Siemens SIMATIC Ident Function Manual page 101

Principle of controlling non-cyclic communication with command and acknowledgment counter
Figure B-1
As can be seen from the diagram above, an acyclic frame triggers the change from one
defined status to the next. A new acyclic frame is not permitted until the next status is
reached. An acyclic frame is either a command to the CM or an acknowledgment from it.
For this reason, it is important to inform the master whether a new acyclic frame can be
executed. Each state is coded in 2 bits and counted up (as shown in the diagram above) The
terms status buts or status counters are also used.
The status bits are transferred cyclically via PROFIBUS DP or PROFINET IO to the master.
The user must evaluate the bits in his program. When the state bit changes, a new state
(new state = old state + 1) is created. Only now can the next acyclic frame be sent.
Two states must be coded.
1. Command status (command counter) to indicate to the user whether a new/next
2. Acknowledgment status (acknowledgment counter) to indicate to the user whether a new
FB 45 for MOBY U, MOBY D, RF200, RF300
Function Manual, 07/2012, J31069-D0166-U001-A4-7618
B.1 Programming the communication modules on PROFIBUS/PROFINET
Command and acknowledgement counter states
command may be transferred to the CM.
acknowledgment from the CM is pending.
Programming communications modules
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