Siemens Sinumerik 840D sl Programming Manual page 477

Note
The preprocessing memory is not filled or filling is interrupted if the machining step contains
commands that require unbuffered operation (search for reference, measuring functions,
etc.).
Note
The control generates an internal preprocessing stop in the event of access to status data
($SA...).
If tool offset or spline interpolations are active, a STOPRE command should not be
programmed, as this will lead to contiguous block sequences being interrupted.
Example: Stop preprocessing
Program code
...
N30 MEAW=1 G1 F1000 X100 Y100 Z50
N40 STOPRE
...
Job planning
Programming Manual, 02/2011, 6FC5398-2BP40-1BA0
CAUTION
8.3 Program sequence with preprocessing memory (STOPFIFO,
Comment
; Measurement block with probe at first
measuring input and linear
interpolation.
; Preprocessing stop.
Path traversing behavior
477