5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip 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
COM20020I. 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 COM20020I 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 COM20020I 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.
SMSC COM20020I 3.3V
Page 21
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
