Motorola MC68824 User Manual page 137

II
A.3 INITIALIZATION
TERMS
Slot Time
A measurement of time consistent throughout the network which is defined as the maximum
amount of time any station need wait for an immediate response from another station. It is
primarily dependent on the worst case station delay in the network. The mathematical formula
is given in 2.1.24 Slot Time.
Bus Idle (Inactivity) Timer
A timer within the Token Bus Controller MAC that determines how long the station will listen
to silence before trying to initialize the logical ring. This timer is set at seven slot times unless
it is the lowest addressed station when it is set to six slot times.
"Claim-Token" Control Frame
Control frame sent to initialize the logical ring. Any station that has not heard anything on
the bus for a specified time (the inactivity timer expired), will send a "claim-token" frame
to start the initialization sequence. A "claim-token" frame has a data field length of zero,
two, four, or eight slot times, depending on the address of the station.
Initialization of the logical ring is a special case of adding new stations. One station on the network
will claim the token and the algorithm is set up such that the station who claims the token is the
highest addressed station. Each active station on the network monitors the medium. If nothing
happens for a specified length of time, that station's bus-idle (inactivity) timer times out. When
the bus-idle timer expires, the station sends a "claim-token" control frame. The length of the
"claim-token" frame information field is in multiples of slot time (zero, two, four, or six) and is
determined (the first time) by the two most significant bits in the station's address. Having different
lengths of the "claim-token" frames is done in anticipation that more than one station will attempt
to establish the logical ring at the same time. After a station sends a "claim-token" frame, the
station waits one slot time for its transmission to die out, then samples the cable. If the station
hears non-silence, it knows that a station with a higher address is also attempting to establish a
logical ring, so the lower addressed station (TS) defers initialization to the higher addressed station.
If the station heard silence, it sends another "claim-token" frame with the next two significant
bits of its address determining the length of transmission. When all the address bits have been
used in the above manner plus one more claim token frame whose length is determined by two
random bits and silence is still sensed, the station has won the initialization sequence and now
holds the token. The logical ring is built from here by adding new stations as described in the
following paragraphs.
A.4 PASSING THE TOKEN
TERMS
"Token" Control Frame
A bit pattern used to determine when a station can transmit
"Who-Follows" Control Frame
Control frame used to determine who the station is that is the successor to this station's NS,
which is contained in the data field of the frame. A "who-follows" frame is used to skip over
a non-working station to delete it from the logical ring. After this frame is sent, the sending
station waits three slot times for a response. An appropriate response is a "set-successor"
frame.
MOTOROLA
A-2
MC68824 USER'S MANUAL