COM20020D Datasheet, PDF (20/72 Pages) SMSC Corporation – ULANC Revision D Universal Local Area Network Controller with 2K x 8 On-Board RAM

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
configuration recommended for applications like
car-area networks or other cost-sensitive
applications which do not require direct
compatibility with existing ARCNET nodes and do
not require isolation.
The Differential Driver Configuration cannot
communicate directly with nodes utilizing the
Traditional Hybrid Configuration. Like the
Backplane Configuration, the Differential Driver
Configuration does not isolate the node from the
media.
The Differential Driver interface includes a RS485
Driver/Receiver to transfer the data between the
cable and the COM20020D. The nPULSE1 signal
transmits the data, provided the Transmit Enable
signal is active. The nPULSE1 signal issues a
200nS (at 2.5Mbps) negative pulse to transmit a
logic "1". Lack of pulse indicates a logic "0". The
RXIN signal receives the data, the transmitter
portion of the COM20020D is disabled during
reset and the nPULSE1, nPULSE2 and nTXEN
pins are inactive.
Programmable TXEN Polarity
To accommodate transceivers with active high
ENABLE pins, the COM20020D contains a
programmable TXEN output. To program the
TXEN pin for an active high pulse, the nPULSE2
pin should be connected to ground. To retain the
normal active low polarity, nPULSE2 should be
left open. The polarity determination is made at
power on reset and is valid only for Backplane
Mode operation. The nPULSE2 pin should
remain grounded at all times if an active high
polarity is desired.
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