COM20020-5 Datasheet, PDF (16/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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 COM20020-5. The
nPULSE1 signal transmits the data, provided
the Transmit Enable signal is active. The
nPULSE1 signal issues a 100nS negative pulse
to transmit a logic "1". Lack of pulse indicates a
logic "0". The RXIN signal receives the data. A
negative pulse on this input indicates a logic "1".
The transmitter portion of the
COM20020-5 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 COM20020-5 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.
Note: The COM20020-5 operates at a 5 Mbps
data rate which is not compatible with previous
ARCNET devices. The COM20020-5 should not
be used with the HYC9088 or HYC9068 media
interface hybrids.
RT
75ALS176B
or Equiv.
COM20020-5
+VCC
RBIAS
+VCC
RBIAS
+VCC
COM20020-5
COM20020-5
RT
RBIAS
FIGURE 4 – COM20020-5 NETWORK USING RS-485 DIFFERENTIAL TRANSCEIVERS
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