Voltage Output with cRIO-9474

[jaredkuper]

I have made a very small program to create a square wave output on pin 0 of the cRIO-9474. I am using a 9103 chassis and a 9004 controller. When I hook a scope to monitor the voltage between pin 0 and common, I can see a voltage change at the correct frequency, but it's not quite right. The voltage change looks like it is only about 80mv, and the waveform somewhat resembles a charging/discharging capacitor. Also, there is another square wave present @ 333kHz and 40mv which only goes away when the cRIO is downloading the program. I cannot find any documentation for the 9474 except for the Operating Instructions located here: http://www.ni.com/pdf/manuals/373509b.pdf and they state that my output range is 5V-30V, even though mine looks to be limited to around 100mv. Also, if/when I do get a cleaner square wave, how can I control the output voltage since the range is 5-30V?

[Jeff M]

First, I suggest that you double-check the wiring configuration of the external power that is being supplied to the cRIO-9474. It seems that your program is operating correctly, but that no external power is being supplied, which is why the output voltage reading is almost 0 V (even though the frequency is correct).

See page 5 of the Operating Instructions you referenced above, which states that "You must connect an external power supply to the
cRIO-9472/9474. This power supply provides the current for the devices you connect to the module. Connect the positive lead of the
power supply to the supply terminal or pin, Vsup, and the negative lead of the power supply to the common terminal or pin, COM."

If you don't want to use an external supply, often times it's possible to daisy chain the power being supplied to the controller, by connecting that same supply to the module as well. This is not the recommended method, so if you choose to do this be sure to take into consideration the current required to drive your device, and how that will be divided between the controller and the module.

The amount of voltage being supplied from the cRIO-9474 is simply the same voltage being supplied from your supply supply to the module. So if you supply any voltage that falls within spec (5-30V), for example 24 V, then the module will apply a square wave from 0 V to 24 V at the frequency you specify.

For comparing power supply specs, an example list of DC power supplies is listed at the following link:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14300

Hope this helps!

Jeff M.

Applications Engineering
National Instruments

[jaredkuper]

The problem was with my external voltage supply, bad grounding connection. The program works like it's supposed to so far, now I just have to get rid of some capacitance problems so I can turn the frequency up. Thanks.