COM20020ILJP datasheet(23/72 Pages) MICROCHIP

23 / 72 page

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

Datasheet

SMSC COM20020I Rev D

Page 23

Revision 12-05-06

DATASHEET

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.

5.2.3

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 COM20020ID. 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 COM20020ID

is disabled during reset and the nPULSE1, nPULSE2 and nTXEN pins are inactive.

5.2.4

Programmable TXEN Polarity

To accommodate transceivers with active high ENABLE pins, the COM20020ID 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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