How to Control a Stepper Motor Using a Computer. Stepper motors produce accurate, computer-controlled motion for applications such as robotic arms and paper-feed mechanisms for printers.

Although the stepper claimed to have a 1:64 reduction gear, the actual ratio wasn’t that precise ( [Rjeuch] estimates it as 1:63.876. The steps he took to fix this are worth a read.

Rather than having the host Pi communicate directly with the stepper drivers, the 3+Pi features an onboard STM32F302CBT6 that handles the actual motor control.