COM20020IP3V datasheet(18/72 Pages) SMSC

18 / 72 page

Backplane Configuration

The Backplane Open Drain Configuration is

recommended for cost-sensitive, short-distance

applications like backplanes and instrumentation.

This mode is advantageous because it saves

components, cost, and power.

Since the Backplane Configuration encodes data

differently

than

the

traditional

Hybrid

Configuration,

nodes

utilizing

the

Backplane

Configuration cannot communicate directly with

nodes

utilizing

the

Traditional

Hybrid

Configuration. The Backplane Configuration does

not isolate the node from the media nor protects it

from Common Mode noise, but Common Mode

Noise is less of a problem in short distances.

The COM20020 supplies a programmable output

driver

for

Backplane

Mode

operation.

push/pull or open drain driver can be selected by

programming the P1MODE bit of the Setup 1

The COM20020 defaults to an open drain output.

The Backplane Configuration provides for direct

connection between the COM20020 and the

media. Only one pull-up resistor (in open drain

configuration of the output driver) is required

somewhere on the media (not on each individual

node). The nPULSE1 signal, in this mode, is an

open drain or push/pull driver and is used to

directly drive the media.

It issues a 200nS

negative pulse to transmit a logic "1". Note that

when

used

the

open-drain

mode,

the

COM20020 does not have a fail/safe input on the

RXIN pin. The nPULSE1 signal actually contains

a weak pull-up resistor. This pull-up should not

take the place of the resistor required on the

media for open drain mode.

COM20020IP3V 数据表(PDF) 18 Page - SMSC Corporation

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