SMSC COM20020 manual Traditional Hybrid Interface, Backplane Configuration

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

reception of data consisting of 1, 1, 0.

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.

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

1Please 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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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 COM20020 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 Register (see register descriptions for details.) The COM20020 defaults to an open drain output.

The Backplane Configuration provides for direct connection between the COM20020 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". ote that when used in the open-drain mode, the COM20020 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.