COM20020_00 Datasheet, PDF (57/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

whenever activity (logic "1") is detected on the
RXIN pin.
The Token Seen (TOKEN) bit is set to a logic "1"
whenever any token has been seen on the
network (except those tokens transmitted by this
node).
The RCVACT and TOKEN bits may help the user
to troubleshoot the network or the node. If unusual
events are occurring on the network, the user may
find it valuable to use the TXEN bit of the
Configuration Register to qualify events. Different
combinations of the RCVACT, TOKEN, and TXEN
bits, as shown indicate different situations:
Normal Results:
RCVACT=1, TOKEN=1, TXEN=0: The node is
not part of the network. The network is operating
properly without this node.
RCVACT=1, TOKEN=1, TXEN=1: The node sees
receive activity and sees the token. The basic
transmit function is enabled. Network and node
are operating properly.
MYRECON=0, DUPID=0, RCVACT=1, TXEN=0,
TOKEN=1: Single node network.
Abnormal Results:
RCVACT=1, TOKEN=0, TXEN=X: The node
sees receive activity, but does not see the token.
Either no other nodes exist on the network, some
type of data corruption exists, the media driver is
malfunctioning, the topology is set up incorrectly,
there is noise on the network, or a reconfiguration
is occurring.
RCVACT=0, TOKEN=0, TXEN=1: No receive
activity is seen and the basic transmit function is
enabled. The transmitter and/or receiver are not
functioning properly.
RCVACT=0, TOKEN=0, TXEN=0: No receive
activity and basic transmit function disabled. This
node is not connected to the network.
The Excessive NAK (EXCNAK) bit is used to
replace a timeout function traditionally
implemented in software. This function is
necessary to limit the number of times a sender
issues a FBE to a node with no available buffer.
When the destination node replies to 128 FBEs
with 128 NAKs or 4 FBEs with 4 NAKs, the
EXCNAK bit of the sender is set, generating an
interrupt. At this point the software may abandon
the transmission via the "Disable Transmitter"
command. This sets the TA bit to logic "1" when
the node next receives the token, to allow a
different transmission to occur. The timeout value
for the EXNACK bit (128 or 4) is determined by
the FOUR-NAKS bit on the Setup1 Register.
The user may choose to wait for more NAK's
before disabling the transmitter by taking
advantage of the wraparound counter of the
EXCNAK bit. When the EXCNAK bit goes high,
indicating 128 or 4 NAKs, the "POR Clear Flags"
command maybe issued to reset the bit so that it
will go high again after another count of 128 or 4.
The software may count the number of times the
EXCNAK bit goes high, and once the final count is
reached, the "Disable Transmitter" command may
be issued.
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