R_LT=LOW
I'd guess right there.
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Why is "setup" empty?
Your code could be considerably simpler if you took the time to combine the individual bits read into numbers, either using "bitWrite" or mask and shift operations.
if(M1_EN==0)
analogWrite(EN1,LOW);
else
analogWrite(EN1,M1_EN);
That's a bit verbose; lose the "if/else" and just write M1_EN to EN1.