Detailed Description

The MAX3291/MAX3292 high-speed RS-485/RS-422

transceivers feature driver preemphasis circuitry, which

extends the distance and increases the data rate of

reliable communication by reducing intersymbol inter-

ference (ISI) caused by long cables. The MAX3291 is

programmable for data rates of 5Mbps to 10Mbps,

while the MAX3292 is programmable for data rates up

to 10Mbps by using a single external resistor.

The MAX3291/MAX3292 are full-duplex devices that

operate from a single +5V supply and offer a low-cur-

rent shutdown mode that reduces supply current to

100nA. They feature driver output short-circuit current

limiting and a fail-safe receiver input that guarantees a

logic-high output if the input is open circuit. A 1/4-unit-

load receiver input impedance allows up to 128 trans-

ceivers on the bus.

Inter-symbol interference (ISI) causes significant prob-

lems for UARTs if the total RS-485/RS-422 signal jitter

becomes 10% or more of the baud period. ISI is

caused by the effect of the cable’s RC time constant on

different bit patterns. If a series of ones is transmitted,

followed by a zero, the transmission-line voltage rises

to a high value at the end of the string of ones (signal 1

in Figure 12). As the signal moves towards the zero

state, it takes longer to reach the zero-crossing,

because its starting voltage is farther from the zero

crossing. On the other hand, if the data pattern has a

string of zeros followed by a one and then another zero,

the one-to-zero transition starts from a voltage that is

takes much less time for the signal to reach the zero-

crossing (signal 2 in Figure 12). In other words, the

propagation delay depends upon the previous bit pat-

tern. This is inter-symbol interference (ISI).

Preemphasis reduces ISI by increasing the signal

amplitude at every transition edge for about one baud

period, counteracting the effects of the cable (see the

section

Setting the Preemphasis Interval). Figure 13

shows a typical preemphasis waveform optimized for

data rates between 5Mbps and 10Mbps. When DI

changes from a logic low to a logic high, the differential

output switches to a strong high. At the end of the pre-

emphasis interval, the strong high returns to a normal

high level. Both levels meet RS-485/RS-422 specifica-

tions, and the strong levels are typically 1.9 times larger

than the normal levels. If DI switches back to a logic

low before the end of the preemphasis interval, the dif-

ferential output switches directly from the strong high to

the strong low. Similarly, this explanation applies when

DI transitions from high to low.

Applications Information

Data Rate vs. Cable Length

In general, preemphasis allows either double the dis-

tance for a fixed data rate or double the data rate for a

fixed existing cable distance over existing RS-485

transceivers that do not feature preemphasis. Figure 14

shows that the MAX3291/MAX3292 transmits approxi-

mately twice as far at the same data rate or twice as

fast at the same cable length as a conventional RS-485

transceiver without preemphasis for 10% jitter.

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

10

______________________________________________________________________________________

SIGNAL 2

SIGNAL 1

BAUD PERIOD

Figure 12. Inter-Symbol Interference among Two Data

Patterns: Signal 1 = 11111110, Signal 2 = 00000010

100ns

001

0

1

0

1

1

1

0

DI

2.5V/div

5V/div

Figure 13. Typical Preemphasis Waveform with a 100ns

Preemphasis Interval