COM20020 Datasheet, PDF (17/72 Pages) SMSC Corporation – COM20020 3.3V ULANC 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
COM20020 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 COM20020. The
COM20020 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 COM20020.
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
COM20020 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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