BECKHOFF BC3100 - Manual (Page 13)

Basic information
BC3100 13
The peripheral data in the process image
After power on, the BC3100 determines the configuration of the plugged-in
input/output terminals. The affiliations between the physical slots of the
input/output channels and the addresses of the process image are defined
automatically or by programming by the bus terminal controller depending
on the settings via the configuration interface. If these affiliations are
programmed, digital and analog signals can be distributed channel by
channel in any order to the process image of the PLC task (global variables
%I* (inputs) and %Q* (outputs)) or of the field bus (process data that is
transferred through the field bus). The setting is defined manually with the
configuration interface or, depending on the field bus functionality of the
bus terminal controller, with the TwinCat System Manager at the variable
level.
By default, automatic allocation is set for the bus terminal controllers. This
is described below:
The BC3100 creates an internal allocation list in which the input/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 and only outputs each are formed. The byte-
oriented channels are located in ascending order at the lowest address in
one group. The bit-oriented channels are located after this block.
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 terminal controller 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 terminal controller 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.
Word Alignment The analog or special signals are mapped with word alignment when the
peripheral signals are allocated into the process image of the PLC task