6501 Output Selection

[dacad]

HI,

I am using a couple 6501 USB DAQ's to control some solid state relays (or trying to) but the modules seem to be defaulted to a push/pull output config while I need the open collector output.  The manual says this is software selectable but 1) I can not find anything in the hardware configuration screens to accomplish this though 2) the properties box is greyed out and not accessible. I have DAQmx 8.1 installed though am trying to download 8.3 (the download is excrutiatingly slow from NI's website) but am not sure if this will address the issue.

Anything I am missing here ? How do you get to the properties screen for these units ?

Thanks

Doug

[stilly32]

Hi Doug,

It sounds like you're looking for the DOOutputDriveType property. you can access it in LabVIEW from a channel property node, Digital Output>> Output Drive Type. You can create a constant and set the desired value. It should look like this in LabVIEW:

If you were using C (for future posts its nice if you include your programming environment) you would use  DAQmxSetDOOutputDriveType();.

Hope this helps, please post back if this wasn't what you were looking for.

Andrew S.

National Instruments 

Message Edited by stilly32 on 11-03-2006 03:43 PM

[MartyD]

Andrew,

Thanks for the reply however this does not appear to work. We are attempting to control these in Labview 7.1

Regardless of how we set up the configuration, we can only get a 3.2 or 3.6 vdc signal from the digital outputs. This is even with nothing connected.

We are attempting to connect a pull-up resistor to get a 5vdc out of the digital outputs but it would be nice to know how to do it without that.

We are driving 13N06 negative fets to control the higher voltage devices. These should draw less than 1/2 milliamp

Any help is appreciated.

Doug

[Jennifer O]

Hi Doug,

Can you verify the voltage coming from your USB bus by measuring your +5V line?  Refer to this Knowledge Base article for information on how the USB bus can affect your output voltage.  Based upon the circuitry, if you do not use an external resistor, your output voltage should be:

 Vout=Vb x R/(R + 4.7k)

Vb is the voltage of your USB bus and R is the resistance of your load. 

Using a test panel I ran this with a 22 kOhm resistor connected between P1.7 and Ground.  With a 4.75V bus I get an expected output of approximately 3.9V.  Note that without an external resistor connected, drawing 0.5mA will result in a 2.35 V drop from your bus voltage (o.5mA x 4.7 kOhm).

Assuming a maximum current draw of 0.5A, you may wish to calculate an external resistance value according to the following equation:

 Vout_min=Vbus-0.5mA[(4.7k x Rexternal)/(4.7k + Rexternal)]

Vout_min is the minimum output high that you require and the contents of the square brackets is the equivalent parallel resistance of the 4.7k and the external resistance.

Hope this helps,

Jennifer O
Applications Engineer
National Instruments

[dacad]

I'm certain I am geting +5v from the USB line. I am using a powered hub and when I check voltage on the daq from the +5v pin to gnd I get a full 5v. I am simply not able to get 5v from any of the outputs. The 6501 specs indicate that the outputs are rated .4mA without a pullup resistor @ 5v. With no load, I am not able to get 5v on any of the output pins regardless of the programming. (I did use the above shown routine and still measured only 3.6v on the output pins with no load attached)

I was able to use a pull-up resistor to achieve +5v to my fet's and make the circuit work. From my position, this is a work around since +5vdc control voltage is pretty standard and to have to add additional components to get it is adding time and cost to my work.

The reason this bothers me is that I can use the comparable Measurement Computing module (1024LS) and am able to get +5v on the ouputs without any special configurations. For this project, I have switched over to this daq. That change is easier than wiring in a bunch of resistors.

At any rate, I thank you for the assistance.

Doug

[Jennifer O]

Hi Doug,

I think the issue you are seeing may be a matter of how you are connecting your signals.  I went back and verified the proper signal connection technique for the Open Collector.  If you connect your load between your port and ground there will always be a measurable drop across the internal resistor assuming that current is flowing from the 5V rail to ground.  The size of this drop was described in my previous post.

For Open Collector mode the proper connection is between the +5V rail and the output port. This is shown in Figure 6 on page 7 of the User Guide and Specifications Manual.  When the port is high, it will be at the same potential as the rail and no current will flow.  When the port is pulled low, the 5V (or bus voltage) is applied to your load. 

Please verify that you are connecting your signals between +5V and the port as shown in the diagram.  Does this enable you to achieve the full 5V in place of 3.2 or 3.6 V? Try several lines to confirm that they are working correctly.  If you are still having difficulties after following this suggestion, could you please post a description of how you are connecting your signals and where the 3.2V is being measured?

Regards,

Jennifer O.

Message Edited by Support on 08-20-2007 09:39 AM