BECKHOFF BK8000 - Manual (Page 13)

Basic information
BK8000 13
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
Analog inputs/ouputs are representative of other complex multi-byte signal
bus terminals (RS485, SSI sensor interface, ...)
Overview of the subdivision of the process image in the bus coupler:
Output data in the bus
coupler
O0
...
byte-oriented
data
...
Ox
Ox+1
bit-oriented
data
Ox+y
Input data in the bus
coupler
I0
...
byte-oriented
data
...
Ix
Ix+1
...
bit-oriented
data
...
Ix+y
The way from I/Os to the
process image in the
application software
The bus coupler automatically assigns the I/Os of the terminals to the
process image in the RS485 communication protocol. These allocations
can be modified with the Beckhoff KS2000 configuration software. Various
mapping parameters (e.g. Motorola/Intel format) can be set in the bus
coupler.
Data consistency Data which contains no contradictions is said to be consistent. The
following consistency is required here: 1. The high byte and low byte of an
analog value (word consistency), 2. The control/status byte and the
corresponding parameter word for accessing the register. The interaction
of the peripherals with the control unit means that data can initially be
guaranteed consistent only within an individual byte: the bits which make
up a byte are read in together, or written out together. Byte-wise
consistency is quite adequate for processing digital signals but is not
sufficient for transferring values longer than eight bits, such as analog
values. The various bus systems guarantee consistency to the required
length. It is important to use the appropriate procedure for importing this
consistent data from the master bus system to the control unit. The
protocol for communication with the BK8000 always exchanges the
coupler's complete process image, thus ensuring data consistency.
Processing complex signals All byte-oriented signal channels such as RS232, RS485 and incremental
encoder, can use byte lengths greater than two. Apart from the actual
difference in length, the procedure is always comparable with that for
analog signals