BECKHOFF BK2000 - Manual (Page 14)

Basic information
14 BK2000
The path from the I/Os to
the Beckhoff-Lightbus
process image
Mapping list Mapping list
Mapping Mapping
by byte by bit and byte
buildt inside the master
by software on the PC
buildt automatically
by the bus coupler
II/O-Lightbus
II/O-Lightbus
BK2000
C1
22
0
I
O
O O O
I I I
The bus coupler creates for itself internal allocations of the peripheral data
to the process image. By means of the KS2000 software, the allocation list
can be modified on a PC. Placeholders are inserted and offsets of the data
in the process image are realized. The figure shows the subsequent "1:1
transfer" via the field bus. The peripheral data can be allocated to the
memory addresses of the controller in the master.
Data consistency Data is referred to as being consistent when it is transferred and processed
coherently in relation to its content.
This is particularly important with regard to
1. the high and the low bytes of an analog value (word consistency over
one or several word(s)),
2. the control/status byte and the affiliated parameter word for access to
the registers.
Fundamentally, data consistency in connection with the periphery and con-
troller, and depending on the bus master used, is only ensured for one byte
or one word. That is to say, the bits belonging to a byte or a word are read
together or are output together. Byte-by-byte consistency is adequate for
processing digital signals. Consistency must be extended in cases when
values with a length in excess of 8 bits, e.g. analog values, are transferred.
Attention must be paid to the correct method of transferring the consistent
data from the master of the bus system to the controller. A detailed
description of the correct method can be found in the corresponding
operating manual for the bus system, in particular the description of the
master interfaces used. With regard to the Beckhoff-Lightbus, data
consistency is ensured for the complete data block of one "CDL". In this
case, the controller can only access one completely transferred data area.
The Beckhoff-Lightbus telegrams are used for access to the data in the
BK2000 and BK2010 bus couplers. You will find application examples in
the annex.
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.