COM20051I Datasheet, PDF (21/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

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 protect it from Common Mode noise, but
Common Mode Noise is less of a problem in short distances.
The COM20051I 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 COM20051I defaults to an open drain output.
The Backplane Configuration provides for direct connection between the COM20051I and the physical medium in
open drain configuration of the output driver. Only one pull-up resistor (for open drain only) 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 COM20051I 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. In typical applications, the serial backplane is terminated at both ends
and a bias is provided by the external pull-up resistor.
The RXIN signal is directly connected to the cable via an internal Schmitt trigger. A negative pulse on this input
indicates a logic "1". Lack of pulse indicates a logic "0". For typical single-ended backplane applications, RXIN is
connected to nPULSE1 to make the serial backplane data line. A ground line (from the coax or twisted pair) should
run in parallel with the signal. For applications requiring different treatment of the receive signal (like filtering or
squelching), nPULSE1 and RXIN remain as independent pins. External differential drivers/receivers for increased
range and common mode noise rejection, for example, would require the signals to be independent of one another.
When the device is in Backplane Mode, the clock provided by the nPULSE2 signal may be used for encoding the
data into a different encoding scheme or other synchronous operations needed on the serial data stream.
Differential Driver Configuration
The Differential Driver Configuration is a special case of the Backplane Mode. It is a DC coupled configuration
recommended for applications like car-area networks (CAN) or other cost-sensitive applications which do not require
direct compatibility with existing ARCNET nodes and do not require isolation. Figure 7 illustrates this configuration.
The Differential Driver Configuration cannot communicate directly with nodes utilizing the Traditional Hybrid
Configuration. Like the Backplane Configuration, the Differential Driver Configuration does not isolate the node from
the physical medium.
The Differential Driver interface includes a RS485 Driver/Receiver to transfer the data between the cable and the
COM20051I. The nPULSE1 signal transmits the data, provided the nTXEN signal is active. The nPULSE1 signal
issues a 200nS negative pulse to transmit a logic "1". The RXIN pin receives the data. A negative pulse on this
input indicates a logic "1". Lack of pulse indicates a logic "0". The transmitter portion of the COM20051I is disabled
during reset and the nPULSE1, nPULSE2 and nTXEN pins are inactive.
Table 2 - Typical Media
CABLE TYPE
NOMINAL
IMPEDANCE
RG-62 Belden #86262
93
RG-59/U Belden #89108
75
RG-11/U Belden #89108
75
IBM Type 1* Belden #89688
150
IBM Type 3* Telephone Twisted Pair Belden #1155A 100
COMCODE 26 AWG Twisted Pair Part #105-064-703 105
ATTENUATION PER 1000 FT.
AT 5MHZ
5.5dB
7.0dB
5.5dB
7.0dB
17.9dB
16.0dB
*Non-plenum-rated cables of this type are also available.
Note: For more detailed information on Cabling options including RS-485, transformer-coupled RS-485 and Fiber
Optic interfaces, please refer to TN7-5 - Cabling Guidelines for the COM20020 ULANC, available from Standard
Microsystems Corporation.
SMSC DS – COM20051I
Page 21
Rev. 03/27/2000