Fail-Safe

The MAX3080 family guarantees a logic-high receiver

output when the receiver inputs are shorted or open, or

when they are connected to a terminated transmission

line with all drivers disabled. This is done by setting the

receiver threshold between -50mV and -200mV. If the

differential receiver input voltage (A-B) is greater than

or equal to -50mV, RO is logic high. If A-B is less than

or equal to -200mV, RO is logic low. In the case of a

terminated bus with all transmitters disabled, the

receiver’s differential input voltage is pulled to 0V by

the termination. With the receiver thresholds of the

MAX3080 family, this results in a logic high with a 50mV

minimum noise margin. Unlike previous fail-safe

devices, the -50mV to -200mV threshold complies with

the ±200mV EIA/TIA-485 standard.

MAX3089 Programming

The MAX3089 has several programmable operating

modes. Transmitter rise and fall times are programma-

ble between 2500ns, 750ns, and 25ns, resulting in

maximum data rates of 115kbps, 500kbps, and

10Mbps, respectively. To select the desired data rate,

drive SRL to one of three possible states by using a

by leaving it unconnected. For 115kbps operation, set

the three-state device in high-impedance mode or

leave SRL unconnected. For 500kbps operation, drive

drive SRL low or connect it to GND. SRL can be

changed during operation without interrupting data

communications.

Occasionally, twisted-pair lines are connected back-

ward from normal orientation. The MAX3089 has two

pins that invert the phase of the driver and the receiver

to correct for this problem. For normal operation, drive

TXP and RXP low, connect them to ground, or leave

them unconnected (internal pull-down). To invert the

invert the receiver phase, drive RXP high or connect it

when RXP is high.

The MAX3089 can operate in full- or half-duplex mode.

Drive the H/F pin low, leave it unconnected (internal

pull-down), or connect it to GND for full-duplex opera-

tion, and drive it high for half-duplex operation. In full-

duplex mode, the pin configuration of the driver and

receiver is the same as that of a MAX3080 (Figure 4). In

half-duplex mode, the receiver inputs are switched to

the driver outputs, connecting outputs Y and Z to inputs

A and B, respectively. In half-duplex mode, the internal

full-duplex receiver input resistors are still connected to

pins 11 and 12.

Applications Information

256 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 MAX3080 family of transceivers have

a 1/8-unit-load receiver input impedance (96k

Ω), allow-

ing up to 256 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.

Reduced EMI and Reflections

or unconnected, are slew-rate limited, minimizing EMI

and reducing reflections caused by improperly termi-

nated cables. Figure 14 shows the driver output wave-

form and its Fourier analysis of a 20kHz signal

transmitted by a MAX3086/MAX3087/MAX3088, and

MAX3089 with SRL = GND. High-frequency harmonic

Fail-Safe, High-Speed (10Mbps),

Slew-Rate-Limited RS-485/RS-422 Transceivers

______________________________________________________________________________________

15

R

R

Y

Z

Figure 5. Driver DC Test Load

RECEIVER

OUTPUT

TEST POINT

1k

1k

S1

S2

15pF

Figure 6. Receiver Enable/Disable Timing Test Load