Token-Passing Method:
In order to achieve the coordination between the active stations (master devices), a token-
message (special data frame) is passed on from one active station to the next (in order of the
ascending addresses). The active station, which possesses the token at the moment, has the
right to access the bus and to communicate with passive stations (slaves) or with other active
stations. When it has finished its communication tasks, it passes on the token to the next
station.
In order to guarantee that each station can send data in any case after a maximum period of
time, it is necessary to set a maximum period of time during which the token must have
completely passed once through the logical ring. For this purpose, the target rotation time TTR
is set during the configuration of the bus system. When a station receives the token, it
compares the target rotation time with the measured real rotation time TRR. The station is
allowed to send messages as long as the target rotation time is not exceeded. In any case the
station has the right to send one message with high priority. Figure 2 depicts the message
processing algorithm.
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