Complete, Isolated, 3.3V

RS-485/RS-422 Data Interface

12

______________________________________________________________________________________

Figure 8. Receiver Propagation Delays

1.5V

0V

1.5V

OUTPUT

INPUT

0V

RO

__Switching Waveforms (continued)

____________________Function Tables

Table 2. Transmitting

INPUTS

DE

´

DI

´

B

A

1

1

0

1

1

0

1

0

0

X

High-Z

High-Z

INPUTS

OUTPUT

DE

´

A-B

–

R

—

O

–

0

≥ +0.2V

0

0

≤ -0.2V

1

0

Inputs open

0

OUTPUTS

Table 3. Receiving

X = Don't care

High-Z = High impedance

goes to approximately double the supply voltage.

Since the circuit is symmetrical, the two outputs can be

combined with diodes, filtered, and used to power the

oscillator itself.

The diodes on the primary side may be any fast-switch-

ing, small-signal diodes, such as the 1N914, 1N4148,

or CMPD2838. The nominal value of the primary filter

capacitor C3 is 0.01µF.

Driver Output Protection

There are two mechanisms to prevent excessive output

current and power dissipation caused by faults or by

bus contention. A foldback current limit on the output

stage provides immediate protection against short cir-

cuits over the whole common-mode voltage range (see

Typical Operating Characteristics). In addition, a ther-

mal shutdown circuit forces the driver outputs into a

high-impedance state if the die temperature rises

excessively.

Resistor R8 provides additional protection by current

limiting between the shield and the two signal wires. In

the event that shielded cable is used and an external

voltage or transient is inadvertently applied between

the shield and the signal wires, the MAX3480 can be

damaged. Although unlikely, this condition can occur

during installation.

The MAX3480 provides electrical isolation between

logic ground and signal paths; it does not provide

isolation from external shields and the signal paths.

When in doubt, do not connect the shield. The

MAX3480 can be damaged if resistor R8 is shorted

out.