This board is based on Allegro Microsystems A3977 Stepper Motor Driver IC.
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The A3977 is a complete microstepping motor driver, with built-in translator. It is designed to operate bipolar stepper motors in full-, half-, quarter-, and eighth-step modes, with output drive capability of 12 V and ±2.5 A. The A3977 includes a fixed off-time current regulator that has the ability to operate in slow-, fast-, or mixed-decay modes. This current-decay control scheme results in reduced audible motor noise, increased step accuracy, and reduced power dissipation.
To set the max current per phase place voltage meter from gnd to REF. Then follow table below to get your desired current by adjusting the Cur pot.
You can also select the Decay mode with the PFD pot. When a STEP input signal commands a lower output current from the previous step, it switches the output current decay to either slow-, fast-, or mixed-decay depending on the voltage level at the PFD input. If the voltage at the PFD input is greater than 0.6 VDD then slow-decay mode is selected. If the voltage on the PFD input is less than 0.21 VDD then fast-decay mode is selected. Mixed decay is between these two levels. Use again your voltmeter to adjust the voltage with the pfd pot.