Siemens SIMATIC FC 45 Function Manual page 121

Command chaining and command buffering on the ASM
Command chaining is indicated when the chaining bit (bit 6 in the command) is set (see also
Section "MOBY commands).
Command buffering is a characteristic of the ASM. A variety of buffers are available to the
ASM for intermediate storage of commands and results.
Use of command chaining and command buffering involves the following characteristics:
• Programming a function block becomes more complex.
• Optimum data throughput to and from the MDS.
The following diagram shows the procedure used for command and acknowledgment
communication between user (DP master) and MOBY-ASM when a chained command is
used.
Figure B-7
The following general conditions apply to the procedures shown in the diagram above.
• It is obvious that data transmission and execution of the commands take place parallel to
• The sequences shown in the diagram above may vary depending on the transmission
• If the PROFIBUS implementation on the DP master only provides limited resources
FC 45
Function manual, Release 03/2006 , J31069-D0167-U001-A2-7618
This is particularly noticeable with large amounts of data ( 1 KB) and slower PROFIBUS
transmission rates.
Command execution: Command chaining and buffering
each other.
speeds of PROFIBUS and the MOBY-MDS.
(buffer) for non-cyclic data transmission, the PROFIBUS data transmission may take
quite a while. This is particularly noticeable in extensive bus configurations with MOBY-
ASM.
Programming the MOBY-ASM on PROFIBUS
B.3 Methods of operation with the ASM
B-9