COM20022ITQFP datasheet(29/83 Pages) SMSC

29 / 83 page

10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM

Datasheet

SMSC COM20022I

Page 29

Rev. 08-18-03

DATASHEET

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 COM20022I. 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 COM20022I 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 COM20022I 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.

Figure 5.10

- Internal Block Diagram

MICRO-

SEQUENCER

AND

WORKING

REGISTERS

STATUS/

COMMAND

RESET

LOGIC

RECONFIGURATION

TIMER

NODE ID

LOGIC

OSCILLATOR

TX/RX

LOGIC

ADDITIONAL

REGISTERS

ADDRESS

DECODING

CIRCUITRY

2K x 8

AD0-AD2,

BUS

ARBITRATION

CIRCUITRY

nPULSE1

nPULSE2

nTXEN

nINTR

nRESET

RAM

A0/nMUX

A2/BALE

nRD/nDS

nWR/DIR

nCS

D3-D15

RXIN

nIOCS16

XTAL1

XTAL2

DMA

DREQ

nDACK

nREFEX

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