COM20020I3V-HT datasheet(21/65 Pages) SMSC

21 / 65 page

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

SMSC COM20020I 3.3V

Page 21

Revision 12-06-06

DATASHEET

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.

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