BECKHOFF BK4000 - Manual (Page 15)

Basic information
BK4000 15
Data consistency Data is described as being consistent if its contents fit together. The follo-
wing contents belong together:
1. the high and low bytes of an analog value (word consistency).
2. The control/status byte and the affiliated parameter word for access to
the registers (e.g. 3-byte input/output for an analog channel).
In the interplay of the peripherals and the controller, data consistency is
fundamentally initially only ensured for one byte (for most IBS masters,
consistency exists for one word). That is to say, the bits of one byte are
read in together or are ouput together. The InterBus transports all data of
all inputs and outputs in one cycle. After an error-free transfer, the data is
available for the controller. In practical terms, the master realises the „ma-
king available“ by switching over from the old data block to the one that is
now the current one. If the controller accesses the data area at the time of
changeover, it reads a portion of the old data and a portion of the new da-
ta. As the controllers only access the memory word by word or even only
byte by byte, the corresponding data consistency is produced. For proces-
sing of digital signals, byte consistency is sufficient. In cases when values
with a length of more than 8 bits are transferred, e.g. in the case of analog
values, the consistency must be extended. Attention must be paid to the
correct method of acceptance by the controller of the consistent data from
the bus system’s master. Refer to the corresponding operating manual of
the master system for a detailed description of the correct procedure.
Complex signal processing All byte-oriented signal channels such as RS232, RS485 or incremental
encoders partly operate with byte lengths of more than two bytes. With the
exception of the length difference, handling of these is always comparable
to that of analog signals.
Calculating the effective
length The greater number of inputs or outputs defines the effective length in the
InterBus system. If the number of inputs and outputs does not correspond,
remaining words are inserted in the process image as „blank words“. The-
se words are read with the contents „0000hex“ and writing to these
addresses has no effect (but the data is transferred and is stored in the bus
coupler’s memory).