For unipolar stepper motors the drive configuration is simply 4 transistors to ground. These are grouped in pairs with each pair driving the ends of a single coil that has its center tap connected to a supply voltage. In addition to the transistors we need to add a clamping circuit to clamp the inductive fly back. The zener in the clamp circuit may be connected to ground as shown in the slide, or may be connected back to Vs either configuration is acceptable. The second will have a lower dissipation in the zener.