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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 COM20020. The
nPULSE1 signal transmits the data, provided
the Transmit Enable signal is active. The
nPULSE1 signal issues a 200nS 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 COM20020 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 COM20020 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.