BECKHOFF BK7000 - Manual (Page 13)

Basic information
BK 7000 12
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 im-
age 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 repre-
sented as "8000” hex and intermediate values are correspondingly propor-
tional 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 con-
figured in the master configuration software. An analog channel is repre-
sented by 2 bytes user data in the process image.
Special signals and
interface A bus coupler supports bus terminals with additional interfaces, such as
RS232, 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 at-
tached bus terminals and sets up a list of assignments. This list distin-
guishes between analog channels and digital channels and between input
and output; which are grouped separately. The assignments begin imme-
diately to the left of the bus coupler. The software in the bus coupler cre-
ates the assignment list by collecting the entries for the individual channels
one at a time, counting from left to right. These assignments distinguish
four groups:
Function type of the channel Assignment level
1. Analog outputs byte-wise assignment
2. Digital outputs bit-wise assignment
3. Analog inputs byte-wise assignment
4 Digital inputs bit-wise assignment