10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM
Datasheet
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. 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 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 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.
A0
/nM
UX
A1
LE
A2/
BA
ADDRESS
DECODING
CIRCUITRY
2K x 8
RAM
nIOCS16
ADDITIONAL
REGISTERS
AD0-AD2,
D3-D15
STATUS/
COMMAND
REGISTER
nPULSE1
nPULSE2
nTXEN
nINTR
TX/RX
LOGIC
MICRO-
SEQUENCER
AND
RXIN
WORKING
REGISTERS
RESET
LOGIC
nRESET
XTAL1
XTAL2
OSCILLATOR
nRD/nDS
nWR/DIR
nCS
BUS
ARBITRATION
CIRCUITRY
DREQ
nDACK
NODE ID
LOGIC
RECONFIGURATION
TIMER
DMA
TC
nREFEX
Figure 5.10 - Internal Block Diagram
Revision 09-27-07
Page 28
SMSC COM20022I
DATASHEET
SMSC [ SMSC CORPORATION ]
