3.3V-Powered, 10Mbps and Slew-Rate-Limited

True RS-485/RS-422 Transceivers

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13

Driver Output Protection

Excessive output current and power dissipation caused

by faults or by bus contention are prevented by two

mechanisms. A foldback current limit on the output stage

provides immediate protection against short circuits over

the whole common-mode voltage range (see

Typical

Operating Characteristics). In addition, a thermal shut-

down circuit forces the driver outputs into a high-imped-

ance state if the die temperature rises excessively.

Propagation Delay

Figures 15–18 show the typical propagation delays. Skew

time is simply the difference between the low-to-high and

high-to-low propagation delay. Small driver/receiver

skew times help maintain a symmetrical mark-space

ratio (50% duty cycle).

(20ns for the MAX3483/MAX3488). The driver skew

times are 8ns for the MAX3485/MAX3490/MAX3491,

11ns for the MAX3486, and typically under 100ns for

the MAX3483/MAX3488.

Line Length vs. Data Rate

The RS-485/RS-422 standard covers line lengths up to

4000 feet. For line lengths greater than 4000 feet, see

Figure 23.

Figures 19 and 20 show the system differential voltage

for parts driving 4000 feet of 26AWG twisted-pair wire

at 125kHz into 120

Ω loads.

Typical Applications

The MAX3483, MAX3485, MAX3486, MAX3488,

MAX3490, and MAX3491 transceivers are designed for

bidirectional data communications on multipoint bus

transmission lines. Figures 21 and 22 show typical net-

work applications circuits. These parts can also be

used as line repeaters, with cable lengths longer than

4000 feet, as shown in Figure 23.

To minimize reflections, the line should be terminated at

both ends in its characteristic impedance, and stub

lengths off the main line should be kept as short as

possible. The slew-rate-limited MAX3483/MAX3488

and the partially slew-rate-limited MAX3486 are more

tolerant of imperfect termination.

DI

RO

DE

RE

A

B

RE

RE

RE

RO

RO

RO

DI

DI

DI

DE

DE

DE

D

D

D

R

R

R

B

B

B

A

A

A

120

Ω

120

Ω

D

R

MAX3483

MAX3485

MAX3486

Figure 21. MAX3483/MAX3485/MAX3486 Typical RS-485 Network