BECKHOFF BC3100 - Manual (Page 19)

PROFIBUS-DP basics
BC3100 19
PROFIBUS-DP basics
Introducing the system
The PROFIBUS enjoys a very wide acceptance in automation technology
due to its openness and its wide manufacturer-independent distribution.
The PROFIBUS was developed in the course of a group project on the
fieldbus concept which aimed at agreeing on a standard. Numerous
different products are now available from independent manufacturers
which all conform to the standard DIN 19245 parts 1 and 2. Standards-
conform PROFIBUS devices can be operated on any bus system.
PROFIBUS specifies the technical and functional characteristics of a serial
fieldbus system which can be used to network distributed digital and
analog field automation devices with low range (sensor/actuator level) to
midrange performance (cell level). PROFIBUS makes a distinction
between master and slave devices; master devices are those which govern
the data traffic on the bus.
A master may send messages without an external request, provided it has
the authority to access the bus. The PROFIBUS protocol also describes
masters as "active stations“.
Slave devices are peripheral devices. Typical slave devices are sensors,
actuators, transducers and Beckhoff bus couplers or bus terminal
controllers. They are not assigned any bus access privileges, i.e. they may
only acknowledge received messages or send messages to a master at its
request. Slaves are also referred to as passive stations.
PROFIBUS DP
PROFIBUS DP is designed for rapid data exchange at sensor/actor level,
where centralized control devices (such as stored program control units)
communicate with decentralized input and output devices by means of a
fast serial connection. The exchange of data with these decentralized
devices is carried out predominantly cyclically. The centralized control unit
(master) reads the input data from the slaves and writes the output data to
the slaves, whereby the cycle time of the bus needs to be shorter than the
program cycle time of the central control unit, which will be under 10 ms in
many applications.
Rapid data throughput alone is not sufficient for the successful
implementation of bus system. Ease of handling, good diagnostic facilities
and fault-proof data transfer technology must all be provided in order to
fulfill the users’ requirements. The characteristics have been optimally
combined in PROFIBUS DP.
At a data transfer rate of 12 Mbits/s, Profibus-DP needs less than 2 ms to
transfer 512 bits of input data and 512 bits of output data distributed to 32
stations. This meets the requirement for a short system reaction time.
System configurations and
device types You can use PROFIBUS DP to implement mono-master or multi-master
systems, which gives you a high degree of flexibility as regards the system
configuration. Up to 126 miscellaneous devices (master or slaves) can be