Remember from our first column, I mentioned the 4-wire bipolar stepper motors. This motors require a proper of sequencing the direction of the current through two sets of coils. We can do this if we have two H-bridges, one for each coil. Now we are using lots of transistors, eight of them, to control a single motor. Fortunately many H-bridge ICs (such as the L293D) are dual H-bridge chips.
Listing 3, bipolar.fth shows the code to drive a bipolar stepper motor through the above sequence.