Adafruit Motor Shield V2.0 Manual page 9

speed
Description
Set speed, control by PWM.
void move(int motor_id, int speed);
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motor_id
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0 - Chanel 0
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1 - Chanel 1
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speed: -100~100, the larger the faster, 0 for stop
stop
void stop(unsigned char motor_id);
brake
void brake(unsigned char motor_id);
Stepper
Note that we use the library provided by Arduino IDE to drive a stepper.
There's something need to be modified, please refer the examples.