COM20019I3V-HT datasheet(21/70 Pages) SMSC

21 / 70 page

Cost Competitive ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

SMSC COM20019I 3.3V

Page 21

Rev. 10-31-06

DATASHEET

5.2

TRANSMISSION MEDIA INTERFACE

The bottom halves of Figures 2 and 3 illustrate the COM20019I 3V 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. (Refer to Note 5.1)

The user may interface to the cable of choice in one of three ways:

Note 5.1

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.

5.2.1

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 COM20019I 3V supplies a programmable output driver for Backplane Mode operation. Apush/pull or

open drain driver can be selected by programming the P1MODE bit of the Setup 1 Register (see register

descriptions for details). The COM20019I 3V defaults to an open drain output.

The Backplane Configuration provides for direct connection between the COM20019I 3V 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 1.6µS negative pulse to transmit a logic "1". Note

that when used in the open-drain mode, the COM20019I 3V 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.

Figure 5.4 - COM20019I 3V NETWORK USING RS-485 DIFFERENTIAL TRANSCEIVERS

In typical applications, the serial backplane is terminated at both ends and a bias is provided by the

external pull-up resistor.

COM20022I 3V

+VCC

RBIAS

+VCC

RBIAS

75176B or

Equiv.

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