". It's probably a good idea to check the zero point if the power is turned off."
Much more than that.
When the controller is turned off you can assume with certainty that the machine position has lost sync with the DRO. in many cases, even if by some luck there was no physical movement, energizing steppers or servos will cause them to "seek" a step detent or encoder position, thereby rendering an erroneous display of actual position. You can assume this happens every time and if you listen carefully, you can hear it. You cannot always see it.
If the controller remains on and the computer and software is power cycled, the onscreen position will be accurate. This is why it is important to home your machine each and every time the control is cycled off.
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