The emergency stop

Hello,
I'm using a PCI-7354 motion control board and a UMI-7774  motion interface. I'm trying to implement an emergency stop button. I think i'm not the first one trying to do this, but i'm a little bit confused about how this is implemented in the NI hardware.
First let me explain how i think an emergency stop should work. It should be a normally closed switch, so if the button is pressed the contact is released (no more electrical connection). If the button is pressed the drive should be disbaled and there should be no more movement. Using a normally closed contact means that the system doesn't work or stops working when the emergency stop button is disconnected or if there is a break in the cable leading to the button. In this way you  make sure that the emergency button is functional before you start moving. I should think this is the standard and most safe way to implement an emergency stop and i know most motor drives have it implemented in this way.
But now back to the NI motion hardware. I know about the shutdown input and this document :
http://digital.ni.com/public.nsf/websearch/622529A109BAD05E862569EA0060304F?OpenDocument
Here it says that the shutdown circuit requires a rising edge to be activated. A rising edge looks like a normally open switch which is closed when pressed. This system doesn't work when the button is not connected or the cable is broken. What am I missing here?
Thanks in advance,
Youri

Thank you both for your replies.
Jochen: I agree with you that there is discussion about what action is taken after the emergency stop button is pressed. This depends largely on the application. But I still think that the way the button is connected to and handled by the motion controller should be in the way i descriped. I've read the threads you mentioned, but they both seem like a lot of work-arounds to me.  In my opinion (and by the knowledge i have on this moment) the shutdown input doesn't respond the way i would like it to do. So i'm wondering how this shutdown input is intended to be used and why there isn't a proper e-stop input on the NI motion controller. These motion controllers are widely used and easy to set up by inexperienced people.  There should be a easy, simple and most of all safe input for an e-stop button. But again, i might be missing something.
 Giridhar: Thanks for the tip. I'm going to look into it.
Kind regards,
Youri

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