COM20020ILJP datasheet(21/72 Pages) MICROCHIP

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

Datasheet

SMSC COM20020I Rev D

Page 21

Revision 12-05-06

DATASHEET

In the MOTOROLA CPU mode (DIR, nDS mode), the same modifications apply.

RBUSTMG BIT

BUS TIMING MODE

Normal Speed CPU Read and Write

High Speed CPU Read and Normal Speed CPU Write

5.2

Transmission Media Interface

The bottom halves of Figure 5.1 and Figure 5.2 illustrate the COM20020ID interface to the transmission

media used to connect the node to the network. Table 5.1 - Typical Media on page 24 lists different types

of cable which are suitable for ARCNET applications.

1 The user may interface to the cable of choice in one

of three ways:

5.2.1

Traditional Hybrid Interface

The Traditional Hybrid Interface is that which is used with previous ARCNET devices.

The Hybrid

Interface is recommended if the node is to be placed in a network with other Hybrid-Interfaced nodes. The

Traditional Hybrid Interface is for use with nodes operating at 2.5 Mbps only. The 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 COM20020ID. The COM20020ID transmits a logic "1" by generating two

100nSnon-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 COM20020ID. 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

COM20020ID can tolerate distortion of plus or minus 100nS and still correctly capture and convert the

RXIN pulses to NRZ format. Figure 5.4 illustrates the events which occur in transmission or reception of

data consisting of 1, 1, 0.

Note:

Please refer to TN7-5 – Cabling Guidelines for the COM20020 ULANC, available from SMSC, for

recommended cabling distance, termination, and node count for ARCNET nodes.

5.2.2

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 COM20020ID 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 COM20020ID defaults to an open drain output.

The Backplane Configuration provides for direct connection between the COM20020ID and the media.

Only one pull-up resistor (in open drain configuration of the output driver) is required somewhere on the

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