When the total amount of time skew becomes 10% or

more of the baud period, the data error rate can

increase sharply.

128 Transceivers on the Bus

The standard RS-485 receiver input impedance is 12k

Ω

(one unit load), and the standard driver can drive up to

32 unit loads. The MAX3291/MAX3292 transceivers have

a 1/4-unit-load receiver input impedance (48k

Ω), allow-

ing up to 128 transceivers to be connected in parallel on

one communication line. Any combination of these

devices and/or other RS-485 transceivers with a total of

32 unit-loads or less can be connected to the line.

Low-Power Shutdown Mode

Initiate low-power shutdown mode by bringing RE high

and DE low. In shutdown the MAX3291/MAX3292 typi-

cally draw only 1µA of supply current.

Simultaneously driving RE and DE is allowed; the parts

are guaranteed not to enter shutdown if RE is high and

DE is low for less than 80ns. If the inputs are in this

state for at least 300ns, the parts are guaranteed to

enter shutdown.

tics tables correspond to when the part is not in the low-

vers and receivers longer to activate from the low-

Line Repeater

For line lengths greater than what one MAX3291/

MAX3292 can drive, use the repeater application

shown in Figure 17.

Figure 18 shows the system differential voltage for the

MAX3292 driving 4000 feet of 26AWG twisted-pair wire

into two 120

Ω termination loads.

Line Termination

The MAX3291/MAX3292 are targeted for applications

requiring the best combination of long cable length and

lowest bit-error rate. In order to achieve this combina-

tion, the cable system must be set up with care. There

are three basic steps:

1) The cable should only have two ends (no tree configu-

ration with long branches), which are terminated with

a simple resistor termination whose value is the

tions anywhere else along the cable. This ensures that

there are no reflections at the end of the cable, and

that all transmitters (whether they are located at the

ends of the cable or somewhere along the length) see

2) Make all branches or stubs short enough so that

twice the propagation delay along the stub (down

and back) is significantly less than one baud period

(around 15% or less). This ensures that the reflec-

tions from the end of the stub (which are unavoid-

able, since the stubs are not terminated) settle in

much less than a baud period. If the application

requires a branch much longer than this, use a

repeater (see the

Line Repeater section).

RS-485/RS-422 Transceivers with Preemphasis

for High-Speed, Long-Distance Communication

12

______________________________________________________________________________________

Figure 17. Line-Repeater Application

2

µs/div

RO

DI

RECEIVER

INPUT

1V/div

5V/div

5V/div

120

Ω

120

Ω

DATA IN

DATA OUT

R

D

RO

RE

DE

DI

A

B

Z

Y

MAX3291

MAX3292

Figure 18. MAX3292 System Differential Voltage Driving 4000

Feet, Using Two 120

Ω Termination Resistors