COM90C66 Datasheet, PDF (43/76 Pages) SMSC Corporation – ARCNET Controller/Transceiver with AT Interface and On-Chip RAM

NODE ID LOGIC
The Node ID code generated by the external
Node ID Select Switches is used to identify this
particular COM90C66. The code, which is input
by the COM90C66 in parallel format, is used by
the COM90C66 during transmission, reception,
reset, and reconfiguration.
Upon reset, the COM90C66 reads the Node ID
code set up on the switches and loads them into
the Node ID Register. For diagnostic purposes,
the Node ID may be read by the microprocessor.
The ARCNET protocol maintains that a
destination Node ID of 00H will allow the source
node to broadcast a message.
In addition, in order to provide the user with
increased flexibility and to eliminate the cost of
the Node ID switch on board, the COM90C66
provides special circuitry which allows the user to
program the Node ID of any node via software.
To software program the Node ID register, the
Node ID switches of that node must be physically
set to 00H. 00H is not recognized by the
ARCNET protocol as a valid Node ID and
therefore is used only to switch to this special
mode. Setting the Node ID switches to all zero's
puts the COM90C66 into reset. The device will
not attempt to join the network and no microcode
is performed. Tokens are not passed and
reconfigurations are not generated by the node.
The device will not attempt to rejoin the network
until a valid Node ID value is placed into the
Node ID Register.
For normal operation, the Node ID switches will
be set to some valid address between 01H and
FFH. Writing a nonzero value to the Node ID
register will allow the COM90C66 to join the
network.
TRANSMIT/RECEIVE LOGIC
Figure 13 illustrates the events which occur in
transmission or reception of data consisting of
1,1,0. The COM90C66, during transmission,
produces two 100 nS nonoverlapping pulses on
the nPULSE1 and nPULSE2 lines to indicate a
logic "1", whereas a lack of pulses indicates a
logic "0". The nPULSE1 and nPULSE2 signals
are used to drive the HYC9058, the HYC9068 or
the HYC9088, which in turn creates a 200 nS
dipulse signal on the transmission media.
During reception, each dipulse appearing on the
transmission media is coupled through the RF
transformer of the LAN Driver to produce a
positive pulse on the RXIN pin. The pulse is
captured by the COM90C66 and is then
converted to NRZ data. Typically, RXIN pulses
occur at multiples of 400nS, plus or minus 100
nS and still correctly capture and convert the
RXIN pulses to NRZ format.
IMPROVED DIAGNOSTICS
The COM90C66 allows the user to better
manage the operation of the network through the
use of improved diagnostics within the device.
A high level on the My Reconfiguration
(MYRECON) bit indicates that the reconfiguration
which has just taken place was caused by this
particular node. After the Reconfiguration
(RECON) bit of the Status Register interrupts the
processor, the interrupt service routine will
typically read the MYRECON bit of the
Diagnostic Status Register. Successive
occurrences of a logic "1" on the MYRECON bit
indicates that something is wrong with this node.
At that point, the transmitter should be disabled
so that the entire network is not continuously
brought down while the node is being evaluated.
The Receive Activity (RCVACT) bit of the
Diagnostic Status Register will be set to a logic
"1" whenever a rising edge of NRZ data is
detected on the RXIN pin.
The Token Seen (TOKEN) bit of the Diagnostic
Status Register will be set to a logic "1" whenever
any token has been seen on the network (except
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