Siemens SIMATIC NET PROFIBUS manual Transmission Techniques, Active and Passive Nodes

PROFIBUS NETWORKS

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:

SAll 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).

SThe right to access the medium, the “Token”, is passed from active node to active node in the order of the logical ring.

SIf 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.

SIf 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).

SPassive nodes never receive the token.

This access technique allows nodes to enter or leave the ring during operation.

1.2.3Transmission Techniques

The physical transmission techniques used depend on the SIMATIC NET PROFIBUS transmission medium:

SRS-485 for electrical networks on shielded, twisted pair cables

SOptical techniques according to the PROFIBUS User Organization guideline /3/ on fiber-optic cables

SWireless techniques based on infrared radiation

SIEC 61158-2 technique for intrinsically safe and non-intrinsically safe electrical networks in process control (PROFIBUS-PA) based on shielded, twisted pair cables.

PROFIBUS Networks SIMATIC NET	1-9
6GK1970-5CA20-0AA1 Release 2 05/2000