BECKHOFF BK8000 - Manual (Page 12)

Basic information
12 BK8000
current consumption. In doing so, pay attention to the maximum output
current of the bus coupler that is available for powering the bus terminals.
Using a special power supply terminal (KL9400), power can be fed back
into the K-bus at any chosen point. If you wish to use a power supply
terminal, please contact Beckhoff’s technical support.
The peripheral data in the process image
When the bus coupler is first switched on it determines the configuration of
the attached input/output terminals and automatically assigns the physical
slots of the input/output channels to the addresses in the process image.
The bus coupler sets up an internal list of assignments in which each of the
input and output channels has a specific position in the process image. A
distinction is made here between input and output and between bit-oriented
(digital) and byte-oriented (analog, or complex) signal processing.
It also forms two groups, whereby one contains only inputs and the other
only outputs. In each group, the byte-oriented channels take the lowest
addresses, in ascending order, and these are then 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) The processing of analog signals is always byte-oriented and analog input
and output values are stored in memory in a two-byte representation. The
values are held as "SIGNED INTEGER” or "twos-complement”. The digit
"0” represents the input/output value "0V”, "0mA” or "4mA”. When you use
the default settings, the maximum value of the input/output value is given
by "7FFF” hex. Negative input/output values, such as -10V, are
represented as "8000” hex and intermediate values are correspondingly
proportional to one another. The full range of 15-bit resolution is not
realized at every input/output level. If you have an actual resolution of 12
bits, the remaining three bits have no effect on output and are read as "0”
on input. Each channel also possesses a control and status byte in the
lowest value byte. If the control/status byte is mapped in the control unit
has to be configured in the master configuration software. An analog
channel is represented by 2 bytes user data in the process image.
Special signals and
interface A bus coupler supports bus terminals with additional interfaces, such as
RS485, RS485, incremental encoder, etc.. These signals can be regarded
in the same way as the analog signals described above. A 16-bit data
width may not be sufficient for all such special signals; the bus coupler can
support any data width.
Default assignment of
inputs and outputs to the
process image When the bus coupler is first switched on it determines the number of
attached bus terminals and sets up a list of assignments. This list
distinguishes between analog channels and digital channels and between
input and output; which are grouped separately. The assignments begin
immediately to the left of the bus coupler. The software in the bus coupler
creates the assignment list by collecting the entries for the individual
channels one at a time, counting from left to right.