System Description S5-95U, SINEC L2
1.3 Procedure for Accessing the SINEC L2 Network
The following section discusses the basic mode of operation of the SINEC L2 network. The goal of
this section is to make you familiar with terms that you will need to configure the S5-95U as a
station and to assign parameters in the S5-95U.
An essential aspect of the network is the access procedure. On the SINEC L2, there are two types
of stations with different access rights: active stations and passive stations.
Active Stations
• Are allowed, when they have the right to transmit data to other stations
• Are allowed to request data from other stations
Passive Stations
• Are allowed to exchange data with an active station only after being requested to do so by the
latter.
Whether a station is active or passive depends on the respective unit. Simple field units such as
motor control units are as a rule passive; smart units such as programmable logic controllers, on the
other hand, are active.
The S5-95U can be a passive as well as an active station on the network. This is possible by
setting parameters.
Station Address
Each station on the bus has a station address that you can assign by setting parameters.
Token / Right to Transmit
So that all active stations do not try to access the bus at the same time, an active station that is
ready to transmit has to wait until it receives the right to access the bus. The station receives this
right through a special frame, the token frame. The structure of the token frame and how its
transmission is controlled will not be discussed here (refer to the PROFIBUS Standard, DIN 19245).
It is important for you to know the following:
• The token frame (and the permission to access) automatically goes from one station to the next
one (according to the ascending sequence of the station addresses).
• The token frame is passed on in the logical ring: The station with the highest address passes
the token frame on to the station with the lowest address. For each active station there is one
token rotation cycle between token frame transmitting and token frame receiving .
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EWA 4NEB 812 6112-02
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