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COM20022I

10Mbps ARCNET controller with 2Kx8 on-board RAM

SMSC -> Microchip 

transformer coupling of the Hybrid offers isolation

for the safety of the system and offers high

Common Mode Rejection. The Traditional Hybrid

Interface uses circuits like SMSC's HYC9068 or

HYC9088 to transfer the pulse-encoded data

between the cable and the COM20022. The

COM20022 transmits a logic "1" by generating two

100nS non-overlapping negative pulses,

nPULSE1 and nPULSE2. Lack of pulses

indicates a logic "0". The nPULSE1 and nPULSE2

signals are sent to the Hybrid, which creates a

200nS dipulse signal on the media. A logic "0" is

transmitted by the absence of the dipulse. During

reception, the 200nS dipulse appearing on the

media is coupled through the RF transformer of

the LAN Driver, which produces a positive pulse at

the RXIN pin of the COM20022. The pulse on the

RXIN pin represents a logic "1". Lack of pulse

represents a logic "0". Typically, RXIN pulses

occur at multiples of 400nS. The COM20022 can

tolerate distortion of plus or minus 100nS and still

correctly capture and convert the RXIN pulses to

NRZ format. Figure 5 illustrates the events which

occur in transmission or reception of data

consisting of 1, 1, 0.

Backplane Configuration

The Backplane Open Drain Configuration is

recommended for cost-sensitive, short-distance

applications like backplanes and instrumentation.

This mode is advantageous because it saves

components, cost, and power.

Since the Backplane Configuration encodes data

differently than the traditional Hybrid

Configuration, nodes utilizing the Backplane

Configuration cannot communicate directly with

nodes utilizing the Traditional Hybrid

Configuration. The Backplane Configuration does

not isolate the node from the media nor protects it

from Common Mode noise, but Common Mode

Noise is less of a problem in short distances.

The COM20022 supplies a programmable output

driver for Backplane Mode operation. A push/pull

or open drain driver can be selected by

programming the P1MODE bit of the Setup 1

Register (see register descriptions for details).

The COM20022 defaults to an open drain output.

The Backplane Configuration provides for direct

connection between the COM20022 and the

media. Only one pull-up resistor (in open drain

configuration of the output driver) is required

somewhere on the media (not on each individual

node). The nPULSE1 signal, in this mode, is an

open drain or push/pull driver and is used to

directly drive the media. It issues a 200nS

negative pulse to transmit a logic "1". Note that

when used in the open-drain mode, the

COM20022 does not have a fail/safe input on the

RXIN pin. The nPULSE1 signal actually contains

a weak pull-up resistor. This pull-up should not

take the place of the resistor required on the

media for open drain mode.

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