HEIDENHAIN PWM 20 - Manual (Page 176)

November 2014 Checking incremental encoders 179
Example:
Linear (or rotary) encoder with one reference mark
Traverse the RM, invert the direction and traverse the RM again (oscillate about the RM; >> <<).
The Reference display value must always be 0, even with high scanning frequencies.
If the value deviates by as little as ±1 signal period, the encoder is faulty.
Schematic representation of incremental linear and circular graduations with one reference
mark:
For more information on reference marks refer to the product brochures available at
www.heidenhain de.