The stepper board consists of a ULN2803 chip.
It consists of 8 Darlingtons to amplify current.
A total of 2 unipolar stepper motors can be controlled with this card.8 bit or 4 bit Data from the Micro Board reaches the inputs from where it is amplified to the outputs.
A visual indication of the incoming data is possible with the help of LED's.

Related items : Download the ULN2803 Texas Instrument's datasheet from here