RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

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13

DI

RO

RE

Z

Z

Z

Y

B

A

YB

A

RE

RE

RO

RO

DI

DI

DE

DE

DE

D

D

R

R

Y

B

A

D

R

MAX3291

MAX3292

Figure 19. Typical Half-Duplex RS-485 Network

3) Don’t overload the cable with too many receivers.

Even though the MAX3291/MAX3292 receives pre-

sent only 1/4-unit load, placing 128 receivers on the

cable will attenuate the signal if spaced out along

the cable and, in addition, cause reflections if

clumped in one spot. The MAX3291/MAX3292 suc-

cessfully drive the cables to correct RS-485/RS-422

levels with 128 receivers, but the preemphasis

effect is significantly diminished.

The MAX3291/ MAX3292 are centered for a load imped-

ance of 54

Ω, which corresponds to the parallel combina-

tion of the cable impedance and termination resistors. If

your cable impedance deviates somewhat from this

value, you still get the preemphasis effect (although the

However, if your cable impedance is significantly differ-

ent, the preemphasis ratio DPER changes, resulting in

significantly less preemphasis. Determine the preempha-

sis ratio versus load by referring to the Driver Differential

Characteristics. Read the strong and normal levels from

the graph (remember that the horizontal units are half

your cable impedance) and divide the two numbers to

Figures 19 and 20 show typical network application cir-

cuits with proper termination.

Preemphasis at Low Data Rates

(MAX3292)

At low data rates (<1Msps), preemphasis operation is

not guaranteed because it is highly dependent on the

system power-supply noise. Minimize this noise by

increasing bypass capacitance and using a power

supply with a fast transient response.

R

D

RO

RE

DE

DI

A

B

Y

DI

DI

DI

RO

RO

RO

DE

DE

DE

RE

RE

RE

Z

Z

Z

Z

Y

Y

Y

AA

A

BB

B

D

D

D

R

R

R

MAX3291

MAX3292

NOTE: RE AND DE ON.

Figure 20. Typical Full-Duplex RS-485 Network