Technical data

PROFIBUS networks
1.2 Basics of the PROFIBUS network
PROFIBUS Network Manual
16 System Manual, Edition 04/2009, C79000-G8976-C124-03
1.2.3 Access mechanism
TOKEN BUS/masterslave method
Network access on PROFIBUS corresponds to the method specified in IEC 61158-2 /
EN 61158-2, "Token Bus" for active and "masterslave" for passive stations.
Token rotation
(logical ring)
Logical token ring
Master-slave relationship
Slave
Master Master Master Master Master
Slave SlaveSlaveSlave
Figure 1-2 Principle of the PROFIBUS medium access technique
Active and Passive Nodes
This access procedure is independent of the transmission medium. Figure 1-1 "Principle of
the PROFIBUS medium access technique" shows the hybrid technique with active and
passive nodes. This is explained briefly below:
● All active nodes (masters) form the logical token ring in a fixed order and each active
node knows the other active nodes and their order in the logical ring (the order does not
depend on the topological arrangement of the active nodes on the bus).
● The right to access the medium, the "token", is passed from active node to active node in
the order of the logical ring.
● If a node has received the token (addressed to it), it can send frames. The time in which it
is allowed to send frames is specified by the token holding time. Once this has expired,
the node is only allowed to send one high priority message. If the node does not have a
message to send, it passes the token directly to the next node in the logical ring. The
token timers from which the maximum token holding time is calculated are configured for
all active nodes.
● If an active node has the token and if it has connections configured to passive nodes
(masterslave connections), the passive nodes are polled (for example values read out) or
data is sent to the slaves (for example setpoints).
● Passive nodes never receive the token.
This access technique allows nodes to enter or leave the ring during operation.