10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
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.
5.2.3 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 or other cost-sensitive applications
which do not require direct compatibility with existing ARCNET nodes and do not require isolation.
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 media.
The Differential Driver interface includes a RS485 Driver/Receiver to transfer the data between the cable
and the COM20022I 3V. The nPULSE1 signal transmits the data, provided the Transmit Enable signal is
active. The nPULSE1 signal issues a 200nS (at 2.5Mbps) negative pulse to transmit a logic "1". Lack of
pulse indicates a logic "0". The RXIN signal receives the data, the transmitter portion of the COM20022I
3V is disabled during reset and the nPULSE1, nPULSE2 and nTXEN pins are inactive.
5.2.4 Programmable TXEN Polarity
To accommodate transceivers with active high ENABLE pins, the COM20022I 3V contains a
programmable TXEN output. To program the TXEN pin for an active high pulse, the nPULSE2 pin should
be connected to ground. To retain the normal active low polarity, nPULSE2 should be left open. The
polarity determination is made at power on reset and is valid only for Backplane Mode operation. The
nPULSE2 pin should remain grounded at all times if an active high polarity is desired.
Revision 02-27-06
Page 28
SMSC COM20022I 3V
DATASHEET
SMSC [ SMSC CORPORATION ]
