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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 protect it from
Common Mode noise, but Common Mode Noise
is less of a problem in short distances.
The COM20051 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 COM20051 defaults to an open drain output.
The Backplane Configuration provides for direct
connection between the COM20051 and the
physical medium in open drain configuration of
the output driver. Only one pull-up resistor (for
open drain only) 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 COM20051 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
configuration recommended for applications like
car-area networks (CAN) or other cost-sensitive
applications
which
do
not
require
direct
compatibility with existing ARCNET nodes and
do not require isolation. Figure 7 illustrates this
configuration.
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
physical medium.
The Differential Driver interface includes a
RS485 Driver/Receiver to transfer the data
between the cable and the COM20051. The
nPULSE1 signal transmits the data, provided the
nTXEN signal is active. The
nPULSE1 signal
issues a 200nS negative pulse to transmit a logic
"1".
The RXIN pin receives
the data.
A
negative pulse on this input indicates a logic "1".
Lack of pulse indicates a logic "0".
The
transmitter portion of the COM20051 is disabled
during reset and the nPULSE1, nPULSE2 and
nTXEN pins are inactive.
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