Siemens SIMATIC ET 200S Operating Instructions Manual page 161

Note
Only one of the following control bits can be set at a particular time:
CMP_VAL1 or CMP_VAL2 or LOAD_VAL or LOAD_PREPARE or C_DOPARAM.
Otherwise, the ERR_LOAD error is reported until all the specified control bits are deleted
again.
The ERR_LOAD error bit is only deleted when the following is carried out correctly.
Acknowledgment Principle in Isochronous Mode
In isochronous mode, exactly 4 bus cycles are always required to reset the status bits and to
accept values during the load function.
Figure 3-20
Technological Functions
Operating Instructions, 06/2010, A5E00124867-07
Acknowledgment Principle in Isochronous Mode
1Count5V
3.6 Count Modes
161