LTC1484CS8 datasheet(13/16 Pages) LINER

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LTC1484

RO4 DE4

RO2

DE2

120

Ω

1.2k

1484 F11

1.2k

120

Ω

1.2k

RO3

DE3

RO1 DE1

receiver output and expects to see perfect agreement

between the two data streams. (Note that the driver inverts

the data, so the transmitted and received data streams are

actually opposites.) If the simultaneously transmitted and

received data streams differ (usually detected by compar-

ing RO and DE with an XOR), it signals the presence of a

second, active driver. The first driver falls silent, and the

second driver seizes control.

If the LTC1484 is connected as shown in Figure 11, the

overhead of XORing the transmitted and received data in

hardware or software is eliminated. DE and RE are con-

nected together so the receiver is disabled and its output

three-stated whenever a “0” is transmitted. A 1k pull-up

ensures a “1” at the receiver output during this condition.

The receiver is enabled when the driver is disabled. During

this interval the receiver output should also be “1”. Thus,

under normal operation the receiver output is always “1”.

If a “0” is detected, it indicates the presence of a second

active driver attempting to seize control of communica-

tions.

The maximum frequency at which the system in Figure 11

can operate is determined by the cable capacitance, the

values of the pull-up and pull-down resistors and receiver

propagation delay. The external resistors take a longer

time to pull the line to a “1” state due to higher source

resistance compared to an active driver, thereby affecting

the duty cycle of the receiver output at the far end of the

line.

Figure 11. Transmit “0” CDMA Application

APPLICATIONS INFORMATION

(b)

Figure 12a shows a 100kHz DE1 waveform for an LTC1484

driving a 1000-foot shielded twisted-pair (STP) cable and

the A2, B2 and RO2 waveforms of a receiving LTC1484 at

the far end of the cable. The propagation delay between

DE1 of the driver and RO2 at the far end of the line is 1.8

µs

at the rising edge and 3.7

µs at the falling edge of DE1. The

(a)

Figure 12. LTC1484 Driving a 1000 Foot STP Cable

1484 F12a

1484 F12b

RO2

DE1

RO2

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