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COM20022I

10Mbps ARCNET controller with 2Kx8 on-board RAM

SMSC -> Microchip 

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 COM20022. 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 COM20022 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 COM20022 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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