BECKHOFF BK2000 - Manual (Page 12)

Basic information
12 BK2000
The peripheral data in the process image
The bus coupler features different memory areas that possess a size of
256 words each. Targeted access to any chosen memory cell is possible
by way of the Beckhoff-Lightbus telegrams. Two relevant areas of the
memory can be distinguished by means of the control and status byte in
the Beckhoff-Lightbus telegram and can be addressed separately. The
value in the control and status byte for triggering a bus coupler update is
10hex and, to do this, the data byte must contain the constant 80hex.
Access to the data in the bus coupler is then possible. To do this, the
control and status byte contains the value 30hex. With one access
operation, two bytes can be written and two bytes can be simultaneously
read. You will find an exact description in the following chapters.
After switching on, the bus coupler determines the configuration of the
inserted input/output terminals. The affiliations between physical slots of
the input/output channels and the addresses of the process image are
established automatically by the bus coupler.
The bus coupler creates an internal allocation list in the input/output
channels that have a specific position in the bus coupler's process image.
Here, a distinction is made between inputs and outputs and between bit-
oriented (digital) and byte-oriented (analog or complex) signal processing.
Two groups are generated, each one only with inputs or only with outputs.
In one group, the byte-oriented channels are at the lowest address in
ascending order followed by the bit-oriented channels.
Digital signals
(bit-oriented) Digital signals are bit-oriented. This means that one bit of the process
image is assigned to each digital channel. The bus coupler sets up a block
of memory containing the current input bits and arranges to immediately
write out the bits from a second block of memory which belongs to the
output channels.
The precise assignment of the input and output channels to the process
image of the control unit is explained in detail in the Appendix by means of
an example.
Analog signals
(byte-oriented) Processing of the analog signals is fundamentally byte-oriented. The
analog input and output values are stored in the memory in a two-byte
notation. The values are represented as "SIGNED INTEGER". The
numeric value "0" stands for the input/output value "0V",0mA" or "4mA". In
the default setting, the maximum value of the input/output value is
represented by "7FFF" hex. The intermediate values are accordingly
proportional with respect to one another. The area with a resolution of 15
bits is not realized with every input or output stage. In the event of an
actual resolution of 12 bits, the last three bits are of no effect for outputs
and, for inputs, they are read as "0". Each channel also has a control and
status byte. The control and status byte is the most significant byte in the
most significant word. An analog channel is represented with four bytes in
the process image, three bytes of which are used. (In the BK3000 and
BK4000, only two bytes are occupied for each analog channel in the
process image of the corresponding bus system. The control and status
bytes of the bus terminals can also be inserted by reconfiguration in the
bus coupler and in the bus terminals.)