VEGA VEGACOM 557 Profibus FMS - Manual (Page 46)

46 VEGACOM 557 Profibus FMS
Supplement
Therefore, the Profibus access procedure
includes the Token-Passing procedure for
communication among complex bus partici-
pants (Master), and underlies the Master-
Slave procedure for communication of the
complex bus participants with low-expendi-
ture peripherals (Slaves). This combined
procedure is called hybrid bus access pro-
cedure (see figure 3).
The Token-Passing procedure empowers
the Token to distribute the bus access au-
thorisation within an exactly defined time
frame. The Token message, a special tel-
egram for transmission of the dispatch au-
thorisation from one Master to the next
Master, must be transferred to each Master
in sequence with a (parameter definable)
max. Token circulation period. The Token-
Passing procedure is used on the PROFIBUS
only between the complex participants (Mas-
ter).
The Master-Slave procedure enables the
master (active participant), currently in pos-
session of the dispatch authorisation, to con-
tact its allocated Slave instruments (passive
participant). The master has here the option
of transferring messages to the slaves or
collecting messages from the slaves.
Figure 3 shows a Profibus structure with
three active participants (masters) and seven
passive participants (slaves). The three
master instruments form a logic token ring.
The PROFIBUS layer 2 operates without a
connection. It enables, beside the logic point-
to-point data transmission, also the multiple
point transmission with broadcast and
multicast communication.
With broadcast communication, an active
participant emits an unacknowledged mes-
sage to all other participants (Master and
Slaves).
With multicast communication, an active
participant emits an unacknowledged mes-
sage to a group of participants (Master and
Slaves).
M M V
Hybrid bus access procedure
Logic Token ring between the master instruments
PLC PLC
PROFIBUS
Sensor Sensor Drive Actor Sensor Drive Trans-
mitter
Passive stations, Slave instruments