How to tell if motorshield has power

I have an Arduino Uno running a single stepper motor using this externally powered motor shield:

http://www.electrodragon.com/w/TB6560_3A_Stepper_Motor_Driver_Board_Single-Axis

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*How can I determine on the Arduino side if there's power to the motor shield? *

I'm definitely a novice, so go easy on me. In my simple mind, I would love it to be as simple as hooking up a wire from the motor shield to the Arduino and reading high voltage on that pin, but given that the voltage to the motor shield is more than 5v, I'm a little worried about frying my uno.

i had to look at the data sheets, the Arduino should not power this board. The inputs are optically isolated and well protected. You must power the board separately, with 12v and use input resistors accordingly.

rcorr: i had to look at the data sheets, the Arduino should not power this board. The inputs are optically isolated and well protected. You must power the board separately, with 12v and use input resistors accordingly.

Yes, I am powering the motor shield from an external source (24v, 3A). What I want to do it detect from the Arduino if power to the motor shield is actually present.

I'll explain a little more to the setup so you can see my dilemma. I have an android application sending commands to the Arduino (via USB, serial) and the Arduino is then running the motor. If there is no power to the motor shield, the Arduino will happily accept commands from the Android and send signals to the motor shield, but the motor shield has no power (thus it does not run the motor). I'd like to detect when the motor shield has no power to it.

In keeping with the inputs, I would use an optical isolator like on your board(the black chips at the inputs) to keep all noise away from the Arduino. Where you connect depends on your security requirements, you can grab the power coming in at the terminals or the run LED as feedback.

Thank you for the suggestion.

I like the idea of tapping into the LED on the motor shield. There's a power LED (in addition to the run LED), so I think that would be simple and straight forward.

Now on to the programming part of it. I guess I have to decide if I'm going to check the pin that is connected to the LED on the motor shield once in the setup procedure, or check it each time in the loop. Since I'm creating the dependency, I might as well enforce it 100% of the time and check it each time in the loop. Any objections?