I have include Cout=0 in the program, why every time when I start to run the program, one of the motor auto run for a short period i.e.0.5 seconds then auto stop? Thank you.
During reset and before you set the pin as an output to zero, the Stamp's pins will float. That particular pin may be floating high, causing your motor driver to turn on. The remedy is a pull-down resistor (~10K) between that pin and ground (Vss), which will hold the pin low before your program has a chance to assert it as an output.
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-Phil