Hi. I think the command is specific for a CAN bus connected accelerometer.
Regarding to the documentation of M956 i changed the command to the following and got sample data:
G1 X-50 G4 S2 G1 X50 F20000 M400 M956 P0 S1000 A0
Note that in this command the P value changed from P124.0 to P0
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