10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM
Datasheet
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 COM20022I. The COM20022I 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 COM20022I. 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 COM20022I 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.
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 COM20022I 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 COM20022I defaults to an open drain output.
The Backplane Configuration provides for direct connection between the COM20022I 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 COM20022I 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.
Revision 09-27-07
Page 26
SMSC COM20022I
DATASHEET
SMSC [ SMSC CORPORATION ]
