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II
SC - Successor Changed
This bit is set when the TBC changes its successor. The new successor's address can be read
via the READ VALUE command. This allows the network to maintain a "live list" of the
stations in the logical ring.
NS - No Successor
NS1 - No Successor 1
When the TBC realizes that it has no successor, it sets either the no successor or the no
successor 1 bit. The TBC can have no successor when it receives a set successor frame with
data unit equal to TS, which means that this is the only station in the ring (no successor 1);
or when it fails to pass the token to any successor, or when it gets the IDLE command and
performs the transition from OFFLINE to IDLE (no successor). Note that no successor 1
represents normal operation while no successor, when not after the IDLE command, may
indicate a problem in the network.
UEXF6 - Unexpected Frame 6
This bit is set when the TBC hears an unexpected frame when expecting a response to a
transmitted request with response frame. An unexpected frame in this case is any valid frame
which is not a response frame addressed to this station. Upon detecting this event, the TBC
sets this bit and goes to IDLE without passing the token.
SA - Solicit Any Arc of the Access Control Machine (ACM) Performed
This bit is set when the TBC thinks it has a successor but reaches the solicit any phase of
the pass token procedure. This means that the successor failed to respond to the token twice,
and no station responded to two who_follows queries. The TBC sends a solicit any frame
and sets this bit.
UNEXF10 - Unexpected Frame 10
The TBC sets this bit when it executes the "unexpected frame 10" transition in the IEEE 802.4
access control machine (ACM). It occurs when the TBC attempted to solicit a new successor
and while waiting for a response, heard either a data frame not sent by itself, a set successor
frame not addressed to it, or another type of control frame not sent by itself. When this event
occurs, it indicates a protocol error, possibly a duplicate token situation. After setting this bit
the TBC will go to IDLE without passing the token.
RECl - Receive Claim Token
This bit is set when the TBC hears a claim token frame sent by another station. This means
that some other station thinks it is the only active station and wants to build a new logical
ring.
NRES - No Response Received (ACM in the Await Response State)
This bit may be used as the MAC "heartbeat" indication to provide a periodic check on the
functioning of the MAC sublayer. The heartbeat indication should occur periodically in a
station where in_ring_desired remains true and sole_active_station is false, in other words
a station which is a member of an active logical ring. If the heartbeat indication does not
occur periodically, a monitor may assume that possibly the station's MAC is malfunctioning
or some other catastropohic network failure has occurred. Also, if the bit is set too frequently,
the maximum inteLsolicit count may be incremented since no new station wants to enter
the ring.
WAS - Win Address Sort
This bit is set when the TBC wins the claim token procedure and now is the one holding the
token. The TBC sends data frames from access class 6 and then opens a response window
to allow other stations to enter the ring.
MOTOROLA
?-1R
MC68824 USER'S MANUAL
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