Motorola MC68824 User Manual page 139

II
New stations attempt to join the logical ring when their any-send-pending or in-ring-desired
parameters become true. New stations are added to the logical ring through a controlled con-
tention process. On each rotation of the token for each station, the current token rotation time
(measured by the ring maintenance timer) is compared to the value set by station management
(max-ring-maintenance-rotation-time). If the ring maintenance timer value is less than the
max-ring-maintenance-time value, that station will let a "new station enter if the new station's
address is between its address and the address of its successor. This occurs by the station sending
out a "solicit-successor-1" control frame with its address and the address of its successor. Then
the sending station waits one response window (one slot time before a station starts transmitting)
for a station to begin to answer that it is within that range. Responding stations send this soliciting
station a "set-successor" frame with its address in the data field so the soliciting station will
make the responding station its new successor. It is possible for more than one station to respond
to the "solicit-successor" at the same time. If this happens, the soliciting station sends a "resolve
contention" control frame which works much like the initialization algorithm.
A.6 PRIORITY
The priority option within the IEEE 802.4 standard is implemented by the token bus controller.
This option allows more important messages to be transmitted the next time the station receives
the token, and less important messages to be transmitted when important messages do not need
to be transmitted. There are four priority queues (access classes) for receive and four priority
queues (access classes) for transmit. These access classes are six, four, two, and zero, with six
being the highest access class. LLC has eight access classes defined. These eight access classes
are mapped into the MAC access classes by the MAC ignoring the least significant bit.
The TBC always receives frames that are destined to it, and places them in the appropriate priority
queue. The TBC only transmits out of enabled queues. The user enables the queues in the transmit
status area of the private area (see 2.1.15 TX Queue Access Class Status). Any combination of
transmit queues may be enabled.
When a station receives a token and all the transmit queues are enabled, the station transmits
out of transmit queue six until it either has nothing more to transmit, or the
hi-priority-token-hold-time expires. If the station has finished transmitting its queue six frames
or the hi-priority-token-hold-time has expired, the TBC will then check to see whether it has
frames in a lower access queue to transmit. If transmit queue four has frames to transmit, the
TBC checks the target rotation time for access class queue four which has been set by station
management. If this target rotation time has already been reached, the station can not transmit
out of this transmit queue and checks the next lower transmit queue. If this target rotation time
has not yet been reached, the station will transmit out of this priority queue until either it has no
more frames of this priority to send, or the target rotation time for the access class queue has
been reached. Then the station checks the target rotation time for the next lower transmit queue
and so on. When the lowest transmit queue is serviced, the station performs any logical ring
maintenence if it is required, and then passes the token to its successor.
NOTE
This is only a summary of a portion of how the IEEE 802.4 standard works. For more
information, refer to the IEEE 802.4 standard 1987. This can be purchased from:
IEEE Headquarters
345E 47th Street
New York, NY 10017-2394
Telephone: (212) 705-7960
MOTOROLA
A-4
MC68824 USER'S MANUAL