LTC2802 datasheet(21/32 Pages) AD

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LTC2873

REV B

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Figure 20. Equivalent RS485 Receiver Input Resistance

Into A/DO and B/RI

2873 F20

A/DO

B/RI

TE485

125k

undriven, to change state momentarily. In these extreme

conditions, high quality shielded cable is recommended.

If necessary, biasing resistors can be used on the bus to

pull the resting signal farther from the receivers failsafe

threshold.

Receiver Outputs

The RS232 and RS485 receiver outputs are internally

driven high (to VL) or low (to GND) with no external pull

up needed. When the receivers are disabled the output pin

becomes Hi-Z with leakage of less than ±5µA for voltages

within the VL supply range.

RS485 Receiver Input Resistance

In RS485 mode, the RS485 receiver input resistance from

A/DO or B/RI to GND is 125kΩ (typical) when the inte-

grated termination is disabled. This permits up to a total

of 256 receivers per system without exceeding the RS485

receiver loading specification. The input resistance of the

receiver is unaffected by enabling/disabling the receiver

or whether the part is in loopback mode, or unpowered.

The equivalent input resistance looking into the RS485

receiver pins is shown in Figure 20.

Figure 21. Typical Resistance of the Enabled RS485

Terminator vs Common Mode Voltage of A/DO and B/RI

VOLTAGE (V)

–10

126

124

122

118

120

116

–5

2873 F21

VCC = 5.0V

VCC = 3.3V

APPLICATIONS INFORMATION

Selectable RS485 Termination

Propercableterminationisimportantforgoodsignalfidel-

ity. When the cable is not terminated with its characteristic

impedance, reflections cause waveform distortion.

The LTC2873 offers an integrated switchable 120Ω ter-

mination resistor between pins A/DO and B/RI.

This termination supports communication over a twisted

paircablewithcharacteristicimpedanceof120Ωor100Ω,

including CAT-5 cables. It has the advantage of being

able to easily change, through logic control, the proper

line termination for correct operation when configuring

transceiver networks. Termination should be enabled on

transceivers positioned at both ends of the network bus

only. However, the driving end of a line does not need to

be terminated. By turning off termination at the driver, the

reduced load results in less power dissipation and a larger

signal swing on the bus. TE485 can be tied to DE485/F232

to logically switch the termination on only when the driver

is inactive if the termination enable/disable delays can be

toleratedintheoverallsystemleveltiming.Ifthedelaysare

not acceptable, tie TE485 low to enable termination for all

modes of RS485 operation, whether driving or receiving.

The termination resistance is maintained over the entire

RS485 common mode range of –7V to 12V as shown in

Figure 21. The voltage across pins with the terminating

resistor enabled should not exceed 6V as indicated in the

Absolute Maximum Ratings table.

RS232 Receiver Input Resistance

In RS232 mode, the receiver input resistance on the

B/RI pin is always 5k to GND. In any other mode, this

resistor is switched out. The 120Ω RS485 termination

resistor between pins A/DO and B/RI is never engaged

in RS232 mode, regardless of the state of TE485 pin.

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