COM20020-5 Datasheet, PDF (15/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

one obtains the contents of COM20020-5
internal RAM. Performing only read from the
Data Register does not load a new data from the
internal RAM. During a write operation, the data
is stored in the data register and then written
into memory. Whenever the pointer is loaded for
reads with a new value, data is immediately
prefetched to prepare for the first read operation.
TRANSMISSION MEDIA INTERFACE
The bottom half of Figure 2 illustrates the
COM20020-5 interface to the transmission
media used to connect the node to the network.
The user may interface to the cable of choice in
one of two ways:
Backplane Configuration
The Backplane 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 COM20020-5 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
Register (see register descriptions for details.)
The COM20020-5 defaults to an open drain
output.
The Backplane Configuration provides for direct
connection between the COM20020-5 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
100nS negative pulse to transmit a logic "1".
Note that when used in the open-drain mode,
the COM20020-5 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. In typical applications, the
serial backplane is terminated at both ends and
a bias is provided by the external pull-up
resistor.
The RXIN signal is directly connected to the
cable via an internal Schmitt trigger. A negative
pulse on this input indicates a logic "1". Lack of
pulse indicates a logic "0". For typical single-
ended backplane applications, RXIN is
connected to nPULSE1 to make the serial
backplane data line. A ground line (from the
coax or twisted pair) should run in parallel with
the signal. For applications requiring different
treatment of the receive signal (like filtering or
squelching), nPULSE1 and RXIN remain as
independent pins.
External differential
drivers/receivers for increased range and
common mode noise rejection, for example,
would require the signals to be independent of
one another. When the device is in Backplane
Mode, the clock provided by the nPULSE2
signal may be used for encoding the data into a
different encoding scheme or other synchronous
operations needed on the serial data stream.
Differential Driver Configuration
The Differential Driver Configuration is a special
case of the Backplane Mode. It is a dc coupled
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