Siemens SINUMERIK 840D sl Programming Manual page 305

Measuring job status for MEASA, MEAWA
If an evaluation is required in the program, the measuring task status can be checked via
$AC_MEA[n], where <n> = number of probe. Once all the trigger events of probe <n> that
are programmed in a block have occurred, this variable returns a value of "1". Otherwise, its
value is 0.
Note
If measurement is started from synchronized actions, $AC_MEA is no longer updated. In this
case, new PLC status signals DB31, ... DBX62.3 or the equivalent variable
$AA_MEAACT[<axis>] must be checked.
Meaning:
$AA_MEAACT==1: Measurement active
$AA_MEAACT==0: Measurement not active
Continuous measurement (MEAC)
The measured values for MEAC are available in the machine coordinate system and stored in
the programmed FIFO[n] memory (circular buffer). If two probes are configured for the
measurement, the measured values of the second probe are stored separately in the
FIFO[n+1] memory configured especially for this purpose (defined in machine data).
The FIFO memory is a circular buffer in which measured values are written to $AC_FIFO
variables according to the circular principle, see the chapter titled "Motion-synchronous
actions".
Note
FIFO contents can be read only once from the circulating storage. If these measured data
are to be used multiply, they must be buffered in user data.
If the number of measured values for the FIFO memory exceeds the maximum value defined
in machine data, the measurement is automatically terminated.
An endless measuring process can be implemented by reading out measured values
cyclically. In this case, data must be read out at the same frequency as new measured
values are read in.
Job Planning
Programming Manual, 06/2009, 6FC5398-2BP20-0BA0
4.8 Extended measuring function (MEASA, MEAWA, MEAC) (option)
Special motion commands
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