused by bus con-

tention or by output shorting is prevented by a thermal shut-

down circuit. This feature forces the driver output into a high

impedance state if, during fault conditions, a significant tem-

perature increase is detected in the internal driver circuitry.

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 limited

drivers, minimizing interference both conducted and radiated.

The ADM488/ADM489 can transmit at data rates up to

250 kbps.

A typical application for the ADM488/ADM489 is illustrated in

Figure 19. This shows a full-duplex link where data may be

transferred at rates up to 250 kbps. A terminating resistor is

shown at both ends of the link. This termination is not critical

since the slew rate is controlled by the ADM488/ADM489 and

reflections are minimized.

The communications network may be extended to include

multipoint connections as shown in Figure 25. Up to 32 trans-

ceivers may be connected to the bus.

Tables I and II show the truth tables for transmitting and

receiving.

Table I. Transmitting Truth Table

Inputs

Outputs

RE

DE

DI

Z

Y

X

1

101

X

1

010

0

0

X

Hi-Z

Hi-Z

1

0

X

Hi-Z

Hi-Z

X = Don’t Care.

Table II. Receiving Truth Table

Inputs

Output

RE

DE

A-B

RO

00

≥ +0.2 V

1

01

≤ +0.2 V

0

0

0

Inputs O/C

1

1

0

X

Hi-Z

X = Don’t Care.

EFT TRANSIENT PROTECTION SCHEME

The ADM488/ADM489 uses protective clamping structures on

its inputs and outputs that clamp the voltage to a safe level and

dissipates the energy present in ESD (Electrostatic) and EFT

(Electrical Fast Transients) discharges.

FAST TRANSIENT BURST IMMUNITY (IEC1000-4-4)

IEC1000-4-4 (previously 801-4) covers electrical fast-transient/

burst (EFT) immunity. Electrical fast transients occur as a

result of arcing contacts in switches and relays. The tests simu-

late the interference generated when, for example, a power relay

disconnects an inductive load. A spark is generated due to the

well known back EMF effect. In fact, the spark consists of a

burst of sparks as the relay contacts separate. The voltage ap-

pearing on the line, therefore, consists of a burst of extremely

fast transient impulses. A similar effect occurs when switching

on fluorescent lights.

ADM488

D

RO

DI

A

B

Z

Y

RE

DE

+5V

0.1 F

RS-485/RS-422 LINK

R

R

ADM489

+5V

0.1 F

D

RO

DI

RE

DE

GND

GND

A

B

Z

Y

Figure 19. ADM488/ADM489 Full-Duplex Data Link