3 transmission techniques – Siemens SIMATIC NET PROFIBUS User Manual
Page 19
PROFIBUS NETWORKS
1-9
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Active and Passive Nodes
The access technique is not dependent on 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:
S
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).
S
The right to access the medium, the “Token”, is passed from active node to
active node in the order of the logical ring.
S
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.
S
If an active node has the token and if it has connections configured to passive
nodes (master-slave connections), the passive nodes are polled (for example
values read out) or data is sent to the slaves (for example setpoints).
S
Passive nodes never receive the token.
This access technique allows nodes to enter or leave the ring during operation.
1.2.3
Transmission Techniques
The physical transmission techniques used depend on the SIMATIC NET
PROFIBUS transmission medium:
S
RS-485 for electrical networks on shielded, twisted pair cables
S
Optical techniques according to the PROFIBUS User Organization guideline /3/
on fiber-optic cables
S
Wireless techniques based on infrared radiation
S
IEC 61158-2 technique for intrinsically safe and non-intrinsically safe electrical
networks in process control (PROFIBUS-PA) based on shielded, twisted pair
cables.