BECKHOFF BK2000 - Manual (Page 17)

Beckhoff-Lightbus coupler BK2000
BK2000 17
Beckhoff-Lightbus coupler BK2000
Presentation of the Beckhoff-Lightbus
system
The Beckhoff-Lightbus is widely accepted in automation technology thanks
to its speed and its cross-manufacturer use. The Beckhoff-Lightbus came
into being within the scope of a comptroller concept for the realization of
NC axes on the industrial PC. The aim of the project was to develop an
industrially suitable, high-speed and reliable I/O level for the PC. A large
number of diverse products from independent manufacturers is nowadays
available. The operation of diverse Beckhoff-Lightbus units on one bus
system is ensured by implementation support and protocol ASICs from the
BECKHOFF company.
The Beckhoff-Lightbus was conceived for a fast exchange of data at the
sensor/actuator level. Here, central controllers (for example, programmable
logic controllers) communicate via a high-speed serial link with
decentralized input and output devices. Data is exchanged with these
decentralized devices cyclically and, if required, with different priorities.
The central controller (the master) reads the input information from the
slaves and sends the output information to them. In doing so, the bus cycle
time must be shorter than the program cycle time of the central controller
which, in many applications, is less than 1 ms.
A high data throughput alone does not suffice for the successful use of a
bus system. Instead, easy handling, good diagnostics possibilities and
fault-free transmission technology must be provided to meet users'
requirements. With the Beckhoff-Lightbus, these characteristics are
optimally combined.
At a data transfer rate of 2.5 Mbits/s, Beckhoff-Lightbus needs
approximately 0.8 ms to transfer 512 bits of input data and 512 bits of
output data distributed to 32 stations. Therefore, the call for a short system
reaction time is ideally met.
System configurations and
device types A mono master system can be realized with the Beckhoff-Lightbus. A
maximum of 254 slaves can be connected to one bus. In the BK2000 bus
coupler, a station address between 1 and 254 is selected automatically in
the startup phase. The system configuration definitions contain the number
of stations, the allocation of the station address to the I/O addresses, data
consistency of the I/O data and the format of diagnostic messages. Every
Beckhoff-Lightbus system consists of different device types.
An Beckhoff-Lightbus slave is a peripheral device (sensor/actuator) that
reads in input information and which sends output information to the
periphery. Devices are also possible that only provide input information or
only output information. Typical Beckhoff-Lightbus slaves are devices that
feature binary inputs/outputs for 24V or 230V, analog inputs, analog
outputs, counters and incremental encoders etc. The quantity of input and
output information depends on the device and is defined by protocol ASIC
as 32 bits of input data and 32 bits of output data. For slaves that handle a
larger amount of data than 32 bits, for example the BK2000, use is made of
an extended method. By means of an addressed access method, it is
possible to read and write up to 256 x 16 bits. This means that one system
is capable of processing up to 254 stations x 508 bytes (not all 512 bytes
are used as user information data) in only one Beckhoff-Lightbus system.
To reduce complexity and for implementation reasons, the currently
available masters operate with a maximum user information data length of
3 Kbytes (24,000 inputs and outputs).