BECKHOFF BK4000 - Manual (Page 13)

Basic information
BK4000 13
The peripheral data in the process image
After power on, the bus coupler determines the configuration of the inser-
ted input/output terminals. The bus coupler automatically assigns the affi-
liations between the physical slots of the input/output channels and the
addresses of the process image.
The bus coupler generates an internal allocation list in which the in-
put/output channels have a specific position in the process image. Here, a
distinction is made according to inputs and outputs and according to bit-
oriented (digital) and byte-oriented (analog or complex) signal processing.
Two groups with only inputs or outputs each are formed. In one group, the
byte-oriented channels are at the least significant address in ascending
order. This block is 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 out-
put 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 ana-
log input and output values are stored in the memory in a two-byte notati-
on. The values are represented as "SIGNED INTEGER". The numeric va-
lue "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 re-
spect 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 correspon-
ding 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 termi-
nals.)
Special signals and
interface The BK4000 supports bus terminals with further interfaces such as RS232,
RS485, incremental encoders or others. These signals can be controlled
just like the above-mentioned analog signals. To some extent, a bit width of
16 does not suffice for the special signals. The bus coupler is capable of
supporting any byte width. The effective byte length of the bus terminals
can be preset with the KS2000 software.
Default assignment of
inputs and outputs to the
process image When the bus coupler is first switched on it determines the number of atta-
ched bus terminals and sets up a list of assignments. This list distinguishes
between analog channels and digital channels and between input and out-
put; 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.