COM20020_00 Datasheet, PDF (20/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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. A
push/pull or open drain driver can be selected by
programming the P1MODE bit of the Setup 1
Register (see register descriptions for details).
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 in 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.
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