ADM488A/ADM489A

Rev. 0 | Page 11 of 16

THEORY OF OPERATION

The ADM488A/ADM489A 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.

The ADM488A/ADM489A 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 differen-

tial line receiver, and are suitable for full-duplex data transmission.

The input impedance on the ADM488A/ADM489A is 12 kΩ,

allowing up to 32 transceivers on the differential bus. The

ADM488A/ADM489A 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).

The ADM488A/ADM489A can transmit at data rates up to

250 kbps. Figure 20 shows a typical application for the

ADM488A/ADM489A, a full-duplex link where data transfers

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 ADM488A/ADM489A and

reflections are minimized.

The communications network can be extended to include

multipoint connections, as shown in Figure 22. 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

1

1

0

1

1

0

1

0

0

0

High-Z

High-Z

1

0

Hgh-Z

High-Z

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 open circuit

1

1

0

High-Z

R

D

RO

DI

ADM488A

A

B

Z

Y

R

D

RO

DI

ADM488A

A

B

Z

Y

NOTES

1. MAXIMUM NUMBER OF NODES = 32.

GND

GND

Figure 20. ADM488A/ADM489A Full-Duplex Data Link