Hi,
You need to look into more details.
You need to SEPARATE the two windings if you want to use the motor as "BiPolar".
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NOTE: This type of "Bipolar" motor with connected-together center taps CAN be changed to "Unipolar" with the use of a different more complex driver board and modifying the motor as shown in THIS PFD FILE
RewireTheSmallStepperMotor.pdfRewireTheSmallStepperMotor.pdf
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See the details. You have to open the motor cover and CUT the connection between the two sets of windings.
BUT: Why do you need/want to do that when there are easy-to-use and cheap driver boards for this motor??