usb-6008 limit switches analog I/O

[newbie09]

hi im using a USB-6008 to identify the position of four switches, two home and two limit, for an XY table. I am using AI0-AI3 for the analog input, and AO1 for the outout voltage. I have the positive side of the channels wired to the switch, and from there, in parallel, to the AO1.The negative side of the channels are wired in series to pin 16, gnd, is this possible?should they be in series or parallel or what? i have downloaded Vi's on continous analog output Vi,s but when i check the values on the screen compared to the voltmeter, they do not correspond. I have tried vi's that use the analog inout, but i cannot get the inout to responde as i wish? any thought or sugggestions greately appreciated. ( note i cannot use to digital side of the USB-6008 as i need those outputs to control the direction and enable of the stepper motors.)

Best Regards

Peter

[smercurio_fc]

I don't quite understand what you mean by "in parallel to the AO1". Are you saying you're trying to measure the output voltage at the same time you're generating it? The USB 6008 does not have isolated inputs/outputs, so the ground is common. It might help if you drew a picture of what you have.

[newbie09]

yes i am trying to gernerate a signal and then measure it at the inputs... is this possible with the usb 6008, or do i need a seperate voltage source?

[Dennis_Knutson]

Sure it's possible but you should really post an image of your circuit since I am also a bit confused on how you are physically doing this. Of course, you can do this without using the analog output at all. If you have a pullup on one side of the switch (the side going to the analog in), then you just have to have the other side of the switch grounded. Then you would read either +5 or 0V.

Do not attempt to read a voltage with the analog input open.

[newbie09]

This is how i have it wired...

[Dennis_Knutson]

That image is next to worthless. What about a schematic version such as the one below?

Message Edited by Dennis Knutson on 01-10-2010 09:24 AM

[newbie09]

This is how uit is wired at the moment, but it doesn't seem to be working. how would the pull up resistor work. I am a mechanical engineering student, struggling with the labview side of a project.

Regards

Peter

[Dennis_Knutson]

The pullup resistor will give you a +5V input when the switch is open. As I said, you cannot leave an analog input open like your design. The 6008 has a +5 output that you can use and your AI configuration should be RSE.

p.s. Try to keep your images to a smaller size if you are going to keep posting things like that. If you don't have a schematic capture program, it would have been better to just use Paint for the schematic.

[newbie09]

cheers i'll know for again. how would i go about closing it.. any chance of a schematic on it? the 5V supply on the digital side is not being used so that can be utilised.

[Dennis_Knutson]

Closing what? The limit switches? They aren't something you would control with the 6008. Aren't they closed by the table?