Unipolar Stepper

[micherte]

My Propscope works well with the Stamp applications but not for measuring a unipolar stepper motor (6V, 200mA) which is controlled by an unknown controller (no Stamp involved); see attachment.

When I want to measure the voltage pattern of a stepper motor the Propscope shows some results but the motor stops turning or turns extremely slowly.·Therefore, I do not know whether the measured signals are right (I·connect the ground probe to the ground of the motor and positive probe to one of non-ground wires). When I disconnect the probes the motor start turning again.

Any help for the right measurement set-up is welcome.