There are three pins each connected to the three stepper motor drivers by a five-way DIN plug, photo 42. There is also a 5 volt positive supply in the cable as well. How the setup works is this, the 5 volts is fed into the driver's control input terminal. The control input terminals are isolated electrically from the rest of the driver to stop anything nasty from the higher voltages from reaching the Arduino. The output from the terminal is connected to a pin on the Arduino via a current limiting resistor. When the pin is high no current flows as everything is at the same voltage level. But when the pin is set low it is now at Ovolts and the current flows and the driver registers this. The pin is then brought high again and the current stops flowing and the driver detects the current dropping and steps the motor. This is known as motor step on falling edge of pulse. All the Arduino has to do is switch a pin from high to low and back to high to step the motors.
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