COM20020D datasheet(17/72 Pages) SMSC

17 / 72 page

The RBUSTMG bit was added to Disable/Enable

the High Speed CPU Read function. It is defined

as:

RBUSTMG=0,

Disabled

(Default);

RBUSTMG=1, Enabled.

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

TRANSMISSION MEDIA INTERFACE

The bottom halves of Figures 2 and 3 illustrate the

COM20020D interface to the transmission media

used to connect the node to the network. Table 1

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:

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 COM20020D.

The

COM20020D 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 COM20020D. 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 COM20020D

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.

1 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.

COM20020D Datasheet(PDF) 17 Page - SMSC Corporation

COM20020D Datasheet(HTML) 17 Page - SMSC Corporation

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