5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
In the MOTOROLA CPU mode (DIR, nDS mode), the same modifications apply.
RBUSTMG BIT
BUS TIMING MODE
0
1
Normal Speed CPU Read and Write
High Speed CPU Read and Normal Speed CPU Write
6.2 Transmission Media Interface
The bottom halves of Figure 2 and Figure 3 illustrate the COM20020I interface to the transmission media used to
connect the node to the network. TABLE 1 - TYPICAL MEDIA lists different types of cable which are suitable for
ARCNET applications. 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 COM20020I. The
COM20020I 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
COM20020I. 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 COM20020I 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.
Please refer to TN7-5 – Cabling Guidelines for the COM20020I ULANC, available from SMSC, for recommended
cabling distance, termination, and node count for ARCNET nodes.
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 COM20020I 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
COM20020I defaults to an open drain output.
The Backplane Configuration provides for direct connection between the COM20020I 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 COM20020I
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.
SMSC COM20020I 3.3V
Page 19
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
