provide the same clock to their respective circuits.

Thus, minimal package-to-package skew is critical. The

skew must be kept well below the period of the clock

signal to ensure that all of the circuits on the network

are synchronized.

128 Receivers on the Bus

The standard RS-485 input impedance is 12k

Ω (one-

unit load). The standard RS-485 transmitter can drive

32 unit loads. The MAX3280E/MAX3281E/MAX3283E/

MAX3284E present a 1/4-unit-load input impedance

(48k

Ω), which allows up to 128 receivers on the bus.

Any combination of these RS-485 receivers with a total

of 32 unit loads can be connected to the same bus.

Thermal Protection

The MAX3280E/MAX3281E/MAX3283E/MAX3284E fea-

ture thermal protection. Thermal protection sets the out-

put stage in high-impedance mode when a short circuit

occurs at the output, limiting both the power dissipation

and temperature. The thermal temperature threshold is

+165°C, with a hysteresis of 20°C.

±15kV ESD-Protected 52Mbps, 3V to 5.5V, SOT23

RS-485/RS-422 True Fail-Safe Receivers

_______________________________________________________________________________________

7

Test Circuits/Timing Diagrams

A

B

1V

RO

-1V

≤ 3ns

OUTPUT

INPUT

Figure 1. Receiver Propagation Delay

OUT

EN

OUT

EN

0

0

0

0

FOR MAX3281E THE ENABLE SIGNAL IS INVERTED.

S1 OPEN

S2 CLOSED

S3 = 1.5V

S1 OPEN

S2 CLOSED

S3 = 1.5V

0.25V

0.25V

OUT

EN

OUT

EN

0

0

S1 CLOSED

S2 OPEN

S3 = -1.5V

S1 CLOSED

S2 OPEN

S3 = -1.5V

GENERATOR

1.5V

-1.5V

R

1k

Ω

S1

S2

50

Ω

S3

Figure 2. MAX3281E/MAX3283E Receiver Enable/Disable Timing