Options Within Ieee - Motorola MC68824 User Manual

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The initialization of a token bus network is accomplished by each station listening to the network.
When a particular station hears nothing for a specified period, it will try to claim the token. A
number of stations can try to claim the token at the same time but only the highest addressed
station will win the token.
Periodically, each station on the network checks to see if a station that has an address between
that of itself and the station it-passes the token to (its successor in the logical ring), wants to enter
the network. If there is such a station, the original station makes the new station the recipient of
the token (its successor), therefore allowing the new station to enter the ring. It is conceivable
that more than one station could try to enter the logical ring at the same time (i.e., have addresses
between the same two stations). The protocol is set up such that the highest addressed station
will be allowed into the ring and the other station(s) will have to try again at the next opportunity.
When a station no longer desires to be a part of the logical ring, it tells the station preceeding it
(previous station) that it is leaving the ring. The previous station will then determine its new
successor, and pass it the token.
For more detailed information on how the IEEE 802.4 standard works, refer to APPENDIX A IEEE
802.4 OPERATION or the IEEE 802.4 standard.
1.3.2 Options Within IEEE 802.4
The token bus controller implements both the priority and request with response (RWR) options
outlined in the IEEE 802.4 standard. These options make the TBe suitable for use in real time
networks as specified in MAP version 3.0.
The request with response option allows any station that is a member of the logical ring to
communicate with any station(s) that is not a member of the logical ring. This is useful in real
time networks because stations that do not have much data to transmit do not add unnecessary
delay to the token rotation time. The longer the token rotation time is, the longer it is between
each station's opportunity to transmit. Not being a part of the logical ring allows these stations
to have less functionality, simpler software, and lower cost. Whenever a station needs data from
a station that is not a member of the logical ring, the requesting station sends a request with
response frame to the station it wants a response from. The responding station then returns a
response while the requesting station still holds the token.
The four levels of priority are optional. If the user chooses not to implement priority, all frames
are received and transmitted out of the highest priority queue. If priority is implemented, frames
are linked into one of the four queues according to their priority. The TBe implements priority
by transmitting out of the highest priority queue until there are no more frames in that queue or
until the timer for that priority has elapsed. The timer for the highest priority queue is set by the
user while the timer for lower priorities is set partially by the user and partially by network history.
The TBe then either passes the token or checks the next lower priority queue for frames to transmit.
If the timer for the next lower queue has not elapsed (the TBe has not passed the token), the TBe
transmits out of that queue until there are no more frames to transmit or until the timer for that
priority (which has been set by the user) has elapsed, and so on through all four queues. Finally
the TBe passes the token to the next station. While receiving, frames of the same priority are
linked together so the host can easily read the data by priority queue.
1.4 TBe ENVIRONMENT
The token bus controller provides ordered access to the token bus medium while under the overall
supervision of a microprocessor. The communication between the microprocessor and the TBe
is primarily through shared memory, but also uses a command structure. .
MOTOROLA
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MC68824 USER'S MANUAL