Setting the Preemphasis Interval

The MAX3291 has an internal fixed preemphasis interval

of 100ns. Use the MAX3291 for existing designs requiring

industry-standard ’75180 pin-compatibility at data rates of

5Mbps to 10Mbps.

The MAX3292 has a resistor-programmable preemphasis

interval for more flexibility. For data rates less than 1Mbps,

emphasis setting resistor):

For example, a baud rate of 500kbps produces a baud

period of 2µs (2µs = 2000ns).

For data rates of 1Mbps to 10Mbps, use the following

For example, a baud rate of 1Mbps produces a baud

period of 1µs (1µs = 1000ns).

(closest standard value is 523k

Ω)

reverts to the nominal 100ns value.

Eye Diagrams

One simple method to quickly determine your circuit

configuration is to view an eye diagram. An eye dia-

gram is a scope photo (voltage vs. time) showing the

transitions of a pseudo-random bit string displaying at

least one bit interval. Use an eye diagram to quickly

calculate the total jitter of a circuit configuration. Jitter is

the total time variation at the zero-volt differential cross-

ing, and percent jitter is expressed as a percentage of

cal eye diagrams for a non-preemphasis device and

the MAX3291/MAX3292. ISI and jitter are often used

interchangeably; however, they are not exactly the

same thing. ISI usually makes up the majority of the jit-

ter, but asymmetrical high and low driver output voltage

levels and time skews of non-ideal transceivers (driver

and receiver) also contribute to jitter.

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

______________________________________________________________________________________

11

10,000

100

0.1

1

10

DATA RATE (Mbps)

1000

10% JITTER

PREEMPHASIS

485 DRIVER

LIMIT

CONVENTIONAL

485 DRIVER

LIMIT

PREEMPHASIS REQUIRED

FOR ERROR-FREE

TRANSMISSION

24-GAUGE

TWISTED PAIR

Figure 14. Preemphasis Driver Performance Compared to a

Conventional Driver Without Preemphasis at 10% Jitter

Figure 15. Eye Diagram of a Typical RS-485 Transceiver

Without Preemphasis, while Driving 1000 feet of Cable at

5Mbps

Figure 16. Eye Diagram of the MAX3292 with a Preemphasis

Interval of 175ns, while Driving 1000 feet of Cable at 5Mbps