COM20020D Datasheet, PDF (17/72 Pages) SMSC Corporation – ULANC Revision D Universal Local Area Network Controller with 2K x 8 On-Board RAM

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
0
1
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.
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