dc motor does not work with monster moto shield VNH2SP30

hi
I want to control a dc motor with monster moto shield, Uno Arduino and Matlab
as the datasheet of the shield the digital pins 7 and 8 are used for running the dc motor, I write this code in Matlab:

a=arduino('com9','uno');
writeDigitalPin(a,'D7',1);
writeDigitalPin(a,'D8',0);

this code is same as digitalWrite(7,HIGH);digitalWrite(8,LOW) in Arduino program
but nothing happened
please help because I have to control the dc motor with Matlab

What's the advantage of using MatLab?

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