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8.3 Command-oriented data exchange
Command-oriented data exchange uses the PCP channel
P
eripherals
C
ommunication
P
rotocol, see [I2]). It is bidirectional and allows parameters to be set for event-
oriented data exchange, as well as the transmission of data from and to data
carriers/the SLK.
The following
functions
are supported:
• Writing data (MDT data only in
CONNECTED
state). This also includes, e.g., also
switching the link state
• Reading data, e.g., also reading the actual link state
• Configuring unbuffered prefetch
• Configuring buffered prefetch
These functions are
communication objects
in the PCP channel and have various
object indexes. An index is therefore the address of a communication object.
Specific parameters of an object are represented by subindexes. The subindex 0
addresses the entire object.
Example:
The object 5FF5 (
direct
reading and writing) has three subindexes.
Subindex 1 contains the SLK address (4 bytes), subindex 2 the desired number
of bytes (2 bytes) and subindex 3 the (octet) string of the bytes in the payload. If
subindex 3 is read, the SLK first takes the data block indicated in subindexes 2 and 2
from the MDT and returns it as the octet string in subindex 3.
If the Interbus master sends the payload in subindex 3 to the SLK, the data is
entered in the data area indicated in subindexes 1 and 2.
Before the payload in subindex 3 is transferred, the desired data block in
subindexes 1 and 2 has to be indicated.
Setting the parameters for the prefetch functions “unbuffered prefetch” and
“buffered prefetch” allows multiple data block requests to be entered. Each
command adds a new SLK data block specification to those already defined. When
the prefetch is executed, the data blocks are read by the MDT in the specified order.
All functions are listed in the table in Chapter 8.3.2 “PCP channel objects”.
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