VEGA VEGACOM 557 Profibus DP User Manual

Page 76

VEGACOM 557 Profibus DP

Supplement C

Therefore, the Profibus bus access proce-

dure includes the T oken-Passing pr ocedure

for communication among complex bus par -

ticipants (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

enpowers

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 transferr ed to each Master

in sequence with a (parameter definable)

max. Token circulation period. The T oken-
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).

Hybrid bus access procedure

Logic token ring between the Master instruments

PLC

PROFIBUS

Sensor

Drive

Actor

Sensor

Drive

Trans-

mitter

Passive stations, Slave instruments