COM20019I Datasheet, PDF (19/65 Pages) SMSC Corporation – Low Cost ARCNET(ANSI 878.1) Controller with 2k X 8 On-Board RAM

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
In the MOTOROLA CPU mode (DIR, nDS mode), the same modifications apply.
RBUSTMG BIT
0
1
BUS TIMING MODE
Normal Speed CPU Read and Write
High Speed CPU Read and Normal Speed CPU
Write
5.2 TRANSMISSION MEDIA INTERFACE
The bottom halves of Figures 2 and 3 illustrate the COM20019I 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 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 defaults to an open drain output.
The Backplane Configuration provides for direct connection between the COM20019I 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 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 COM20019I
Page 19
DATASHEET
Rev. 04-15-05