Siemens SIMATIC Ident Function Manual page 106

Programming communications modules
B.3 Methods of operation with the communications module
The following diagram shows the procedure used for command and acknowledgment
communication between user (PROFINET/PROFIBUS master) and communications module
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 bus
● If the PN/PB master module only provides limited resources (buffers) for acyclic data
● If the master module can be set to permit several acyclic frames between cyclic data
● When commands need to be processed by the CM or reader for which there is not
● The CM does not necessarily need the chaining bit in the command. However, from the
● The number of buffers on the CM/reader depends on the CM or reader type. Refer to the
106
Command execution: Command chaining and buffering
each other.
transmission rate and the tag transmission rate.
transfer, data transmission may take quite some time. This is particularly noticeable in
extensive bus configurations with CMs.
exchange, data transmission can be speeded up in a bus configuration with many CMs.
However, this has a negative effect on the cyclic data exchange of I/O modules that are
also part of the same PROFIBUS line. The cycle time of PN/PB becomes irregular and
sporadically may become very high.
enough buffer space on the CM/reader, the user must first fetch results from the CM
before new commands can be sent to the CM.
user's point of view, it is an elegant way to identify related partial commands. A chaining
bit set in the command is returned by the CM in the acknowledgment.
table in the section "Command repetition (Page 37)" and section "Brief description of the
hardware (Page 81)".
FB 45 for MOBY U, MOBY D, RF200, RF300
Function Manual, 07/2012, J31069-D0166-U001-A4-7618