COM20019I_0610 Datasheet, PDF (21/70 Pages) SMSC Corporation – Cost Competitive ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

Cost Competitive ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
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.
RT
75176B or
Equiv.
+VCC
RBIAS
+VCC
RBIAS
RT
+VCC
RBIAS
COM20022I 3V
COM20022I 3V
COM20022I 3V
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.
SMSC COM20019I 3.3V
Page 21
DATASHEET
Rev. 10-31-06