47
47
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 Setup
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 may be 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.
The New Next ID bit permits the software to
detect the withdrawal or addition of nodes to the
network.
The Tentative ID bit allows the user to build a
network map of those nodes existing on the
network.
This feature is useful because it
minimizes the need for human intervention.
When a value placed in the Tentative ID Register
matches the Node ID of another node on the
network, the TENTID bit is set, telling the
software that this NODE ID already exists on the
network. The software should periodically place
values in the Tentative ID Register and monitor
the New Next ID bit to maintain an updated
network map.
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