ADM488/ADM489

Rev. D | Page 11 of 16

THEORY OF OPERATION

The ADM488/ADM489 are ruggedized RS-485 transceivers

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

against radiated and conducted interference and are ideally

suited for operation in electrically harsh environments or where

cables can be plugged/unplugged. They are also immune to

high RF field strengths without special shielding precautions.

They are intended for balanced data transmission and comply

with both EIA RS-485 and RS-422 standards. They contain a

differential line driver and a differential line receiver, and are

suitable for full-duplex data transmission.

The input impedance on the ADM488/ADM489 is 12 kΩ,

allowing up to 32 transceivers on the differential bus. The

ADM488/ADM489 operate from a single 5 V ± 10% power

supply. A thermal shutdown circuit prevents excessive power

dissipation caused by bus contention or by output shorting.

This feature forces the driver output into a high impedance state

if, during fault conditions, a significant temperature 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 com-

ponents such as tranzorbs or surge suppressors. Furthermore,

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 24 showing a full-duplex link where data is

transferred at rates of 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 ADM488/ADM489

and reflections are minimized.

ADM488

D

RO

DI

A

B

Z

Y

DE

5V

0.1µF

RS-485/RS-422 LINK

R

R

ADM489

5V

0.1µF

D

RO

DI

DE

GND

GND

VCC

VCC

A

B

Z

Y

RE

RE

Figure 24. ADM488/ADM489 Full-Duplex Data Link

The communications network can be extended to include

multipoint connections, as shown in Figure 30. As many as

32 transceivers can be connected to the bus.

Table 6 and Table 7 show the truth tables for transmitting and

receiving.

Table 6. Transmitting Truth Table

Inputs

Outputs

RE

DE

DI

Z

Y

X1

1

1

0

1

X1

1

0

1

0

0

0

X1

Hi-Z

Hi-Z

1

0

X1

Hi-Z

Hi-Z

1 X = Don’t care.

Table 7. Receiving Truth Table

Inputs

Output

RE

DE

A to B

RO

0

0

≥ +0.2 V

1

0

0

≤

−0.2 V

0

0

0

Inputs O/C

1

1

0

X1

Hi-Z

1 X = Don’t care.

EFT TRANSIENT PROTECTION SCHEME

The ADM488/ADM489 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) 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 simulate 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 appearing on the line,

therefore, consists of a burst of extremely fast transient impulses.

A similar effect occurs when switching on fluorescent lights.

The fast transient burst test, defined in IEC1000-4-4, simulates

this arcing, and its waveform is illustrated in Figure 25. It

consists of a burst of 2.5 kHz to 5 kHz transients repeating at

300 ms intervals. It is specified for both power and data lines.

Four severity levels are defined in terms of an open-circuit voltage

as a function of installation environment. The installation

environments are defined as

•

Well protected

•

Protected

•

Typical industrial

•

Severe industrial